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The Reactivity of 1,3,2-Diheterophosphacyclanes Containing a Pentacoordinate Phosphorus

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1985 Russ. Chem. Rev. 54 1126

(http://iopscience.iop.org/0036-021X/54/11/R06)

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1126

Translated from Uspekhi Khimii, 54, 1899-1939 (1985)

Russian Chemical Reviews, 5 4 (11), 1985

U.D.C. 547.341

The Reactivity of 1,3,2-Diheterophosphacyclanes Containing aPentacoordinate Phosphorus Atom

N.A.Polezhaeva and R.A.Cherkasov

Studies on the reactivity of X5-1,3,2-diheterophosphacyclanes obtained during the last ten years are examined and describedsystematically. Data concerning new prototropic and elementotropic processes involving derivatives of pentacoordinatephosphorus are presented. Attention has been concentrated on the elucidation of the influence of the incorporation of aphosphorus(V) atom in the ring on the reactivity of phosphoranes in reactions of various types. The role of phosphorus(V)compounds as intermediates in reactions of phosphorus compounds involving different types of coordination of the phosphorusatom is demonstrated.The bibliography includes 176 references.

CONTENTS

I. Introduction

II. Hydrophosphoranes

III. Hydroxyphosphoranes. The phosphate-phosphorane tautomerism

IV. Aminophosphoranes. The phosphimide-phosphorane tautomerism

V. Other tautomeric transformations of phosphoranes. Phosphorotropic processes

VI. Thermolysis of phosphoranes

VII. Nucleophilic substitution at a pentacoordinate phosphorus atom

1126

1126

1130

1137

1139

1140

1142

I. INTRODUCTION

The study of the relation between the structure and reac-tivity of 1,3,2-diheterophosphacyclanes containing tricoor-dinate x and tetracoordinate2 phosphorus atoms revealed thespecific features of the chemical behaviour of these organo-phosphorus compounds (OPC) associated with the inclusionof a phosphorus atom in the cyclic molecular skeleton. Thereactivity of five-membered cyclic OPC of the above type hasbeen found to be particularly unusual, which was manifestedin the kinetic, synthetic, and stereochemical results of thereactions involving them.

In the present review an attempt is made to give a system-atic account of the survey of the literature data on the reac-tivity of heterocyclic compounds of pentacoordinate phos-phorus containing in the cyclic fragment, together with thephosphorus(V) atom, two other identical or different hetero-atoms (Ο, Ν, S), namely λ5-1,3,2-diheterophosphacyclanes.In a number of instances data on the reactivities of acyclicphosphorus(V) compounds and 1,2-heterophosphacyclaneswith a similar ligand environment around the phosphorus atomare adduced for comparison.

In the review literature available hitherto attention hasbeen concentrated on the structural 3 " 5 and to a lesser extentthe synthetic 6" 8 chemistry of compounds of pentacoordinatephosphorus. Although modern ideas about the structure, theligand reorganisation processes, and the role of intermediatesof the phosphorane type in the chemical reactions of OPChave been developed precisely in relation to λ5-1,3,2-diheterophosphacyclanes,9 the relation between the structureand reactivity of these OPC has been studied systematicallyonly for a few examples.1 0 During 1970—1980, new experi-mental data were obtained which deepened our knowledgeabout the chemical behaviour of compounds of pentacoordinatephosphorus. New types of phosphoranes were synthesised;some of them are stable models of the intermediates formedin reactions of compounds with different types of coordinationof the phosphorus atom. New types of prototropic andelementotropic tautomeric systems, including phosphorotropicsystems, containing the phosphorus(V) atom have been dis-covered. Data concerning the mechanisms of nucleophilic

substitution reactions at a pentacoordinate phosphorus atomhave been obtained for the first time. New possibilities forthe employment of phosphorus(V) compounds in organic syn-thesis have been demonstrated.1 1 The "hydroxyphosphoraneconcept", which is a compilation of empirical rules governingthe structure and character of the ligand and valence isomer-isation of the phosphorane molecules, has been developedfurther. 1 2

These and other results of the studies during the last tenyears are included in the present review. The syntheticand stereochemical aspects of the chemistry of X5-l,3,2-diheterophosphacyclanes are used only to the extent thatthey are necessary for the elucidation of the characteristicsof the reactivity of OPC of this type.

I I . HYDROPHOSPHORANES

Information about the methods of synthesis and the chemicalproperties of hydrophosphoranes obtained in the early stagesof the development of the chemistry of these compounds ofpentacoordinate phosphorus, containing a P—Η bond, arepresented in the relevant reviews1 3"1 5 and are not includedin the present review. The principal characteristic featuresof the tautomeric processes involving such compounds andthe results of the study of their reactivity obtained duringrecent years are considered here.

1. The Phosphate—Phosphorane Tautomerism

The isolation of the first spirophosphoranes and somewhatlater of monocyclic phosphoranes with a P-H fragment (seePudovik et a l . l and Arbuzov and Polezhaeva10 and theliterature quoted therein) made it possible to establish theexistence of the hydrophosphorane—γ-hydroxyalkyKaryl)phosphite or γ-aminoalkyKaryl) phosphite tautomeric equi-librium:

•*r H. V -"V -sfS \YH

X, Y= NR, 0 .

Russian Chemical Reviews, 5 4 (11), 1985 1127

The position of this equilibrium depends on the structure ofthe compound, the medium, and temperature. The phos-phorane form is more characteristic of spirocyclanes sub-stituted at the carbon atoms, those containing a secondaryamino-group, and unsaturated and aromatic systems. Adecrease of temperature promotes an increase of the contentof the phosphorus(V) isomer. The introduction of substitu-ents at the endocyclic nitrogen atom in A5-oxazaphospholansshifts the equilibrium towards the phosphite. 1 0 ' 1 6

The ease of the phosphorus(V) -*• phosphorus(III) transi-tion is determined to a large extent by the nature of theendocyclic heteroatoms. For X = Υ = Ο, the equilibrium isdisplaced towards the spirophosphorane even at room temper-ature, particularly in the case of substituted rings. 1 7 Forasymmetrically substituted rings, two and sometimes threephosphorus(III) forms can exist in equilibrium with thehy drophosphorane.

Although a correlation between the polarity of the solventand the content of the phosphite form at equilibrium has notbeen observed, it has been established that basic solventsshift the equilibrium towards phosphorus (I II), while acidsolvents shift it towards phosphorus( V).

An analogous equilibrium for monocyclic phosphoranes hasbeen detected by 3 1 P NMR; in this case too a decrease oftemperature favours the formation of the phosphorus(V) form:

(I)

Hydrophosphoranes of type (I) are interesting becausethey are intermediates in many reactions of derivatives oftervalent phosphorus with proton-donating reagents (seePudovik et al. * and the literature quoted therein) and alsoserve as the starting compounds for the synthesis of alkyl-spirophosphoranes with a P-R bond. 1 8

The type of tautomerism described is characteristic mainlyof hydrophosphoranes, but the observation of the phos-phite—phosphorane tautomerism in the phenylphosphonite(II)—poly eye lie phenylphosphorane (III) system has beenreported: 1 9

RC(O) P h C(O)R

0 — Ρ — O s

(ID

(III)

When the polycyclic phosphorane (III) is dissolved in CHC13,the equilibrium is established in 1 h at 25 °C.

The reactivity of hydrophosphoranes is determined mainlyby the phosphite-phosphorane equilibrium and the presenceof a fairly mobile Ρ—Η proton in the phosphorane isomer. Theelectrophilic properties of the pentacoordinate phosphorusatom, which predetermine the behaviour of spirophosphoranesand in general of phosphorus(V) derivatives as Lewis acids,play a significant role. In the phosphorus(III) form the

reactions can involve one of two nucleophilic centres—thetricoordinate phosphorus atom and the YH(XH) group.

Estimation of the acid properties of hydrophosphoranes2 0

demonstrated a decrease of the Η-donating capacity followingthe replacement of endocyclic oxygen atoms by the NH group[apparently as a consequence of the greater (ρ—(ί)π donorcapacity of the nitrogen atom compared with oxygen 2 0] andon transition from spirocyclic to monocyclic hydrophos-phoranes. The reactivity of hydrophosphoranes in thereaction with diphenyldiazomethane increases in parallel withthe enhancement of the acid properties. 2 0

The ease of the abstraction of the P-H proton on treatmentof hydrophosphoranes with various metallating agents (metalhydrides and amides and organometallic compounds) is alsodetermined by the structure of the phosphorane. 2 1 ' 2 2 Whereasa proton can be abstracted from hydrophosphoranes which arederivatives of pinacol by metal amides (but not hydrides),hydrophosphoranes containing an endocyclic or exocycliccarbonyl group are deprotonated even by diethylamine

According to 3 1P NMR data, the metallic derivatives ofhydrophosphoranes are Ο anions and not Ρ anions (the :JpHdoublet disappears and the signal of tricoordinate phos-phorus appears).2"1 However, a dual reactivity is character-istic of these compounds. Their alkylation by alkyl halides,alkenyl halides, and alkyl sulphates gives rise to a productwith a spiran structure (IV). In the reactions with aromaticaldehydes the products are a-hydroxyalkylspirophosphoranes(V) which are thermally unstable and on heating regeneratethe initial spirophosphorane and the aldehyde. The phos-phoranes (V) can be formed also without the participation ofmetals (see below). Silylation and acylation involve theterminal oxygen atom of the phosphite group and lead tothe phosphites (VI) and (VII) respectively.2 1 We may notethat the reactions of the hydrophosphoranes themselveswith acyl chlorides in the presence of tertiary amines alsolead to γ-acetoxyalkyl phosphites (VII), 2 3 while acylation ofX5-oxazaspirophospholans by acetic anhydride ruptures bothP-N bonds :Zk

(VII)

An analogy with dialkyl phosphites is manifested in thesereactions: metallation takes place at the oxygen atom andthe subsequent reactions with electrophiles can proceed viaboth the oxygen and phosphorus atoms.

Like dialkylphosphorous acids, hydrophosphoranes interactwith aliphatic aldehydes, 2 1 ' 2 5 activated ketones, and animals2 3 ) 2 6 and add to electrophilic unsaturated systems. 2 7 ' 2 8

As already stated, although the a-hydroxyalkylspiro-phosphoranes formed on addition of hydrospirophosphoranesto aldehydes can be isolated, they are unstable and readilydecompose. It has been suggested2 6 that these reactionsinvolve the nucleophilic addition of the phosphorus(III) atomof the form (VHIb) to the carbon atoms of the carbonylgroup; the dipolar ion, whose formation is promoted by anexcess of the carbonyl reagent, is stabilised in two ways:

1128 Russian Chemical Reviews, 54 (11), 1985

the elimination of the epoxide leads to cyclic a-hydroxy-phosphonates (IX), while cyclisation gives rise to spiro-phosphoranes (X):

(Villa) (Vlllb)

χ.(IX)

\tΜ

(X)

Imines are less reactive in the addition reactions of hydro-spirophosphoranes than the aldehydes with a similar struc-t u r e ; 2 8 in the presence of alcohols cyclic enamines oxidisehydrospirophosphoranes to the corresponding pentaoxaspiro-phosphoranes. 2 9 The oxidation products (XII) are obtainedultimately also on interaction with azodicarboxylic ester inthe presence of alcohols:

NHCOOEt

NCOOEt

(NHCOOEt).

