Toluenee Amination

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ISSN 20799780, Review Journal of Chemistry, 2011, Vol. 1, No. 4, pp. 359384. Pleiades Publishing, Ltd., 2011.Original Russian Text A.V. Aksenov, N.A. Aksenov, O.N. Nadein, Yu.I. Smushkevich, 2011, published in Obzornyi Zhurnal po Khimii, 2011, Vol. 1, No. 4,

pp. 350375.

359

CONTENTS

1. Introduction2. Twostep Methods For Amination3. Nucleophilic Amination of Arenes4. Direct Electrophilic Amination of Arenes4.1. Electrophilic Amination of Arenes with Hydroxylamine and Its Derivatives

4.2. Electrophilic Amination of Arenes with Haloamines

4.3. Electrophilic Amination of Arenes with Hydrazoic Acid and Azides

4.4. Other Reagents for the Electrophilic Amination of Arenes

5. Conclusions

1. INTRODUCTION

Amines are important intermediates in the synthesis of different substances widely used in diverse areasof human activity. These are pharmaceuticals, for example, paracetamol, acetophenetidine, and acetoacetanilide, as well as various dyes and pigments [14]. This review is therefore dedicated to methods for thesynthesis of these compounds, with the main attention devoted to direct electrophilic amination.

2. TWOSTEP METHODS FOR AMINATION

The substitution of a hydrogen atom in the aromatic ring by an amino group is usually a twostep process that includes the introduction of an intermediate functional group or atom [57]. The classical variant of these processes is provided by a nitrationreduction sequence (Scheme 1).

Scheme 1.

At present, there is a great number of systems for nitration, which are not considered in this review,because many of them are included in textbooks on organic chemistry.

A large number of different systems was also suggested for reduction. Among them are metals in acidicmedium, for example, [810]; metals in neutral medium, for example, ZnCaCl2EtOH[11, 12]; metalsin alkaline medium, for example, ZnNaOHEtOH [13]; metal salts in alkaline medium, for example,FeSO4NH4OH[14]; hydrazine hydrate, for example, [15]; and others.

ArH ArNO2 ArNH2HNO3H2SO4

[H]

Methods for the Amination of Arenes

A. V. Aksenova#, N. A. Aksenova, O. N. Nadeina, and Yu. I. Smushkevichb

aStavropol State University, ul. Pushkina 1a, Stavropol, 355009 Russia

bMendeleev University of Chemical Technology of Russia, Miusskaya pl. 9, Moscow, 125820 Russia#email: [email protected]; [email protected]

Received January 25, 2011; in final form July 4, 2011

AbstractThis review covers literature data on the amination of arenes and considers both multistepmethods for the introduction of an amino group in the aromatic ring and the direct amination (amidation) of aromatic hydrocarbons. The main attention is drawn to the electrophilic amination (amidation) of aromatic hydrocarbons because of the lack of comprehensive reviews on recent advances inthis area.

Keywords:direct electrophilic amination and amidation, aromatic amines, anilides, arenes, methodsof functionalization.

DOI: 10.1134/S2079978011040017


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AKSENOV et al.

In many cases, an acylation of the amino group occurs in the course of reduction, for example, [8, 9,16, 17] (Scheme 2).

Scheme 2.

Metals in the presence of acids or acid chlorides [8, 9], formic esters [16], thioacids [17], and otherreagents were used in these reactions.

Another twostage reaction sequence includes azo coupling and the reduction of the resulting azocompound, for example, [18] (Scheme 3).

Scheme 3.

One more variant of the twostep process is the sequence of the acylation reaction and the Schmidt(Beckmann, Lossen, and other) rearrangements. We will consider these reactions in more detail, becausethey are the most akin to the onestep reactions under consideration. It should be noted that the low reactivity of acylating agents is the disadvantage of this method. Therefore, the scope of this method is limitedto the area of application of the acylation reaction.

Carboxylic acids and their derivatives are used as acylating agents: mixed anhydrides, anhydrides,esters, and nitriles. Lewis acids, polyphosphoric acid (PPA), and Broensted acids are employed as catalysts. The acylation reaction is reversible; therefore, its regioselectivity is dependent on the type of reagent,catalyst, and reaction conditions, for example [1923].

The second stage of the method is the Schmidt or Beckmann rearrangement or related reaction. Thesehave been studied rather well [24, 25]. The mechanism of the Schmidt reaction for unsymmetrical ketonesseems to be more complex than that for symmetrical ones on account of the presence of isomeric forms.In this case, the preferable product has a structure where a migrating hydrocarbon substituent occupiesthe antiposition toward the diazo group [24, 25] (Scheme 4).