(XI) (XII)

It has been suggested3 0 that the adduct (XI) is formed ini-tially, but it is unstable and has not been isolated.

The reactions of hydrospirophosphoranes with aldehydes,ketones, and imines are reversible and their thermodynam-ically controlled products are phosphoranes with an exocyclicP—C bond. Like the interaction with phenyl azide3 1 and withcertain other unsaturated systems, 2 3 ' 2 5 ' 2 8 ' 2 9 these reactionsare characteristic only of the hydrophosphoranes which arein equilibrium with the phosphite form; in the absence ofthe latter, there is no reaction.

In the general case the reactivity of λ 5-1, 3, 2-diheterophos-pholans with Ρ—Η bonds in reactions with electrophilesdepends to a large extent on the nature of the heteroatomsattached to the phosphorus atom. Spirophosphoranes con-taining even one 1, 3,2-oxazaphospholan ring are more reac-tive than tetrahydroxyspirophosphoranes. 2 8

The nature of the endocyclic heteroatom of hydrospiro-phosphoranes influences also the synthetic result of thereactions involving addition to acetylenic reagents. Tetra-hydroxyhydrophosphoranes readily add to acetylenedicar-boxylate esters and proprionates with formation of vinyl-phosphoranes (XIII) . 3 2 The adduct (XIII), which exists inthe form of two stereoisomers, which can be separated bycrystallisation, combines with a second molecule of hydro-phosphorane at an elevated temperature:

r ROOCC=CCOOR 'OH)OR

(XIII)

ΛCOOR

' _ C H _ P

Ό '2 COOR X Ο

It is striking that the P-C bond in the vinylphosphorane isfairly labile and is readily broken on treatment with analcoholic solution of sodium alkoxide.3 3 This can beaccounted for by the high electrophilicity of the phosphorus-(V) centre.

If the spirophosphorane contains even one endocyclic Ρ—Νbond, then the reaction products are the stereoisomericphosphites (XV). However, ring-substituted A5-oxazaphos-pholans and X5-dioxaphospholans form compounds of penta-coordinate phosphorus of type (XIV). It has been sug-gested31* that the phosphite formed is involved in the additionprocess (Scheme 1); hydrophosphoranes which are not inequilibrium with the phosphorus (I II) isomer are inert insuch reactions.

Scheme 1

IIX"^

R'C=CCOOR

\e Ι_Θ—C=CCOOR V,/ι\

HCCOOR

(xiv)

NHCR =CC00Rφ θ

NCR'=CHCOOR •

Z, E-(XV) .

The mechanism presented in Scheme 1 is supported by thefact that alcohols do not react with these acetylenic com-pounds, whereas secondary amines readily add to them. Theinteraction of the γ-aminoalkyl cyclic phosphite isomer there-fore involves the terminal amino-group like the addition ofamines, while the oxa-analogues add as a result of thenucleophilic attack by the tervalent phosphorus atom on theB-carbon atom of the acetylenic group. 32~31>

Vinylphosphoranes of types (XIII) and (XIV) combine witha second hydrospirophosphorane molecule slowly, while theaddition of dialkyl phosphites (in the presence of bases) israpid. This difference has been attributed 3 3 to the differentnucleophilic properties and steric hindrance of the phos-phorus centres of the addends. In the addition of vinyl-phosphoranes, the nucleophile is subjected to a 3-directingeffect [in relation to phosphorus(V)], as in the analogousreactions of the vinyl derivatives of tricoordinate and tetra-coordinate phosphorus. Only the vinylphosphorane obtainedfrom propiolic acid esters (XIV, R' = H) combines with aminesat the α-carbon atom.3 3 Presumably the β-directing effectunder conditions involving competition between the electron-accepting substituents in compound (XIV) is more pronouncedfor the alkoxycarbonyl group than for the spirophosphoranylgroup.

Numerous data demonstrating the manifestation of the Lewisacidity of the phosphorus atom in hydrophosphoranes havebeen obtained. The following series, based on the weakeningof this acidity, which agrees with the decrease of the overallelectron-accepting capacity of the substituents in X5-di-heterophospholan rings, have been found: 2 2

K<XC>CK)

Russian C h e m i c a l R e v i e w s , 5 4 (11), 1985 1 1 2 9

The Lewis acidity of hydrophosphoranes predetermines theircapacity for coordination to bases, which increases the coor-dination number of the phosphorus atom to six: 3 5

Tautomerism involving the formation of the forms (XXI,b - d ) is potentially possible for the bicyclic hydrophos-phoranes (XXIa):

(XVI)

According to 3 1 P NMR data, the reversible reaction ofcompound (XVI) with pyridine leads to compounds of hexa-coordinate phosphorus, whose content in the equilibriummixture increases with increase of temperature. An analogousphenomenon obtains also when other spirophosphoraneswithout the P-H fragments are used, but in these cases itis less marked. 3 6

The interaction of substituted dibenzohydrospirophos-phoranes (XVI) with bases stronger than pyridine (triethyl-amine, triethylenediamine, and diisopropylamine) leads to thephosphoranes (XVII) as a result of the interaction of the twotautomeric forms of the hydrophosphorane: 3 7

(XVI) -OX* i:t3N

(XVII)

The above data indicate the ability of hydrophosphoranesto be involved in a tautomeric equilibrium with γ-hydroxy-alkyl(aryl) phosphites, on the one hand, and (in basicmedia) with derivatives of hexacoordinate phosphorus, onthe other. The presence of both types of equilibria hasbeen established in the (XVIII) ^ ( X I X ) ^ (XX) system: 3 6

(XX)

With increase of the amount of pyridine, the equilibriumshifts towards the "phosphorate" (XX).

(XXIb) (XXIa)

Η—Ρ Ν

HO\

(XXIc) (XXId)

Only the phosphorane (XXIa), which has been found to beinvolved in the Ρ—Η P-D deuterium exchange, has beendetected in solution; 38~1*0 the form containing a phosphorus-(III) atom has been identified in the gas phase by photo-electron spectroscopy. kl Bicyclohydrophosphoranes combinewith phenyl azide.1*2 They react with transition metals toform complexes in which the coordinated ligand can exist ina monodentate or bidentate monocyclic phosphite form of type(XXII).1*3"1*6

co σ

(XXII) (XXIII)

The adduct (XXIII), containing two molecules of the complex-forming agent, has been obtained by the interaction of(XXIa, R = Ph) with Me2S .BH3. "

2

2. Phosphoranide Anions

As pointed out above, the deprotonation of hydroxyspiro-phosphoranes results in the formation of phosphoranideanions, which may be involved in a tautomeric equilibriumwith phosphite-alkoxides. l47'1*8 This method of generatingstable cyclic phosphoranide anions has been used1*8"52 forthe synthesis of stable cyclic anionic systems (XXIV)-(XXVII)from the corresponding hydrophosphoranes; the basesemployed were butyl-lithium5 0 '5 1 or triethylamine: HB'52

(XXV) (XX.VI)

X-Ray diffraction (XRD) data for the anion (XXVII) haveshown that it has the structure of a distorted trigonalbipyramid (TBP) with the unshared electron pair (LEP)in the equatorial (e) position and with a large differencebetween the lengths of the axial (α) Ρ—Ο bonds (1.77 and2.02 A)." 8

Species of the type of phosphoranide anions have beenpostulated as unstable intermediates in bimolecular nucleo-philic substitution reactions at a tricoordinate phosphorusatom: 1 ' 5 3

>: +

1130 Russian Chemical Reviews, 5 4 (11), 1985

The isolation and identification of stable phosphoranideanions is of fundamental importance for the understandingof the mechanisms of nucleophilic substitution at the atomsof not only tricoordinate1 but also tetracoordinate 2 andpentacoordinate (see Section VII) phosphorus atoms, which,at least in cyclic OPC, can proceed via a stage involving theformation of intermediates with an increased, comparedwith the initial phosphorus compound, coordination numberof the central atom.

A phosphoranide centre can be formed also intramolecu-larly: 5"

(XXXI)

(XXVIII) (XXIX)

The equilibrium (XXVIII) ^ (XXIX), detected by 3 1P NMR, isstrongly displaced towards the form (XXVIII). Intermolecularnucleophilic substitution in this system takes place via anintermediate with a hexacoordinate phosphorus atom. 5 5

Unusual polycyclic phosphoranides have been synthesisedby the reaction of the hydrophosphorane (XXX) with transi-tion metal compounds.5 6 According to XRD data, the phos-phorus ligand in these compounds is bidentate and is coor-dinated to the metal atom via both the phosphoranide andamine centres.

III. HYDROXYPHOSPHORANES.PHOSPHORANE TAUTOMERISM

THE PHOSPHATE-

The development of the chemistry of hydroxyphosphoranesis closely related to modern ideas concerning the mechanismof nucleophilic substitution reactions at a tetrahedral phos-phorus atom [ S N 2 P ( I V ) ] , which are based on the rulesassociated with the hydroxyphosphorane concept—intermedi-ates having a TBP structure with a pentacoordinate phos-phorus atom are formed in these processes (see Cherkasovet al. 2 and the literature quoted therein). The character ofsuch intermediates determines to a large extent the reactivityof phosphorus(IV) compounds and the synthetic and stereo-chemical results of nucleophilic substitution reactions. Onhydrolysis of phosphonium salts [Eqn.d)] or solvolysis ofphosphoryl compounds by proton-containing nucleophiles[Eqn.(2)], the above TBP intermediates should have thehydroxyphosphorane structure:

• Iproducts; (1)

(XXX)

We may note, incidentially, that the first diphosphoranecompound with a P(V)-P(V) bond has been obtained from apolycyclic hydrophosphorane: 5 7

Compound (XXXI) contains a direct bond between thephosphonium and phosphoranide phosphorus atoms. Theinteraction with chloranil results in the formation of a"phosphoniaphosphorate"—the first compound with aPdV)-P(VI) bond: 5 8

»ip=o + HNU —*- > *- products; (2)

Nu = nucleophile.

The identification of the hydroxyphosphoranes and the studyof their properties are therefore of decisive importance for thedemonstration of a two-stage mechanism of the nucleophilicsubstitution reactions at phosphorus(IV), including biochem-ical phosphate transfer processes. 2 ' 5 9 ' 6 0

Nevertheless, until the end of the 1970's, the demonstrationof the formation of hydroxyphosphoranes in the nucleophilicreactions of phosphorus(IV) remained an unsolved problem.The first unambiguous evidence for the formation of hydroxy-phosphoranes was obtained in the study of the interaction ofcyclic phosphoryl compounds with proton-containing nucleo-philes2 and the hydroxyphosphoranes were trapped directlyand successfully characterised in a study of the isomerisa-tion of γ-hydroxyalkyKaryl) esters of cyclic quinquevalentphosphorus acids (see Cherkasov et al. and the literaturequoted therein 2 ) . Methods for the synthesis of stablehydroxyphosphoranes were subsequent developed.

The 5-hydroxy-2-methylphenyl ester of the cyclic four-membered phosphinic acid (XXXII) shows signs in solutionof a dynamic equilibrium due to the isomerisation (XXXII) ^(XXXIV) involving the intermediate formation of the hydroxy-phosphoranes (XXXIII, a and b ) : 6 1

Russian Chemical Reviews, 54 (11), 1985 1131

(XXXII) (XXXIlIo) (XXXIII b) (XXXIY)

The activation energy for the isomerisation process is17 kcal mol"1. The concentration of compounds (XXXIII) isinsufficient for the i r direct detection by NMR, but thepossibility of the i r formation has been demonstrated by thereaction of a mixture of compounds (XXXII) and (XXXIV)with diazomethane: the methyl es ter of the acid (XXXIII)was obtained in 5% yield. It is s t r ik ing that the activationb a r r i e r to the isomerisation of the acyclic analogues (XXXII)is 24 kcal mol"1 for diphenylphosphinate and 28.3 kcal mol"1

for diphenylphosphinothioate, which reflects the well knownsequence in the decrease of the energy of the TBP on pass ingfrom acyclic to monocyclic and spirocyclic s t r u c t u r e s 2 andthe lower stability of mercaptophosphoranes compared withh y d r o x y p h o s p h o r a n e s . 6 2

In this connection we may also note t h a t , when the mono-cyclic si loxyphosphorane (XXXV) is acidified or cautiouslyhydrolysed, the product is not the hydroxyphosphorane(XXXVI) but the isomeric γ-hydroxyalkyl phosphinate(XXXVII): 6 3

(CF,),

(CF 3) 2

(CF 3) 2

H20, -(Me3Si)2O

HCl,-Me3SiC)

S O

USiMe3

(XXXV)

(CF 3) 2

OH

(XXXVI)

Me2P(O)O OH

( C F 3 ) 2 C — C ( C F 3 ) 2

(XXXVII)

However, it is of interest that the reaction of the phosphinate(XXXVII) with the chlorophosphorane (XXXVIII) proceedswith transfer of the reaction centre and leads to theanhydride of the hypothetical P(V)-OH acid (XXXIX):

(CF 3) 2

(XXXVII) + B ' \ l >(GF3)2

(CF3)S

(CF3) : \

Cl

(XXXVIII) (xxxix)

However, the acyclic chloride of dimethylphosphinic acidconverts compound (XXXVII) into the symmetrical bisphos-phinate (XLI) and not into the mixed anhydride (XL): 6 3

These results indicate the possibility of the existence of afairly mobile phosphate—phosphorane equilibrium involvinghydroxyphosphoranes, which has been demonstrated for thefirst time by Ramirez et al.61* and by Chang Bui Cong et a l . 6 5

The product of the desilylation of bis(phenylenedioxy)siloxy-phosphorane (XLII) has the structure of the phosphate(XLIV) in the crystalline state 6 6 [cf. the hydrolysis of thesiloxyphosphorane (XXXV) to the phosphinate (XXXVII) 6 3 ] .However, in acetonitrile at —48 °C the 3 l P NMR spectrumshowed two signals due to the phosphorus nucleus: thehydroxyphosphorane (XLIII) [6( 3 1P) = 27.0] was foundtogether with the phosphate (XLIV) [δ( 3 1Ρ) = +6.7] inequilibrium with the latter and its amount was greater thanthat of the phosphate—the ratio P(IV) : P(V) = 1 : 1.5. Thecontent of the phosphorus(V) form increases in the morebasic acetone (3 : 1 at -48 °C):

Scheme 2

(XLIII1

(XLIV)

C H 2 N 2

(XLV)

OC(0)Me

MeCi-'Ori

^ (and/or) NN.

(ΧΙΛΊ) iXI.VH) C(O)Me

(xxxvii) + M. 2P(O)CI ^ A °P^

(XL·)

- Me2P(O)OC(CF3)2C(CF3)2OP(O)Me : .

(XLI>

The formation of hydroxyphosphoranes has been postulatedalso in other reactions of γ-hydroxyalkyKaryl) esters ofphosphorus(IV) acids. 2

Treatment of the (XLIII) ** (XLIV) mixture with diazo-methane affords a qualitative yield of the methoxyphos-phorane (XLV); acetylation leads to a mixture of theacetyl derivatives of the phosphorane (XLVI) (20%) andthe phosphate (XLVII) (80%) (Scheme 2). However, oneshould note that the synthetic result of the acetylation ofcompound (XXXV) or the (XLIII) =^(XLIV) mixtures doesnot reflect the contents of the phosphorus(IV) and phos-phorus(V) forms in the solutions: the reactions can proceedwith transfer of the reaction centre [this has been confirmed

1 1 3 2 Russian Chemica l R e v i e w s , 5 4 (11), 1985

particularly strikingly by the reaction (XXXVII) +(XXXVIII) -+ (XXXIX) or may be subject to thermodynamiccontrol. It has been established that the acetoxyphosphorane(XLVI) can be partly isomerised to the phosphate (XLVII)under the influence of the HC1 evolved in the reaction.66

In the general case an increase in the content of thephosphorus(V) form in the hydroxyphosphorane—phosphateequilibrium is promoted by a decrease of temperature, byan increase of the basicity of the medium, and by theenhancement of the electron-accepting properties of thedioxyalkylene(arylene) substituent.

The intensity of the hydroxyphosphorane signal in the 31PNMR spectra of the P(V) =?* P(IV) mixture begins to increasewith decrease of temperature in those cases where individuallines correspond to each of the two forms P(V) and P(IV)[ slow, in terms of the NMR time scale, equilibria of the type(XLIII)^(XLIV) and (LVI)^(LVIa) (Table 1)]. Theaveraged signal of the two equilibrium structures appears inrapid equilibria [hydroxyphsophorane (XLVIII) ?* (LI)]. Adecrease of temperature entails a successive upfield shift ofthe resonance peak until the attainment of a fixed positionin the region characteristic of hydroxyphosphoranes67

(Fig.l).

The opening of the five-membered ring and the formationof phosphtate isomers are catalysed by acids. On the otherhand, the introduction of bases into the P(V) ^ P(IV) mix-ture favours the hydroxyphosphorane structure up to forma-tion of stable hydrogen-bonded complexes or salts ofhydroxyphosphoranes, which can be isolated.65"68

However, the structure of the cyclic skeleton of the mole-cules of hydroxyphosphoranes is of decisive importance forthe increase of their stability. Hitherto stable hydroxy-phosphoranes have been identified only for a series of spiro-cyclic phosphoranes made up of five-membered oxa- anddioxa-phospholan rings (the effect of the five-memberedring). 1 2 > 6 9 NMR evidence has been adduced for the formationof relatively unstable acyclic, 7 0 monocyclic,71 and condensedbicyclic hydroxyphosphoranes72 (Table 1).

The stability of hydroxyphosphoranes increases sharply upto the formation of the sole P(V) form in solutions after theintroduction of electron-accepting oxo-groups [compounds(LII), (LIII), (LV), and (LVIII)] or trifluoromethyl groups[compounds (LVII) and (LIX)]. A significant stabilisationof hydroxyphosphoranes is ensured by the presence ofgeminal dimethyl, bis(trifluoromethyl), and diphenyl frag-ments in rings (the Ingold—Thorpe effect2).

The estimation of the factors controlling the relativeamounts of the forms involved in the hydroxyphosphorane^γ-hydroxyalkyKaryl) phosphate equilibrium reduces, otherconditions being equal, to the analysis of the sequence ofequilibria (A)=^ (B)^ (C)^(D) [Eqn.(3)]: 6 7

(A) (B) (C) (D)( 3 )

The position of the hydroxyphosphorane—phosphate equi-librium is largely determined by two equilibrium constants—the dissociation constant Kx and the cyclisation constantproper K2, i.e. Κ = ΚλΚ2.

6 7 The determination of thethermodynamic parameters of the ring closure stages(B) ^(C) (K2) for compounds (XLVIII)-(L) with hydroxyacidligands showed that the P(IV) form is slightly preferablefor the cyclic oxo-compound (XLVIII) (AG° = +0.08, Table 2),whereas the spiran forms are more stable for the monosub-stituted homologues (XLIX) and (L) under these conditions.The parameters obtained are close to those for otherP(V) 6 7 and P(V)**P(VI) 8 0 spirocyclisation processes.

The derivatives of the gem-disubstituted hydroxyacids(LII) and (LIII) and the phosphorane (LIV), synthesisedwith participation of benzohydroxamic acid, exist in thephosphorane forms over a wide temperature range (Fig.l).It is striking that the cyclic derivative of phenylethanediol(LX), whose structure is close to that of the spirophos-phorane (LIV), does not pass to the spirophosphorane formunder any of the conditions investigated. This has beenattributed to the much lower dissociation constants K1 of thephosphate (LX) compared with the phosphorylated derivativeof benzohydroxamic acid (LIV):67

y ^

(LX)

The hydroxyphosphoranes (LVI)—(LVIII) containingendocyclic P—C bonds are exceptionally stable. These com-pounds have been isolated and exist as hydroxyphosphoranesin the crystalline state; the equilibrium with the phosphinateisomer is observed only in solutions of the tetrasubstitutedhydroxyphosphorane (LVI).6 9 '7 6 The perfluoropinaconederivative (LIX) is so stable under the usual conditions thatit can be easily recrystallised in the presence of moisture;the five-membered rings are not ruptured by sulphuric acidand potassium hydroxide.79 In the latter case the poassiumhydroxyphosphoranide is formed readily.

-60ISO Z50

Figure 1. The influence of temperature on 6(31P) forhydroxyphosphoranes:67 1) (XLVIII) (DMF); 2) (XLIX)(DMF); 3) (L) (DMF); 4) (LI) (DMF); 5) (LII) (CD3CN);6) (LII) (PhCN); 7) (LII) (DMF); 8) (LIII) (DMF); thenumbering of the compounds is the same as in Table 1.

The XRD data for the triethylammonium salt of the hydroxy-phosphorane (LIII) 7I* and the salts of oligomeric hydroxy-phosphoranes75 confirmed that they have the TBP structurewith five-membered rings in the α and e positions and theoxy-anion in the e position. The exocyclic P-CT bond inthese compounds is shortened, which reflects its double bondcharacter. 7 3 This is caused in its turn by the influence ofthe oxo-group, as a result of which the phosphorus atomacquires a significant positive charge and the hydroxy-phosphorane acquires fairly pronounced acid properties(Table 1).

Russian Chemical Reviews, 5 4 (11), 1985 1133

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1134 Russian Chemical Reviews, 54 (11), 1985

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Russian Chemical Reviews, 5 4 (11), 1985 1135

The fact that XRD data 6 3 ' 6 6 for the hydroxyphosphinate(XXXVII) and the hydroxyphosphate (XLIV) confirm thefavourable entropy of the hydroxyphosphorane ring closureis striking. The hydroxy-group in the 2-position in thealkylene (arylene) ether group is in direct proximity (~3 A)to the P(IV) atom.