Scheme 4.

The authors failed to reveal a clear relationship between migratory aptitude and the electronic effect ofthe substituent in the benzene ring (Scheme 5) [26].

[H]ArNO2 RCOX

X = OH, Cl;

Ar N

R

O

H

SnCl2

HClPhN2

+

NH2 NH2

N NH2

NH2

NPh

NH2 NH2

NH2

N NH2

NH2

NPh

NNPh

NH2 NH2

NH2 NH2

SnCl2

HCl

PhN2+

96% 86%

31%94%

O

RL

RS

N

RL

RS

N+

N

RL

NH

RS

O

N

RL

RS

N N+

RL

NH

RS

O

major

minor

HN3

H+

N2

HN3

H+N2


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METHODS FOR THE AMINATION OF ARENES 361

Scheme 5.

The dependence on the sterical hindrances of the substituent is more clear. The migratory aptitude ofalkyl substituent R relative to the phenyl group increases with size; however, the best yields were obtainedin the case of phenyl group migration (Scheme 6).

Scheme 6.

The possibility of formation of amides from ketones through their reaction with hydroxylamine andoxalic acid was demonstrated in [27]. The authors considered the possibility of the reaction course viaalternative mechanisms (Scheme 7).

Scheme 7.

The Schmidt reaction with aldehydes and ketones leads simultaneously to nitriles and formamides.The yield of the latter rises with molar concentration of H2SO4in the reaction mixture [28] (Scheme 8).

Scheme 8.

The conditions of the Schmidt and Beckmann reactions are close to those of the acylation reaction[2931], and they can therefore be combined. The tandem of the acylationBeckmann rearrangement

was realized in [32] (Scheme 9). The corresponding acetanilides were obtained in 4595% yield.

O

X

HN

OX

+ NH

X

O

5060C

A

59

51

54

61

52

B

41

49

46

39

48

X = Cl

NO2Me

OMe

Ph

Cl3CCOOH

NaN3(3 equiv)

H2SO4(3.6 equiv)

R

OHN R

O

+ N

H

R

O

R A : B Yield, %

Me 95 : 5 81

Et 85 : 15 80

iPr 51 : 49 57

tBu 0 : 100 11

5060C

Cl3CCOOH

NaN3(3 equiv)

H2SO4(3.6 equiv)

A B

O

R R'

N

R R'

OCOCO2H OCOCO2H

NHR'

RHO

R'O

NH

R

NH2OH*HCI

(CO2H)2

100C412 h

or

6096%

O

R H R

HN ORCN +

HN3

6096%

H2SO4


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AKSENOV et al.

Scheme 9.

A onepot process using the sequence of the Schmidt and acylation reactions is exemplified in the work[33] (Scheme 10).

Scheme 10.

Another approach to electrophilic amination consists in the transformation of the initial arene into anorganometallic compound followed by reaction with an Nelectrophile [3439]. Arylsulfonylhydroxylamines were used as such electrophiles in [3436] (Scheme 11).

Scheme 11.

Another reagent based on hydroxylamine is N,Obis(trimethylsilyl)hydroxylamine, which is used incombination with lithium cyanocuprates [37] (Scheme 12).

Scheme 12.

There are reagents for such an amination on the basis of phosphorus compounds [38], for example,hydroxylamine 1(Scheme 13). Phosphorus reagents are less efficient than compounds based on sulfur orsilicon, so amination products form in a lower yield.

RHN

OR

MeO

MeO

HN

O

4595%

72%

1) AcOH/PPA

2) NH2OH

1) AcOH/PPA

2) NH2OH

HN N

R

N NH

R

NH

O

2) NaN33) H2O

1) AcOH/PPA quantitativeyield

R

MgBr NR 2

R

Li NEt2

ArSO3NR2

ArSO3NEt2

3791%

42%

MeO2

CuCNLi2+Si O

NH

Si

NH2

MeO

CuCNLi2+Si O

NH

SiS

2S

NH2

70%

58%


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Scheme 13.

Diphenoxyphosphoryl azide provides an example of a reagent based on organic azide [39]. In this case,aryl halides are used as initial compounds. The onepot process initially leads to an organomagnesiumcompound that then produces phosphorylated triazene 2in a yield up to 91%. Hydrochloric acid hydrol

ysis of the latter results in amine salt (Scheme 14).