Table 2. Thermodynamic parameters of the equilibriumP(IV) *> P ( V ) - C r . 6 7

Compound

(XLVIIIa)(XLIXa)

(La)

Δ//0, kcal mol"1

—3.9—5—4.8

AS®, e.u.

—13- 1 3—11

AG0*, kcal mol"

+0.08—1- 1 . 3

The isolation of stable hydroxyphosphoranes permittedwell-founded inferences about their acid properties. Thisproblem is of fundamental importance because hydroxyphos-phoranes are partial esters of the hypothetical pentacoor-dinate phosphorus acid P(OH) 5 . 8 1 At different stages of theinvestigation of hydroxyphosphoranes, their acid propertieswere estimated within wide limits—from pKa = 10-11 6I*'66

to claims that the strength of these acids is fairly h igh. 6 5

The latter were based on the ability of hydroxyphosphoranesto form stable hydrogen-bonded complexes with dimethyl-formamide (DMF) and dimethyl sulphoxide (DMSO) and ammo-nium salts with tertiary amines. 6 5 ' 6 7 ' 6 8

Indeed direct determination of the pKa (Table 1) demon-strated their fairly marked dependence on the character ofthe substituents. It was found that the dissociation con-stants of the oxo-substituted hydroxyphosphoranes (LIII)and (LV) are higher than those of the strong dichloroaceticand diphenylphosphoric a c i d s . 7 3 7 5 As expected, in thegeneral case the acid properties of the hydroxyphosphoranesincrease sharply after the introduction of electron-acceptingsubstituents into the five-membered rings (Table 1).

The study of the comparative reactivities of the hydroxy-phosphoranes (LVI) —(LVIII) also demonstrated a significantrole of the nature of the substituents in the ring. Whereasthe methylated derivatives (LVI) is readily converted intothe quasi-phosphonium compound (LXI) on treatment withthionyl chloride, the analogous salts are not formed fromcompounds (LVII) and (LVIII). In these cases the electron-accepting ring substituents apparently destabilise the positivecharge on the phosphorus atom. On the other hand, thedifference between the acid strengths is such that the sodiumsalt of the acid (LVII) can be formed on treatment withalkali, whereas compound (LVI) is deprotonated only withsodium hydride. The oxo-8nalogue (LVIII) is converted intoa phosphorus(IV) derivative in aqueous alkaline solutions. 6 9

The anion exchange between compound (LXI) and trifluoro-methanesulphonic acid leads to the triflate (LXII); the latteris reduced to the phosphoranide anion (LXIII), which givesrise to the hydroxyspirophosphoranes (XLIV) on acidification.It is of interest that the first representatives of hydroxy-phosphoranes have been obtained by oxidising hydrophos-

The description of the TBP structure of hydroxyphos-phoranes is based on the concept of hypervalent bonds: theapical and central atoms of the TBP are linked by a three-centre four-electron delocalised bond, which consists ofone unoccupied and two occupied MO made up of a single ρorbital of each atom involved in the hypervalent bond. 6 9 > 8 2

Such a structure presupposes the presence of long bondsbetween the electronegative α-atoms and the central atomcarrying a partial positive charge.

(LVI) -f- soci2 — • * - h=< y =

phoranes. Alkylation (arylation) of the cations (LXII)with Grignard reagents affords the alkyl(aryl)spirophos-phoranes (LXV).6 9

(LXY)

The stability of the TBP increases after the introduction ofelectron-accepting substituents at the α-atoms and isincreased by the five-membered ring and Ingold—Thorpeeffects. It is believed6 9 '8 2 that a significant differencebetween the electronegativities of the a- and e-atoms, whichis achieved by introducing σ-donor carbon-centred orPiy-donor heteroatomic equatorial ligands, is an importantfactor stabilising hypervalent structures. It has also beensuggested 7 9 that steric shielding by bulky substituents,for example trifluoromethyl substituents in the structure ofcompound (LVII), can also promote an increase of the sta-bility of such systems. This is evidently likewise true ofhypervalent compounds of other elements (S, Si, I ) . 8 2

The importance of the synthesis and study of the structureand properties of hydroxyphosphoranes goes beyond theframework of the investigation of this new class of OPC alone.After the isolation and identification of hydroxyphosphoranes,the hydroxyphosphorane concept and the hypothesis of therole of phosphorus(V) intermediates in the nucleophilicsubstitution reactions at P(IV) received powerful experi-mental support. Data for the new type of phosphorus(V)acids combined with data for the strong acids of tetracoor-dinate phosphorus and weak acids of tricoordinate phos-phorus constitute an integral picture of the complex problemof the relation between the structure and acidity of organo-phosphorus acids. It is extremely significant that thepathways leading to the approach to hydroxyphosphoranespresupposes the "opening" of the very strong (120-150kcal mor 1 8 3 ) P=O bond (in essence addition to the P=Obond). Apparently energy is then gained as a result of thespirocyclisation of the reaction products . 2 ' 6 9 ' 7 6 This makes itnecessary to look again at the character of the driving forcesin many reactions of OPC: it may be that the formation ofstrong P=O bonds (or their analogues), as, for example, inthe Arbuzov or Wittig reactions, is not the only factor which

1 1 3 6 Russian Chemical R e v i e w s , 5 4 (11), 1 9 8 5

determines the energy relations of chemical processes involv-ing OPC and that account must be taken also of other factors,for example, the spirocyclisation and the effects accompany-ing it (see above).

IV. AMINOPHOSPHORANES.PHORANE TAUTOMERISM

THE PHOSPHIMIDE-PHOS-

As stated above, the P=O double bond can participate inthe addition reactions of Η-containing nucleophiles withformation of hydroxyphosphoranes. In intermolecular addi-tion reactions [Eqn.(l)] the latter are not detected—theinteraction models the first stage of the nucleophilic sub-stitution process at the tetrahedral phosphorus atom[Sjy/2P(IV)] which proceeds via an addition-elimination mech-anism. Intramolecular addition to the P=O bond, particu-larly when it is accompanied by spirocyclisation, in somecases leads to stable hydroxyphosphoranes and this prin-ciple can evidently serve as the basis of the synthesis ofstable cyclic phosphoranes.

Indeed the intramolecular addition of the γ-ΖΗ functionalgroup in alkoxy- or aryloxy-substituents to the P=N bondof phosphorus imides makes it possible to obtain stableaminophosphoranes (see Cherkasov et al. 2 and the literaturequoted therein):

atom fully displace the equilibrium towards aminophosphoranes.Electron-donating substituents in the imine fragment (R3 =t-Bu) favour the phosphorus(V) form, while electron-accept-ing substituents (R3 = Ph3C) increase the content of thephosphimides (LXVI) in the tautomeric mixture.85

Thus the enhancement of the electron-donating propertiesof the atom X (O and N) to which a mobile hydrogen atom isattached and of the electron-accepting properties of thephosphorus atom promotes cyclisation. This is seen particu-larly clearly in the diazaphospholan series:8 7 the NMR spec-tra of N-unsubstituted acyclic phosphimides (LXVII), R1 = H,show only the phosphorus(IV) signal. These results aresatisfactorily consistent with the mechanism involving thenucleophilic addition of the heteroatom X to the phosphorusatom in the cyclisation process.8 5

(LXVI), (LXVII) •(LXVIII), (LX1X)

NIIX

The intramolecular addition reactions involving the phos-phimino-group and leading to spirocyclic aminophosphoranesare as a rule irreversible (see, however, Dahl et al.81*),whereas in the series of monocyclic X5-diaza- and X5-oxaza-benzophospholans it is possible to observe the ring-chainphosphimide—phosphorane tautomerism [Eqn.(4)] after theappropriate selection of substituents at the phosphorus andnitrogen atoms and in the benzene ring:

/'

XH

, ir N " P - R ' "R4/X/\R3

R»n* Ι γ A*

P—R'

N̂H Λ (4)R3 R«

(LXVI): X = 0 ; (LXVII): X = NRi (LXVIII): X = O; (LXIX); X = N R ' .

The study of the thermodynamics of equilibrium (4) inseveral reaction series8 5"8 7 demonstrated the appreciableinfluence of temperature and of the nature of the solventon the ratio of the tautomeric forms. Thus in nitrobenzenemore than 90% of the acyclic phosphorus(IV) is present inthe equilibrium mixture, whereas in tripropylamine the degreeof cyclisation reaches 100%. With decrease of temperature,the equilibrium constant Κ = k1/k-1 increases [the content ofthe cyclanes (LXVIII) and (LXIX) increases].

The electronic influence of substituents is clearly shownwhen the groups in the phosphorus and imine components ofthe molecules are varied. Hydrogen atoms at the phosphorus

However, the steric characteristics of the substituentsplay a decisive role. In the series of oxaza-derivatives theequilibrium (LXVI)^ (LXVIII) is displaced towards X5-oxaza-phospholans only for R1* = t-Bu. 8 5 ' 8 6 Appreciable amountsof the phosphoranes (LXIX) appear in the mixture when thefree rotation of the phosphimino-group in phosphimides ishindered (R2 = t-Bu). 8 7

This reaction series also illustrates the spirocyclisationeffect which stabilises the phosphorus(V) form. The inclu-sion of phosphorus atoms in the five-membered ring, as incompound (LXVII, R7 = R8 = 2,2'-biphenylene), makes itpossible to detect appreciable amounts of X5-diazaphospholansin the mixture even in those cases where the nitrogen atomis unsubstituted (R1 = H ) . 8 7

The observed features of the interconversions of phos-phimides and aminophosphoranes are also manifested inthe synthetic results of certain reactions of X5-oxazaphos-pholans. 8 e Thus the ligand exchange of fluorine in theaminofluorophosphorane (LXX) for an alkoxy-group (Scheme3, reaction a) does not alter the coordination of the phos-phorus—the phosphorane (LXXI) is obtained, while hydroly-sis (reaction b) leads to the phosphinamidate (LXXII) andnot to an aminohydroxyphosphorane. The substitution ofthe fluorine atom by carbon-centred nucleophiles (reaction c)for R3 = Η induces the appearance of a mixture of products[(LXXIII, R3 = H)^(LXXIIIa)]; when R3 = alkyl, suchtautomerism is naturally impossible and the only reactionproduct is the X5-oxazaphospholan (LXXIII, R3 = Me or Et).The interaction of compound (LXX) with lithium amides(reaction d) leads to diazadiphosphetidines (LXXIV), evi-dently via the stage involving the formation of the phos-phimine (LXXVI) with an endocyclic P=N bond: 8 8 t

t For information about the ease of dimerisation of cyclicphosphimides, see Dahl et al.81* and Sheldrick et a l . 8 9 andthe literature quoted therein.