Scheme 14.

3. NUCLEOPHILIC AMINATION OF ARENES

Nucleophilic amination of arenes containing electronwithdrawing substituents and deficienthetarenes and their salts is a well studied process. A.E. Chichibabin first reported such processes almost100 years ago, and at present this reaction is named after the discoverer [4042] (Scheme 15).

Scheme 15.

The disadvantage of the method is the use of rather severe reaction conditions that substantially confined the scope of its application.

About 30 years ago, van der Plas proposed a method of homogeneous oxidative amination with aRNH2NH3KMnO4 system, which at present is widely used in organic synthesis, especially for thefunctionalization of heterocyclic compounds. This method provides an opportunity to carry out reactionsunder mild conditions and to involve compounds containing labile groups as substrates. This reaction isexemplified by the amination of pyridazine reported in the work [45] (Scheme 16). Previously, it wasshown by NMR in [46] that the reaction proceeds through the intermediate formation of 3,4dihydropyridazine 3.

R

MgBr(Li)

+O

PO NH2

NH2

Li+

+

OP

O NH2

NH2

R

2235%

37%

1

1

BrHN N

N

P(O)(OPh)2

NH3+

Cl

Br HN NN

P(O)(OPh)2

NH3+

Cl

R RR1) Mg

2) (PhO)2P(O)N3

HCl

MeOH

3351%

28%

1) Mg

2) (PhO)2P(O)N3

HCl

MeOH

2

N

NH2

N

HetArH HetArNH2 for example1) NaNH2

2) H2O

1) NaNH2

2) H2O

58%


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AKSENOV et al.

Scheme 16.

When an additional electronwithdrawing group is present in the molecule of the heterocyclic compound, the amination can be carried out in the absence of metal amide [47] (Scheme 17).

Scheme 17.

Air oxygen can act as an oxidizing agent in this reaction, for example, [48] (Scheme 18).

Scheme 18.

At present, theAgPy2MnO4complex is the most efficient oxidizing agent for such reactions. For example, this complex provides even the diamination of 6,8dimethylpyrimido[4,5c]pyridazine5,7(6,8)dione 4[49] (Scheme 19).

Scheme 19.

Vicarious substitution is one more variant of nucleophilic amination. As a rule, hydroxylamine and,less frequently, hydrazine are used as aminating reagents in these reactions [48, 50] (Scheme 20).

Scheme 20.

Examples of amination by vicarious substitution followed by the closure of the imidazole ring werereported in [51] (Scheme 21).

NN

NH

N

NH2H

NN

NH2

KNH2 KMnO4

91%

NH3

3

NN

NO2

R'

R

NN

NO2

R'

RH2N

KMnO4

NH39398%

NN

N

NN

N

RN

R R'

RR'NH

O2

3060%

NN

N

N

O

O NN

N

N

O

O

HNR

HN

RRNH2AgPy2MnO4

7377%4 5

NN

NO2

N

N

NO2

NH2

NN

N

NN

N

NH2

NH2OH

50%[50]

NH2OH

48%[48]


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Scheme 21.

Hydrazine is used in these reactions much less frequently. An example of this type of transformation isthe reaction of pyridazinones with hydrazine hydrate [52], which results in a mixture of isomeric amines(Scheme 22).

Scheme 22.

4. DIRECT ELECTROPHILIC AMINATION OF ARENES

As shown above, the substitution of the hydrogen atom in the aromatic ring by an amino group is usually a twostep process that requires the preliminary introduction of a functional group or atom. Exceptions are azines, azoles, and aromatic compounds with strong electronwithdrawing substituents that can

be aminated by the nucleophilic substitution of a hydrogen atom. Therefore, in recent years there has beengrowing interest in the study of reactions of direct electrophilic amination of arenes. These reactions have

been known for more than 100 years (see, for example, [5, 5355]). Over this time, different reagents and

their combinations have been used, but no universal and truly efficient method has been designed to date.

4.1. Electrophilic Amination of Arenes with the Use of Hydroxylamine and Its Derivatives

One of the first works in this direction is the paper by Grabe published in 1901 [55] (Scheme 23). Thiswork describes the amination of benzene and alkylbenzenes with hydroxylamine in the presence of aluminum chloride. Toluene forms a mixture of p and otoluidines in a 9 : 1 ratio in 2% yield. orthoXyleneafforded a yield of 7%.

Scheme 23.