Russian Chemical Reviews, 54 (11), 1985 1137

Scheme 3

•Ph

(LXXIV)(LXX). R3=H. Me, Et

RLi

(LXXII)

(Lxxnn

The phosphimides (LXXVI, a and b) are also formed whenthe phosphimides (LXXV, a and b) are heated above themelting point, which is accompanied by the evolution of thecorresponding hydrocarbons: 9 0

(LXXVI, a and b)

The ability of cyclic aminophosphoranes to be convertedinto phosphimides has found a synthetic application in thereactions with carbonyl compounds.9 1 The interaction ofphosphorane (LXXVII) with aldehydes, ketones, and form-amides proceeds via a stage involving the formation of thephosphimide (LXXVIII), which is converted into the phos-phinates (LXXX) via a mechanism analogous to that of theWittig reaction:

(LXXIIIa)

The formation of the phosphimide (LXXVIII) has been con-firmed by the isolation of its dimer (LXXIV, R\R 2 = H) onthermolysis of compound (LXXVII) in solution or in theliquid state. The formation of the same final products(LXXX) as a result of the reaction of diazaodiphosphetidine(LXXIV) with carbonyl compounds indicates the reversibilityof the dimerisation reaction (LXXVIII) ** (LXXIV).

The transition state (LXXIX) (or the intermediate) in thesereactions is modelled by the stable adducts (LXXXII, R 1 =Meor CF3; R2 = Me, CF3 or Ph), obtained and isolated as aresult of the consecutive [2 + 2]-cycloaddition reactions, 8 9 ' 9 2

accompanied by a change in the coordination number of thephosphorus atom [P(III) -+ P(IV) in compound (LXXXI) andP(III) -s- P(V) in compound (LXXXII)]:

Λ

ΛCOOMe

(LXXXI)

The stability of the cyclic adducts (LXXXII) with anaminophosphorane structure increases with decrease of tem-perature and with increase of solvent polarity. Acyclicphosphimides do not give rise to stable aminophosphoranes intheir reactions with carbonyl compounds. 8 9# The influenceof the electron-accepting properties of the substituents inthe carbonyl partner must also be optimal: the equilibriumin the reaction with acetone is wholly displaced towards theinitial compounds. The adducts (LXXXII, R1 = CC13, R 2 = H )with chloral and (LXXXII, R1 = C6H4NO2-p, R2 = H) withp-nitrobenzaldehyde are converted into the aldiminophos-phine oxides (LXXXIII, R = CC13 or C6H4NO2-p, R2 = H)with rupture of the phosphimide bond even at room tempera-ture.

(LXXIV), R' = R2 = H

# A version of the [2 + 2]-cycloaddition of hexafluoro-acetone to an unstable phosphimide with an exocyclic P=Nbond has been discussed by Storzer et a l . 9 3

1138 Russian C h e m i c a l R e v i e w s , 5 4 (11), 1985

The driving force of the cycloaddition reaction is thereduction of the angular strain in the TBP of the phos-phorane (LXXXII) compared with the phosphimide (LXXXI).According to XRD data, 8 9 the structure of the phosphorane(LXXXII) deviates only slightly from the ideal TBP, whereasthe initial compound (LXXXI), in which the endocyclic CPNangle differs greatly from the tetrahedral value, is desta-bilised by an appreciable steric strain. 9 2

Interesting P(IV)=?iP(V) interconversions have beenobserved in the series of spirocyclic tetraaminophosphoranes.91*The thermal condensation of equimolecular amounts ofJV-methylbenzamidrazone hydrochloride (LXXXIV) withphosphorus pentachloride leads to diaminotrichlorophos-phorane (LXXXV), which is converted into the diazadiphos-phetidine (LXXXVI) on treatment with bases. The samereaction with a twofold excess of compound (LXXXIV) iscompleted by the formation of the immonium chloride(LXXXVII); the latter is dehydrochlorinated by a tertiaryamine to the spirocyclic phosphazene (LXXXVIII). Thesame compound can be arrived at by treating the dimer(LXXXVI) with the initial compound (LXXXIV) (Scheme 4):9 1*'9 5

Me.NH NHo

(LXXXIV)

PCI 5

- H C 1Μ

C 1 Cl

Scheme 4

y.

Cl

(LXXXV)

Kt 3N

-κι3Ν·ιΐα

. / =

(LXXXIX)

= PNMe9, PCI, P(O)Ph, P(O)C1 .

On passing from compounds (LXXXV) and (LXXXVI) to thespirocyclanes (LXXXVII) and (LXXXVIII), the phosphorusatom becomes a weaker electron acceptor (replacement of twochlorine atoms by amino-groups), so that the triaminophos-phazene (LXXXVII) has an ionic structure, in contrast tothe diamine analogue (LXXXV), and its dehydrochlorinationproduct is monomeric [cf. the transition (LXXXV) -»• (LXXXVI)].

Phosphorylation of the phosphazene (LXXXVIII) bydichlorophosphines and dichlorophosphine oxides enabledSchmidpeter et al. 91* to observe a new type of reversibleP(VI) =^=P(V) transitions—the phosphazene-diazadiphos-phetidine rearrangement, involving the migration of chlorine(and not of a proton as in the previous examples) betweenthe P(III) and P(IV) atoms in compound (LXXXIX) and anincrease of the coordination number of one of the phosphorusatoms in compound (XC). This is accompanied by a pro-nounced shift of the δ( 3 1 Ρ) signal from ~50 p.p.m. for theP(IV) atoms in compound (LXXXIX) to -10 to -40 p.p.m. forthe phosphorane phosphorus in compound (XC), the P(V)chemical shift depending significantly on the structure of thefragment Υ.

The position of the tautomeric equilibrium is also determinedby the nature of the Ρ substituent in the fragment Υ: thestronger the electron-accepting properties of the P(III) inthe tricyclic tautomer (XC), the greater the shift of theequilibrium towards this tautomer. For the group Υ = PPh,the tautomer (XC) is not detected and the introduction ofthe dimethylamino-group at P(III) lowers appreciably thecontent of the phosphorane: at room temperature in chloro-form the ratio (LXXXIX) : (XC) is 73 : 27 for Υ = PNMe2.

Each of the tautomeric forms (LXXXIX) and (XC) is chiraland can exist as two diastereoisomers and therefore in thegeneral case four isomers may be present in the equilibriummixture.91* Thus, when crystalline compound (XC, Υ = PCI)(one stereoisomer) is dissolved in chloroform, only a mixtureof the diastereoisomers of compound (XC) in proportions of92 : 8 is initially observed but after a week a mixture ofthree isomers is detected in the solution—one isomer of com-pound (LXXXIX) and two isomers of compound (XC) (32:63: 5).The equilibrium mixture of the four isomers of compounds(LXXXIX) and (XC) with Υ = POPh in benzene (17 : 30 : 27 : 26)is converted into a single crystalline isomer (XC, Υ = POPh)after the removal of the solvent. 91*

It is of interest that the isomerisation described above takesplace not only as a rsult of the transfer of a halogen betweenthe P(III) and P(IV) atoms but also in the phosphazene-phosphonium system:91*

^ b

Ph

(LXXXIX*)

Ζ1 H

(XG1) (XCII)

Bromination of the phosphazenes (LXXXIXa, R1 = Ph orNMe2) leads to tricyclic phosphorane-phosphonium isomers(XCII) and not to the expected spirophosphorane phos-phazene-phosphonium isomers (XCI).

An even more complex reorganisation of the molecules takesplace as a result of the bromination of the mixture of com-pounds (LXXXIX) and (XC, Υ = PCI). The diphosphorane(XCIII), which exists only for a short time in solutions,undergoes in succession two types of isomerisation—theskeletal rearrangement to the hypothetical phosphazene(XCIV) as a result of the opening of the four-membered ringwith transfer of the hydrazone group and subsequent closureof a new diazadiphosphetidine ring with formation of thedimer (LXXXVI) :91*

(LXXXIX) 7 » (XC) Cl Cl,

\ M -

Fh

(XCIII)

Me—N. xN

Ph

Cl

Ph'

rN—-I'd,

-Me

(LXXXVI)

(XCIV)

Russian C h e m i c a l R e v i e w s , 5 4 (11), 1985 1 1 3 9

The amination of the halogenophosphoranes (XC) withammonia, primary or secondary amines, and hydrazine leadsto stable pentaaminophosphoranes (XCV) which are incapableof the P(V), P(IV)^P(IV), P(IV) tautomerism:

(XC),Y=P(O)Ph + 2R2NH Χ Γ ·Me—-IT y—P<O)Pb

(XCV)

On reaction with aldehydes and ketones, the hydrazide[XCV, R2N = N(He)NH2] is converted into the correspondinghydrazones. It has been noted91* that compounds of penta-coordinate phosphorus linked to five N-ligands as a rule tendto be converted into phosphorus(IV) derivatives. Forexample, the substitution of the halogen atoms in the dimer(LXXXVI) by amino-groups induces its conversion into themonomeric form. 9 5 In this sens compounds (XCV), whichare not converted into the isomeric phosphonatophosphimides(XCVI), constitute a fairly rare example of stable penta-aminophosphoranes. §

V. OTHER TAUTOMERIC TRANSFORMATIONS OF PHOS-PHORANES. PHOSPHOROTROPIC PROCESSES

Various types of tautomeric transformations into compoundsof tricoordinate, tetracoordinate, pentacoordinate, and hexa-coordinate phosphorus have been observed for compounds ofpentacoordinate phosphorus. Equilibrium transitions betweencompounds of pentacoordinate and dicoordinate phosphorusare so far unknown. The discovery and investigation of thephosphite—phosphorane and phosphimide—phosphoranetautomerism indicates the relative ease of the dissociation ofthe P=O and P=N bonds and the stability of the phos-phoranes arising in such a process. All these equilibriumprocesses are characteristic only of cyclic, mainly X5-l,3,2-diheterophospholan, systems; in this case there is noanalogy with acyclic compounds.

The dissociation of the P=C bond with formation of aphosphonium centre also proceeds fairly readily but thephosphorane—ylide equilibrium could not be observed for along time, although some examples of the dual (both as aphosphorane and as an ylide) behaviour of phosphoranesystems have been noted. Thus the A5-l,2-oxaphospholan(XCVII) has the structure of a compound with a pentacoor-dinate phosphorus atom (XCVIIa), but in some reactions itbehaves as the betaine (XCVIIb), while in reactions withaldehydes and ketones it also reacts as a phosphonium ylide(XCVIIc): 9 7 ' 9 8

Ph 8P-CH a

/ \0 CH2;i

\ /CH2

(XCVIIa)

θ θPh 3P(CH 2) sO ?

(XCVIIb)

i Ph sP—CH(CH 2) 2OH

(XCVIIc)

§ The synthesis of other aminophosphoranes containingtwo phosphorus atoms with different types of coordinationhas been described. 9 6

The equilibrium between an ylide and a phosphorane hasbeen reported but it is associated with the addition-elimina-tion of the methanol molecule: "

P—OMe .

The first piece of evidence for the existence of the ylide—phosphorane equilibrium has been obtained 1 0 0 in a study ofthe interaction of cyclic phosphites with acetylenedicarbox-ylate ester in the presence of benzoic acid:

P O M e + MeOC(O)C=CC(O)OMe + PhC(O)OH 2 ° c c , °

—C\

— 0 κ

XOOMe

Ph

COOMe

(XCVIIIa), 18%

- O -

O

OC(O)Ph

XP—C(COOMe)=CHCOOMe .