The effect of the nature of hydroxylamine salt on the yield and regioselectivity of toluene aminationwas studied in [56]. Hydroxylamine sulfate in the presence of a 10% excess of aluminum chloride provedto be the most efficient salt for this purpose.

Other derivatives of hydroxylamine such as alkylhydroxylamine [57] and hydroxylaminesulfonicacid [5860] were successfully used for electrophilic amination.

Thus, the amination of toluene with hydroxylaminesulfonic acid was reported to result in a mixtureofp and toluidines in a 40% yield toward the aminating reagent [58, 60]. The yield toward initial toluene was about 4% (Scheme 24). It was noted in [58, 59] that the content of theparaisomer in the toluidinemixture is about 96%. It was later shown that these data were erroneous [60]. A more detailed study of thereaction (catalyst amount, reaction time, solvent) showed the formation of mtoluidine in 1113% yield

N

XN

O

Y

W

+

N

NN

W

Y

X

NH2OH

AcOHAcONa

quantitativeyield

Br

N

N

R

OR'

N

N

R

OR'H2N

N2H4

1537%

NH2R

R

NH2OH

AlCl3 27%


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AKSENOV et al.

along withp and toluidines formed in approximately equal amounts or with a slight predominance ofthe orthoisomer.

Scheme 24.

The application of alkylhydroxylamines allowed the direct introduction of the amino and dialkylaminogroup into the arene molecule [57] (Scheme 25). Similarly to the reaction of hydroxylaminesulfonicacid, toluene forms a mixture of all isomers but with the predominance of the para isomer. In certaininstances, its content in the reaction mixture reaches 60%.

Scheme 25.

There are papers in which Nphenylhydroxylamine with PPA or trifluoroacetic anhydride in trifluoroacetic acid was used as the aminating reagent, for example, [61] (Scheme 26).

Scheme 26.

In this case, along with the amination product, a mixture of products of arylation at the orthoandparapositions relative to the amino group is also formed. Amination product prevails in the case of benzeneandpxylene. Its yield in the reaction with benzene is 25% for PPA and 45% for trifluoroacetic anhydride.In the reaction withpxylene, the yields under the same conditions were 47 and 66%, respectively.

ArylNbenzoylhydroxylamines undergo intramolecular electrophilic amination under the actionof aluminum chloride in methylene chloride [62] (Scheme 27). The products of electrophilic aminationat the orthoposition form in 2285% yield.

Scheme 27.

The authors postulate a reaction mechanism that involves the coordination of the catalyst at the oxygenatom of the hydroxylamine group at the first stage of the process [62] (Scheme 28).

NH2R

R

240%

NH2OSO3H

AlCl3

R

R

NR2

242%

NR2OR'

AlCl3

HNOH

R

R

+

R

R

HN

NH2

R

R

R

R

NH2++

PPA/TFA

or TFAA/TFA

R = H, Me; total yield is 7865%

O

HN O

Ph

OHHN O

PhR R

2285%AlCl3

CH2Cl2


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Scheme 28.

In some cases, this reaction leads to side reactions: substitution of the hydroxy group and halogenationwhen a halogen is present in theparaposition, as well as the displacement of a halogen at the orthopositionand chlorination (Scheme 29).

Scheme 29.

In our opinion, these side processes involve chemical reactions shown in Scheme 29.

O

HN O

Ph

Cl3Al +

O

N O

Ph

Cl3Al

+

AlCl2 O

N O

Ph

Cl3Al

AlCl2

OHHN O

Ph

O

N O

Ph

Cl2Al AlCl2

O

HN O

Ph AlCl3CH2Cl2

R R R R

R R

AlCl3

HCl

HCl

H2O

O

HN O

Ph

Cl

ClHN O

PhCl

Cl

OAl

NH

PhOCl

Cl

Cl+

Cl

OAl

Cl

Cl

Cl

HHN

O

Ph

O

HN O

PhF

Br

OH

Br

Cl

HN O

Ph

OAl NH

PhOCl

ClCl

+

F

Br

OAl

NH

PhOCl

Cl +

Br

F

Cl

OAl

HN

Cl

Cl

Br

Cl

Ph

O

AlCl3

CH2Cl2

AlCl3

19%

Al(OH)Cl2

AlCl3

CH2Cl246%

AlCl3

H2O

F H+

Cl

OAl

NH

PhO

Cl

Cl +

Cl

H

OAl

HN

Cl

Cl +

BrH Cl

Ph

O


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AKSENOV et al.

There are examples of acetamination of arenes with the use of hydroxylamine derivatives, namely,hydroxamic acids [63, 64] (Scheme 30).

Scheme 30.

The yield of acetanilides 6was from 8 to 63%. The lowest yield of the product is observed for compounds with electronwithdrawing substituents like iodine or a carbonyl group.

The intramolecular reaction of electrophilic amination with participation of hydroxylamine derivatives, namely oximes, has been used many times in the synthesis of quinoline derivatives (Scheme 31) [64].

Scheme 31.

A thermal cyclization ofmethylated oximes into quinolines was reported in [65] (Scheme 32).

Scheme 32.

Chemical and photochemical variants of such transformations are possible foracylated oximes [55,

67] (Scheme 33).

Scheme 33.

The reaction reported in [68] may provide an example for the use of ketoxime with a saturated sidechain for the synthesis of quinoline (Scheme 34).

Scheme 34.

R'

OR

NHO

R'

PPA

863%

OR

+NH

OH

O

6

O NH

OHNH

O

PPA

57%

N

O

NPhMereflux

93%

Et2N

Ph

O Ar

NO

O

N

O NEt2

Ar

Ph

ON

Ph

PhO N Ph

Ph

h

9660%

[66]

[67]52%

O

O

NOH

O

O N

Bu4NReO4CF3SO3H

chloranil,dichloroethane

75%


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4.2. Electrophilic Amination of Arenes with the Use of Haloamines

Another type of reagents used for direct electrophilic amination is bromo and chloroamines [54, 6977]. Preparative procedures have been developed on their basis in many instances.

The amination of benzene with Nchloropiperidine was the first use of haloamines [70] (Scheme 35).A small amount ofNphenylpiperidine was detected among the reaction products.

Scheme 35.

The amination of phenol with chloroamine, which led to small amounts ofpaminophenol along with4,4'dihydroxydiphenylamine, was studied later [71] (Scheme 36).

Scheme 36.

The amination of 2,5disubstituted phenols with chloroamine is accompanied by ring expansion [72,73]. The suggested mechanism of this reaction involves the ipsoattack by chloroamine on the substitutedphenols to form intermediate 7, whose oxo group then undergoes intramolecular nucleophilic attack bythe amino group. The resulting bicyclic compound 8undergoes expansion of the sixmembered ring viapericyclic reaction (Scheme 37).

Scheme 37.

The preparatively significant methods of amination with the use of haloamines are presented in theworks [7477] (Scheme 38).

Scheme 38.

The reaction of chloroamines with toluene was shown to result mainly in a paraisomer along withsmaller amounts of orthoand metaisomers. The latter prevails in the presence of iron salts. In this case,

N

Cl

+

N

Ph traces

OH

NH2

Cl+ HO NH2+ HO N + ...OH

H

R

O

R R

O

R

NH2R

OH

R

NH

NH

OH

R

R

NH2Cl

5055%7 8

R

NR2 Cl

+

NR2

R RClNR2

ClNR2

1938%

21%


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AKSENOV et al.

there is no byproduct of arene halogenation, which is present when other systems are used, except for thereactions carried out in nitromethane and nitrobenzene, the best solvents for this process. The effect ofacid nature was also studied. Sulfuric acid proved to be more efficient than aluminum chloride. The effectof acid concentration and metal ion was also shown. A conclusion was drawn on the possibility of reactioncourse via a freeradical mechanism.

4.3. Electrophilic Amination of Arenes with Hydrazoic Acid and Azides

Another system long used for direct electrophilic amination is hydrazoic acid in the presence of different catalysts: Lewis and Broensted acids. The paper by Schmidt [78] published in 1924 was one of the firstworks that addressed amination with the use of this reagent. It describes the amination of benzene withhydrazoic acid in the presence of sulfuric acid. Aniline was obtained in this reaction in a low yield (Scheme39).

Scheme 39.

Hydrazoic acid proved to be one of the most promising reagents for direct electrophilic amination.Thus, HN3in the presence of aluminum chloride is more efficient for toluene amination than hydroxylaminesulfonic acid [58]. Since toluene was used as a solvent, the yield of isomeric toluidines towardtoluene does not exceed 5%. The reaction leads to a mixture of orthoandparaisomers with considerablepredominance of the latter (Scheme 40).

Scheme 40.The authors of [79] reported the direct electrophilic amination of mesitylene with hydrazoic acid in

concentrated sulfuric acid. They described the formation of three amines in this reaction: mesidine,diaminomesitylene, and 3amino2,4,6trimethylbenzenesulfonic acid (Scheme 41). The main reactionproduct is 3amino2,4,6trimethylbenzenesulfonic acid formed in 42% yield.