(XCVIII), 82%

In methylene chloride the fraction of compound (XCVIIIa)increases to 37%, but, after the removal of this solvent anddissolution in CCl^, the previous ratio is restored.

The phosphorane—phosphonium betaine tautomerism hasbeen postulated for the monocyclic A5-l,2-oxaphospholens(XCIX) on the basis of the exchange of the methyl groupsof the acetylacetonate system observed from XH NMR spec-tra. 3>1*'1 0 '1 0 1 An analogous phenomenon was believed 1 0 2 ' 1 0 3

to be characteristic also of spirophosphorane systems:

=CC(O)Me

Ο C—R! θ ι

( 5 )

JMe

(XCIX)

PR(OMe),

The existence of the two tautomeric forms (Ca) and (CHa),observed in solutions at room temperature [the ratio(Ca) : (Clla) = 1 : 2 and is independent of the nature ofthe solvent ] , is apparently associated with the appearanceof an intermediate dipolar ion (CI), which, however, is notdetected in the spectra owing to its short life span: 101*~106

He c

i. JCCCOJR1

P(OMe)3

(C,a-d)

K— c — C — C — R

$P(OMe)3

(CI,a-d)

( 6 )

P(OM*)3

(CH,a-d)

(a) R=Ph; R1 = Me; X = CHPh, ΟΗΟ,Η,ΝΜε^ρ, CHCeH4NO2--p;

(b) R = Me; R» = Me; X = CHMe, CHPh, CHC,H4NMe,-ρ, 0Η0βΗ4ΝΟ2-ρ; }

(c)R = R1 = Ph; X = O; (d) R = R' = Ph; X = NPh

Detailed studies of the Ή , 1 3 C, and 3 1P NMR spectra ofthe (C, a—d) —(CII, a—d) systems as a function of tempera-ture showed than an intramolecular transfer of the phos-phorus-containing group takes place in such systems betweentwo oxygen atoms; 1 0 7 ' 1 0 8 the tautomeric process is degener-ate for the phosphoranes (C, b - d ) - ( C I I , b - d ) .

The chemical shifts of the phosphorus nuclei in all thesolvents 1 0 7 ' 1 0 8 remain unchanged over a wide temperaturerange (from -35° to +125 °C) and are observed in theregions characteristic of the resonance of the nuclei of penta-coordinate phosphorus(-27 to -51 p.p.m.) . Thus the phos-phonium structure is not an equilibrium component of thesesystems—the equilibrium is between two phosphorane forms

1140 Russian Chemical Reviews, 54 (11), 1985

Eqn.(6) and the phosphorane—phosphonium tautomerism[Eqn.(5)] is not observed for compounds of this kind. Thetemperature-induced changes in the NMR spectra of phos-phoranes described in other investigations3'1*'101""103 shouldapparently be interpreted analogously.

For exactly the same region, it is impossible to give anunambiguous answer concerning the possibility of thephosphorane—phosphonium equilibrium in aminophos-phoranes : 1 0 1 ' 1 0 9 ' 1 1 0

PhC=CPh

i ο\

P

PhC=CPh

D o o

P(NMe s), e P ( N M e s ) 3 .

Detailed study of these systems by ΧΗ, 1 3 C, and 3 1P NMRover a wide temperature range in solvents of differentpolarity led the authors 1 1 1 ' 1 1 2 to the conclusion that theobserved spectral changes are associated with the differentpolarities of the P-0 bond, which depend on the dielectricconstant of the medium.

The existence of the equilibrium between the phosphoraneand phosphonium structures has been demonstrated by low-temperature 13C NMR in a study of the halogenation of thecyclic phosphite (CIII): 1 1 3

Ο Me

(CVI)

At low temperatures (-100 °C) the dibromophosphorane(CIV) [δ( 3 1 Ρ) = 110 p.p.m.] and the phosphonium tri-bromide (CVI) (35 p.p.m.) were detected. The intermediatebromide (CV) ultimately symmetrises to form bromospiro-diphenylenetetroxyphosphorane (CVII) (-28 p .p .m.) ; inessence this is an unusual version of the Arbuzov reaction,the second stage involving the dealkylation of the alkoxy-group separated from the phosphorus atom by the phenoxy-bricige. The driving force of this version of the Arbuzovreaction is spirocyclisation to compound (CVII) (see SectionIII) and not the formation of the thermodynamically favour-able P=O bond as in the classical Arbuzov reaction.

The phosphorane—phosphonium equilibrium is character-istic also of acyclic phosphoranes. In acyclic systems thisequilibrium is as a rule displaced towards the phosphoniumform, while in cyclic systems it is displaced towards thephosphorane form, owing to the pronounced ring strain aris-ing in cyclic, specially spirocyclic, phosphonium cations.Thus only the product with pentacoordinate phosphorushas been detected on chlorination of ethylene phosphoro-chloridite.111*

The structure of the monocyclic phosphoranes (CVIII) and(CIX) is intermediate between the halogenophosphorane andphosphonium halide s t ruc tures : 1 1 5 ' 1 1 6

(GIX)

The appreciably ionic character of the phosphorus—halogenbond shifts the 6(3 1P) signals of these compounds to theregion characteristic of phosphonium structures, while theortho-protons of the aromatic ring and the diastereotopicbenzyl protons in compound (CIX) are manifested in the"phosphorane" region. By varying the substituents at thephosphorus atom, it is possible to alter the polarity of thephosphorus—halogen bond, whereupon the -"Ή NMR spectraof such structures become characteristic in many cases ofionic compounds. This behaviour is very close to thatobserved for the NMR spectra of the adducts of a-dicarbonylcompounds with tris(amino)phosphines. 1 1 1 > U 2

VI. THERMOLYSIS OF PHOSPHORANES

It has already been stated that the stability of X5-l,3,2-diheterophosphacyclanes increases in the sequence acyclic <monocyclic < spirocyclic. This is valid also for their thermalstabilities. The series based on the influence of exocyclicgroups at the phosphorus(V) atom on the thermal stabilitiesof monocyclic phosphoranes has been established: 1 1 7

(RO),P' > (R2N)3Po—

> R , PO—

NO—

Many cyclic phosphoranes have such high thermal stabil-ities that they can be vacuum distilled, but some phos-phoranes, obtained from phosphites or their analogues andα-dicarbonyl or αβ-unsaturated carbonyl compounds, decom-pose on heating intot the initial reactants, which indicatesthe reversibility of their formation reactions. Thephosphoranes formed after prolonged heating (50—60 °C) oftrimethyl phosphite with dibenzylideneacetone, benzylidene-acetophenone, and benzylidene-a-tetralone dissociate intothe starting materials at a higher temperature (>90 °C). 1 2 2 > 1 2 3

The thermolysis of X5-l,3,2-oxaphospholans (CX) also leadsto the ready elimination of trialkyl phosphites and theregeneration of the initial o-quinone monoxime:1 2 h

NOH

+ P(OR),20°, 2 h

OH

/ \ / N \P(OR)3 .

\o/

(CX)

The rate of decomposition of A5-l,3,2-diheterophospholansis determined to some extent by the stereochemistry of thecyclic molecular skeleton. Thus the study of thethermal decomposition of the stereoisomeric spirophosphoranes(CXI) established that steric factors exert a decisive influ-ence on the rate of stereospecific fragmentation with forma-tion of a diene and phosphonite (CXII): for the cis-disposi-tion of the substituents R and Me in the phospholen ring,the decomposition proceeds much more readily; the bulkyphenyl group at the phosphorus atom also accelerates the

Russian Chemical Reviews, 5 4 (11), 1 9 8 5 1141

fragmentation. The influence of electronic factors on thisprocess is insignificant:1 2 8

(CXI) (CXII)

The thermal stability of X5-l,3,2-diheterophosphacyclanesvaries within very wide limits depending on the structureof the rings and the nature of the substituents at the phos-phorus atom (see Arbuzov and Polezhaeva10 and the refer-ences quoted therein). As an example of extremely lowstability, one may quote the spirophosphorane formed oninteraction of phenylphosphiran with dioxetan. 1 2 9 TheA5-dioxaphospholan (CXIII), containing a three-memberedring, is stable only at -80 °C; at room temperature itdecomposes with elimination of ethylene. An analogousbehaviour evidently obtains also in the case of the dithia-phospholen (CXIV): 1 3 0

Although in the latter case (R = Cl) the compound doesform, it decomposes immediately with formation of oligomericsulphides. The authors explain this instability by theunfavourable position of the chlorine atom in the e-positionin the TBP or in the apical position of a square pyramid. 1 3 1

The monocyclic phosphoranes (CXVI) containing the 1,2-thiaphospholen ring cannot be isolated. They readily isomer-ise already in the course of their synthesis with migrationof the methyl group to the nucleophilic sulphur atom: 1 3 2 > 1 3 3

CC(O)R

CHMe

ο Ί

ί!CHMo

®P(OMe)3J

The first instance of reversible insertion in the disulphidelinkage has been described.131* The mode of the reaction ofthe cyclic phosphonite (CXVII) with the cyclic aromaticdisulphides (CXVIII) depends on the nature of the solventemployed:

(CXIV)

These reactions involve a kind of "transcyclisation" of thephosphorus(III) derivatives.

Phosphoranes containing sulphur atoms linked directly tothe phosphorus atom have been investigated much less thantheir oxygen analogues. The stability of the spiro-phos-phoranes with four sulphur atoms (CXV) depends greatly onthe exocyclic substituent R and decreases in the sequenceMe > Ph » Cl:

(CXV)

•s—

C~*OC*> ·ο(CXVII)

-A

(CXVIII) (CXIX)

In acetonitrile the spirophosphoranes (CXIX) are formedreadily and can be obtained in a crystalline state. In othermedia (CH2C12, CHC13) the reverse reaction predominates.Both processes proceed quantitatively without the formationof side products and are specific to the (CXVII) + (CXVIII)pair only.

The influence of the nature of the solvent on the stabilityof phosphoranes has been noted fairly frequently. Thereversible formation reaction of the phosphoranes (CXX) hasbeen observed for the interaction of 2-monoanils of 1,2,3-tricarbonyl compounds with trialkyl phosphites. For

R = H, Me .

X = H, the phosphorane (CXX) was isolated in a crystallineform. For X = NMe2, the formation of phosphoranes insolution has been detected by 3 1 P NMR at room temperature.The attempts to isolate them in a crystalline state wereunsuccessful: either the initial compounds are recovered(in ether) or the betaines (CXXI) are formed (in methylenechloride or in an excess of the phosphite). The betaines(CXXI), which are ammonium inner salts, are formed as aresult of the Ο •> Ν migration of the methyl group. Therearrangement of the phosphoranes into the stable betainesystem, observed for the first time in Refs. 135—137, showsthat phosphoranes have the capacity to alkylate tertiaryamines.

1 1 4 2 Russian Chemical Reviews, 5 4 (11), 1 9 8 5

RC—CC(O)R'

0 NC6H4X-nP(OMe)3 C-=i=C=uC

NC6H4X

®P(OMe)

[C6H4X-n

' 3 _

Ρ(ΟΜβ)3

(CXX)

0

-CR1

OMe

R # = = C — " O R 1 MeC=CC(O)Me= NMe2 Φ Mel I „ ?}-:

*• NC6H4NMe3 t _ >· NC6H4NMe3

O=P(OMe)2 O=P(OMe)a Ι θ

.(CXXD

This was also confirmed by the model reaction 1 3 8

PhC :CC(O)Ph

I | + PhNMe2

0 NPh

P(OMe)3

ο ο

IS Θ :ΙNPh PhNMe3

PhNMe3

BPh4

The monoanil of benzothiophen-2,3-dione does not reactwith tr ia lkyl phosphi tes in a d r y solvent; in the presenceof moisture, the reaction proceeds via a protonat ion, but notalkylation, m e c h a n i s m : 1 3 9

CD + P(OR),

OR

/ \ /

- A I I I/ >/\s / N>N-P(O)(OR),

/

OP(O)(OR),

S^NHAr IAr = Ph, C,H4NMe,

The thermolysis of X5-l,3,2-dioxaphospholans (CXXII)leads to the formation of epoxides with inversion of theconfiguration at the carbon atom1"*0"1"*3 (see Arbuzov andPolezhaeva10 and the references quoted therein). Themechanism of this reaction involves the heterolytic dissocia-tion of the P-0 bond, rotation about the C-C bond, andirreversible intramolecular substitution of the sp3-carbonatom with formation of an epoxide:

O5(CXXII) (CXXIII)

The study of the kinetics of these reactions by NMR 1 W 5

showed that they are of first order with respect to the phos-phorane and that the thermolysis rate constants vary linearlywith the σ constants of the substituents in the exocyclicaromatic groups (p = -3.5). The negative sign of ρ indicatesthe acceleration of the decomposition process with enhance-ment of the electron-donating properties of the variedgroups, which facilitate the dissociation of the Ρ—Ο bond andevidently stabilise the phosphonium centre in the intermedi-ate (CXXIII).

At the same time the introduction of electron-donatinggroups should hinder the nucleophilic substitution reactionat the irreversible epoxide elimination stage, so that, whenaccount is taken of effects which compensate one another, a

fairly large absolute value of ρ is somehwat unexpected. Theactivation energy for the decomposition process (-20kcal mol"1) is close to that for the reactions of phosphoranesinvolving the rupture of the endocyclic P-O bond,11*1* andthe heterolytic dissociation reaction (CXXII) -> (CXXIII) istherefore most important. l l > 5

The thermal and photolytic decomposition of λ5-1,4,2-oxazaphospholens leads to the formation of nitrile ylides11*6

(see Arbuzov and Polezhaeva10 and the literature quotedtherein). Their formation was detected with the aid ofdipolarophiles. The reaction with these substances leadsto products of [3 + 2]- and [3 + l]-cycloaddition: l l + 7 ' l l t 8

F,C- P(OMe),[ F»CX Θ Θ Ι

)C-N=CRFsCx J

+ O=P(OMe)3

The pyrolysis of X5-l,5,2-oxazaphospholens is accompaniedby the formation of azirines in high yields. 1 4 9 Bearing inmind the ready availability of such initial phosphoranes,which are obtained from α-bromoketones or ketoylides andtriphenylphosphine, and also the possibility of carrying outall the stages without isolating the intermediates, the abovemethod of synthesis of azirines can be regarded as fairlygeneral and suitable for preparative purposes:

Ν PPhs

The formation of phosphoryl compounds on thermal decom-position of cyclic oxaphosphoranes has been frequentlyobserved. The formation of compounds of tetracoordinatephosphorus as a result of thermal rearrangements of phos-phoranes is rarer. Thus, on heating, the 1,3,2-thiaza-phospholane (CXXIV) is converted into the phosphinamidate(CXXVI). On the other hand, the corresponding λ5,1,3,2-oxazaphospholans (CXXV) are thermally very stable. 1 5 2

Their thermolysis in the gas phase 1 5 3 leads to the formationof products with retention [compound CXXVII)] or cleavage[compound (CXXVII)] of the oxazaphospholan ring dependingon the character of the exocyclic substituents at the phos-phorus atom: for R1 = OMe, the main product is compound(CXXVIII), which has a cyclic structure:

X = S (CXXIV);

X=O(CXXV) . (CXXVIII)

V I I . NUCLEOPHILIC SUBSTITUTION AT A PENTACOOR-DINATE PHOSPHORUS ATOM

The mechanism of nucleophilic substitution reactions at apentacoordinate phosphorus atom [ S N 2 P ( V ) ] has not beenstudied in quite such detail as the mechanism of substitutionreactions at a phosphorus(IV) atom. To some extent this isassociated with the fact that the chemistry of phosphoraneshas developed mainly during the last two decades. However,the main difficulty consists in the exceptionally high ratesof hydrolysis (solvolysis) of phosphorus(V) compounds.

Russian Chemical Reviews, 54 (11), 1985 1143

The kinetic study of these processes is complicated by thepresence of a large number of stages—the reaction rarelystops after the rupture of one of the two bonds betweenphosphorus(V) and the heteroatom and usually developsfurther.

Two probable mechanisms of the S#2P(V) reactions havebeen discussed: 1 5 4 " 1 5 6 an associative mechanism involvinghexacoordinate transition states and (or) intermediates:

χ

>—

X

••>-' products, (7)

and a dissociative mechanism with intermediate formation ofphosphonium ions:

x

Λ Λ (8)

The study of the kinetics of the hydrolysis of pentaaryl-oxyphosphoranes demonstrated the exceptional sensitivity ofthe rate of reaction to the electronic and especially stericeffects of the substituents at the benzene ring in neutraland alkaline media (Table 3). 1 5 1 f The rate of reactionincreases with enhancement of the electron-accepting proper-ties of the substituent and is sharply retarded after theintroduction of ortho-groups. This influence of the substi-tuent X, the magnitude of the kinetic isotope effects (KIE)(k _/ίο_ = 3 in hydrolysis by water and approximately

unity in acid hydrolysis), and the absence of a salt effectare not characteristic of a dissociative process [Eqn.(8)]but agree well with the rate-determining attack by the watermolecule or the hydroxide anion on the pentacoordinatephosphorus atom in pentaaryloxyphosphoranes:151*

(ArO)6P + H2O

(ArO)6POH f a S ' •-

(ArO)5POH + Η

The acid-catalysed reaction proceeds with protonation ofthe phosphoranes and the subsequent rate-determining sub-stitution of the phenol by water:

(ArO)sP + H e (ArO),POAr

Η

Η,Ο, slew products.( 9 )

The acid hydrolysis reaction is less sensitive to the influenceof substituents but in this case too the steric effect of thegroup X is manifested distinctly by a decrease of /cH+(Table 3). The rate of reaction varies linearly with the con-centrations of H+ and OH~ in the strongly acid and stronglyalkaline media respectively (Fig.2).

lg*c

7

- 1

- J

~ 5

\

1 1 1

1 >

Ζ

\lVι ι ι10

pH

(ArO)sP=O + ArOH + ArO® .

Figure 2. Dependence of the rate of hydrolysis of phos-phorane (fcobs) o n t n e pH of the medium: 1) penta-aryl-oxyphosphoranes; 2) spirophosphoranes (CXXIX).1 5 1"1 5 5

Table 3. Kinetics of the hydrolysis of pentaaryloxyphos-phoranes (XC6HltO)5P at 25 °C in 25% dimethoxyethane.151*

Reaction rate constant

*OH~> m o 1 litre"1 s"1

X

Η

0.202

16.4

11.4

o-Me

1.1-ΙΟ"5

1.3

7.5-10"1

p-Me

0.0197

79

3 4

o-CI

0.106

p-Cl

390

Notation: k is the rate constant for neutral hydrolysis

("plateau" on curve 2 in Fig.2), k + the rate constant for

acid hydrolysis (left-hand branch of curve 2), and ^OH~the rate constant for base hydrolysis (the right-hand branchof curve 2).

Another type of influence of the pH on the rate wasestablished in a study of the kinetics of the hydrolysis ofspirocyclic phosphoranes. 1 5 5 The alkaline reaction (in thepH range pH > 10, Fig.2) of the phosphoranes (CXXIX) isalso described satisfactorily within the framework of theassociative mechanism. In these reactions, involving theopening of the rings in A5-l,3,2-dioxaphospholans, and alsoin exocyclic substitution reactions involving alkoxy(aryloxy)-spiro-phosphoranes,1 5 6 an increase of the reactivity of theorganophosphorus substrates is observed after the introduc-tion of electron-accepting substituents into the ligands.

However, in the range pH < 9 the hydrolysis of compound(CXXIX) resembles very closely the analogous reactions ofdioxolan orthoesters, for which a dissociative carboniummechanism has been established. It has been suggested 1 5 5

that the experimental results agree best with the acid-catalysed rate-determining dissociation reaction (CXXIX) ^(CXXX) and the rapid cleavage of one or both rings:

An increase of the steric hindrance by the ligands desta-bilises the octahedral intermediate (or the transition state).By imparting a positive charge to the phosphorus atom,electron-accepting substituents accelerate the slow stage ofthe reaction, namely the attack by the nucleophile on thephosphorus(V) atom.

(CXXIX)

Ar = Ο6Η4Μβ -ρ, Ph, C6H,C1 -p.

- ArPOCH2CH2OH + H O C H J C H J O HΙ ο

1 1 4 4 Russian Chemical Reviews, 5 4 (11), 1 9 8 5

(CXXUO

(cxxx)

№wT A r P\ Ι ϊ

»- ArP(OKOCH2CH2OH)2 .

The pre-equilibrium protonation of the neutral oxaphos-phorane (CXXIX) takes place with the simultaneous ruptureof the P-0 bond, so that the existence of the protonatedphosphorane [Eqn.(9)] as an intermediate in cyclic systemsis not obligatory. 1 5 5 This may be associated with the con-siderable ring strain in the protonated cyclic phosphorane.In this sense the acid hydrolysis of the phosphoranes(CXXIX) resembles the nucleophilic substitution reactionsof five-membered cyclic phosphates. 2 ' 1 5 7 It is believed1 5 6

that in the general case the contribution of the dissociativemechanism should increase with enhancement of the electron-accepting properties of the ligands in the phosphorane.

Little is known about the existence of protonated phos-phoranes. The formation of the protonated structure of aspirobicyclic phosphorane in the reversible acid catalysedhydroxyphosphorane-phosphate reaction (XLIII) ** (XLIV)has been reported. 6 " The study of the temperature varia-

Ή 1 3 3tion of the

3 C, and 3 1 P NMR spectra of certain mono-cyclic phosphoranes in the presence of small amounts ofHSO3F showed 1 5 8 ' 1 5 9 that the protonated hydroxyphospho-ranes are real intermediates in the reversible transitionbetween dihydroxyphospholens and the isomeric enolphos-phonium ions:

θFSOn

(10)

Table 4. Activation parameters for the phosphorane—dipolarion equilibrium [Eqn.(10)].

Phosphorane

45-AH*

kcal mol *

2.8

3.0

2.6

AS*, e.u.

—37

•—40

—44

kcalmol"1

12.0

13-0

13.5

Refs.

[159]

[160]

[1601

The rate of this process, which can be regarded as a modelof intramolecular phosphorylation, is limited by structuralfactors and is entropy-controlled (Table 4). The equilibrium[Eqn.(10)] involves the bimolecular proton transfer between

the protonated and neutral X5-l,3,2-dioxaphospholens.Equilibration is rapid when the ring contains a double bondand slow for saturated rings. It follows from the activationparameters for the phosphorane—phosphonium ion equilib-r ium 1 6 0 ' 1 6 1 that the rigidity of the five-membered ring facili-tates the ring closure to form the bicyclic phospholan struc-ture, while in the case of a six-membered ring the processis less effective (Table 4).

The equilibria described here [Eqn.(10)] play an importantrole in the mechanism of the intramolecular phosphorylationof the 2-OH group in RNA, which is facilitated by the riboser i n g . 1 6 0 It has also been noted that phosphorane-betaineinterconversions of this kind are responsible for the ligandreorganisation processes involving spirocyclic arylphos-phoranes, which are promoted by methyltrifluoromethane-sulphonate. 1 6 2

The associative mechanism has been demonstrated for thehydrolysis of the monocyclic phosphoranes (CXXXI) and(CXXXIII) in aqueous organic media.163>161* The high valuesof the KIE (greater than 3), the large negative activationentropies, data concerning· the influence of solvent polarityon the reaction rates, and the second (or higher) order ofthe reaction with respect to water suggest the formation ofoctahedral intermediates with phosphorus(VI). The subse-quent involvement of a second water molecule in the processleads to ring opening products. The phosphorane (CXXXI)can form under these conditions the trimolecular complex(CXXXII):1 6 3

(CXXXI)

H.O

Ό — Η

(CXXXII)

products.

CH2P(OEt)3

The hydrolysis of compound (CXXXIII) includes two pre-equilibrium stages—the formation of a hexacoordinate inter-mediate and exothermic opening of the X5-dioxaphospholanring: 1 6*

(CF3)2r \ | Γ η

( C F * l /[OCH(CF3)2J2-+-H2O

(CXXXIII)

ΡΪ1Ν\ o

EUK

HjO \ .

The formation of intermediate hexacoordinate phosphorateanions in the base hydrolysis of phosphoranes is especiallyprobable in the series of X5,l,3,2-diheterophospholans. This

Russian Chemical Reviews, 54 (11), 1985 1145

has been confirmed by numerous examples of the isolation oridentification in solutions of phosphorus(VI) anions 1 6 6 " 1 6 9

(see Sarma et a l . 1 6 5 and the literature quoted therein).It has also been suggested that hexacoordinate intermedi-

ates can arise in the hydrolysis of ethylene methyl phosphatein highly alkaline media. 1 7 0 ' 1 7 1 However, recent NMR studiesof this process failed to confirm such a possibility.1 7 2

The stereochemical relations in the process involving anincrease of the coordination number [P(V) ->• P(VI)] areclearly demonstrated by the comparison of the molecularstructures of the phosphorane (CXXXIV) and the phosphateanion (CXXXV) (Fig.3), formed as a result of the additionof the phenoxide ion to the phosphorus atom in compound(CXXXIV).165 It is usually suggested that the attack by thenucleophile on the central atom takes place in the equatorialplane of the TBP: 1 6 7 ' 1 7 3

ο. ,ο-OPhX: Θ Θ

0 + Et,NH PhO0 Et3NH

(CXXXIV) (CXXXV)

The approach of the phenoxide anion to the 0(3) atom fromthe rear (Fig.3a) should lead to a "phosphorate" with thetrans-disposition of the PhO groups; the cis-configurationis a result of the addition of PhO" to 0(2) and 0(4) fromthe rear. It has been established1 6 5 that the most favourablesteric disposition of the ligands in the "phosphorate"(CXXXV) (Fig.3b) is obtained following the attack on O(4)from the rear in the last sbustituted region of the phosphor-ane (CXXXIV). The alternative attack on O(3) from the reargives rise to the sterically less favourable "phosphorate".

trans-substitution via pathway α leads to the inversion ofthe configuration of the phosphorus atom, while syn-attack(pathways b and c) affords a product with retention of theconfiguration. Determination of the ratio of the stereoisomers(CXXXVIIa) and (CXXXVIIb) (7 : 1) by low-temperature 3 1 PNMR showed that the reactions of compound (CXXXVI) withdimethylamine and lithium dimethylamide proceed with thepreferential inversion of configuration: ka = 7(k{j + kc).

On substitution of PhO" at -78 °C, the stereoisomericcomposition corresponds to (CXXXVIIa) : (CXXXVIIb) = 1 : 1 ,while at room temperature the composition changes in favourof the isomer (CXXXVIIb) (0.64 : 1). Methyllithium givesrise to a mixture of equal amounts of the isomeric phos-phoranes. The equilibration of the stereoisomers can beachieved via the hexacoordinate intermediate (CXXXVIII).

These results might have been interpreted on the assump-tion that the nucleophilic substitution at P(V) does notrequire the attack by the nucleophile of the leaving group. 1 7 3

However, it was later established for the interaction ofphosphoranes of type (CXXXIX) with ionic nucleophiles atlow temperatures (between -60° and -100 °C) that only theless stable trans-anions (CXLa), which isomerise to the "cis-phosphorates" (CXLb) on heating, are formed initially:

Scheme 5

(a)'

(CXXXV 0

Figure 3. Molecular structures:b) "phosphorate" (CXXXV).165

a) phosphorane (CXXXIV);

The stereochemical result of the Sjy2P(V) reactions dependsto some extent on the nature of the nucleophile added tophosphorus(V) and on the leaving group. 1 7 3 The attack bythe nucleophile on the phosphorus atom in exocyclic substi-tution reactions involving the phosphorane (CXXXVI) canproceed in principle via one of three pathways in the equa-torial plane of the TBP (pathways a-c, Scheme 5). The

(CXXXIX) (CXLa)

Y = F, p-FC 6 H 4 O; Z = O, S.NR.

\ / Y ( x )

CF 3 CF3

(CXLb)

The ease of the trans-attack, which is a kinetically con-trolled process, is due to the stereoelectronic effect of theLEP of the heteroatoms adjoining the P-Nu bond. 2 ' 1 7 3 Thetrans-attack on spirocyclic phosphoranes of the type(CXXXIV) and (CXXXIX) is facilitated by the appropriateorientation of the LEP orbitals of the four heteroatoms,while the cis-attack is facilitated by only two of theseorbitals. 17l> The formation of the more stable cis-isomers(CXXXV) and (CXLb) is a thermodynamically controlledprocess, determined by the steric situation at the phos-phorus (VI) atom [see the above remarks concerning the"phosphorate" (CXXXV)]. It has been noted 1 7 3 that thedissociative mechanism of the exocyclic nucleophilic substitu-tion in spirocyclic phosphoranes is apparently impossible due

1146 Russian Chemical Reviews, 54 (11), 1985

to the pronounced angular strain in the intermediate spiro-cyclic phosphonium cation. An analogous cis—trans isomer-isation has been established also in a study of the interactionof the fluoride anion and the p-fluorophenoxide anion withA5-oxathia- and A5-oxaza-phospholens.175

As a consequence of the highly electrophilic properties ofthe phosphorus(V) atom, the attack by the nucleophile inthe Ξ^ 2P(V) processes is almost always directed at thephosphorus centre, in contrast to the nucleophilic substitu-tion reactions at P(IV), where both the tetracoordinatephosphorus atom and the endo- or exo-cyclic carbon atomscan serve as centres for the nucleophilic interaction.2 How-ever , an instance of the selective pseudoequatorial attackby nucleophiles on the carbon atom in the TBP structure of thetricyclic phosphatranes (CXLI) has been observed recently: 1 7 6

/ OCH a CH, x 1

RXP-OCHaCHa-N

^ O C H J C R / J

OH 9RXP(OCHaCH2)aNCHaCHaOH

Ο (CXLII)

The transannular Ν -»• Ρ bond in these compounds ensuresquinary coordination of the phosphorus atom. In all cases(X = Ο or S) the products of the alkaline hydrolysis arethe phosphates (CXLII) and nucleophilic attack is directedselectively to the pseudoequatorial carbon atom. It isbelieved1 7 6 that such interaction is a consequence of theinternal properties of the electronic configuration of theTBP: as a consequence of the dative transfer of electrondensity from the pseudoequatorial oxygen atom to the phos-phorus atom, in the initial structure there is already somepredisposition towards the formation of the P=O π-bond,which arises as a result of the reaction. A deficiency ofelectron density is therefore produced at the pseudoequa-torial carbon atom, which renders this centre most sensitiveto nucleophilic attack.

—oOo—-

The data presented in this review demonstrate the multiplicityof the chemical properties of A5-l,3,2-diheterophospha-cyclanes. The discovery of the role of cyclic phosphoranesand related structures with a phosphorus atom in the hetero-cycle (phosphonium cations, phosphoranide and hydroxy-phosphoranide anions, "phosphorate" anions, and phospho-rane—base complexes) as intermediates in many reactions ofOPC, including those which have become classical, is offundamental importance in this connection. This permitswell-founded conclusions about the mechanisms of the reac-tions of OPC and indicates the usefulness of their investiga-tion , particularly in relation to cyclic OPC. Various tauto-meric processes and the relative ease with which thecoordination number of the phosphorus atom can change inchemical reactions of OPC [P(III)-P(IV)-P(V)-P(VI)],which are particularly clearly manifested in the chemistry ofcyclic phosphoranes, to a large extent predetermine theunity and wide variety in the phosphorus chemistry.

At the same time numerous questions concerning the reac-tivity of A5-l,3,2-diheterophosphacyclanes, such as theinfluence of the size of the heterocycle, of endocyclicheteroatoms, and of exocyclic functional groups and thequantitative estimation of the electronic and steric charac-teristics of phosphorus(V)-containing groups still await amore far reaching elucidation.

REFERENCES

1. Μ.A.Pudovik, V.V.Ovchinnikov, R.A.Cherkasov andA.N.Pudovik, Uspekhi Khim., 1983, 52, 640 [Russ.Chem.Rev., 1983(4)].

2. R.A.Cherkasov, V.V.Ovchinnikov, Μ.A.Pudovik, andA.N.Pudovik, Uspekhi Khim., 1982, 51, 1305 [Russ.Chem.Rev., 1982(8)].

3. R.Luckenbach, "Dynamic Stereochemistry of Penta-coordinated Phosphorus and Related Elements",G.Thieme, Stuttgart, 1973.

4. R. R. Holmes, "Pentacoordinated Phosphorus Structure andSpectroscopy", Amer.Chem.Soc, Washington, 1980,Vol.1.

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