Scheme 41.

A number of works deal with the thermal decomposition of hydrazoic acid or its ammonium salt in aromatic hydrocarbons, for example, [80]. The decomposition was carried out in benzene andpxylene in a

bomb at 220260to give the corresponding amines in a low yield.The results of comparison for the reaction of hydrazoic acid with toluene in the presence of sulfuric

acid and aluminum chloride are described in [81]. In all cases, the reaction leads to a mixture of three possible isomers with the predominance of the orthoisomer. The total yield in the presence of aluminum chlo

NH2NH3H2SO4

NH2

+

NH2

40% (with respect to the aminating agent)

NH3

AlCl3

NH2

+

NH2

NH2

+

NH2HO3SHN3

H2SO4

42%


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ride as catalyst is higher than in the presence of sulfuric acid. The reaction of hydrazoic acid with chlorobenzene in the presence of aluminum chloride was also studied; theparaisomer prevailed in this case.

G. Olah and colleagues studied the electrophilic amination of benzene and its derivatives in a NaN3AlCl3HClsystem [82]. The authors suggested the reaction mechanism shown in Scheme 42 [82].

Scheme 42.

Study of the solvent effect showed that the best result was achieved when an excess of the aromatic

compound was used. Its replacement with hexane leads to reduction in the yield of amines, while in theauthors opinion 1,2dichloroethane and nitromethane are inappropriate solvents on account of strongcomplexation with the Lewis acid. However, it was shown in [83] that the amination of mesitylene with aNaN3AlCl3HClsystem in hexane and 1,2dichloroethane as solvents provided moderate (40%) andhigh (86%) yields of mesidine, respectively.

The same authors performed an elaborate study on the effect of solvent and crown ether additives onthe yield and regioselectivity of amination of xylene, mesitylene, durene, and pentamethylbenzene in aNaN3AlCl3HClsystem [84] (Scheme 43).

H2N H

+

NH2

H2N H

+

N+

N

N H

+

NH2N

NaN3+ AlCl3 AlCl2N3+ NaCl

AlCl2N3+ 2HCl NH2N2+AlCl4

A

NH2AlCl4

+ N2

B

H+

N2

A

B

A

+


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AKSENOV et al.

Scheme 43.

According to the data of this work, methylene chloride and 1,2dichloroethane are the most efficientsolvents for these processes. Upon oxylene amination, the ratio of 2,3 and 3,4dimethylanilines varydepending on solvent nature from 0.56 to 0.9. In all cases, the 3,4isomer is the main reaction product.Crown ethers have little effect on the amination of arenes [84]. The amination of solid arenes, durene andpentamethylbenzene, was also reported in [84]. The yield of 2,3,5,6tetramethylaniline and pentamethylaniline in methylene chloride was 30 and 52%, respectively.

The effect of Lewis acid nature on the amination of mesitylene with a NaN3MHlgnHClsystem wasstudied in [85]. The following Lewis acids were used: GaCl3, ZrCl4, AlCl3, AlBr3, FeCl3, SbCl5, SnCl4,TiCl4, SbCl3,and GeCl4. The use of SnCl4and TiCl4leads to very low conversion of the initial arene, whileno reaction was observed with SbCl3and GeCl4. Such Lewis acids asAlBr3, FeCl3, and SbCl5show mod

erate activity, while GaCl3, ZrCl4, andAlCl3are the most efficient.In [86], the authors described the use of ionic liquid ([bmim]Cl) in the amination reaction of andmxylene and mesitylene in a NaN3AlCl3HClsystem. Xylene produces a mixture of isomeric dimeth

ylanilines in 65% total yield for xylene and 78% for mxylene. Mesitylene under these conditions givesmesidine in 76% yield.

G. Olah and colleagues proposed the use of trimethylsilyl azide in trifluoromethanesulfonic acid fordirect electrophilic amination [87]. This reagent allows preparation of corresponding anilines from alkyland halobenzenes in 73 to 95% yield with respect to the aminating reagent (Scheme 44). The disadvantages of the method are the use of an excess of aromatic substrate as a solvent and low regioselectivity formonosubstituted benzenes.

Scheme 44.

It proved very efficient to use a N3CF3COOHCF3SO3Hsystem for direct electrophilic amination[88]. The system allows the amination of a large number of alkyl and halobenzenes, diphenyl, naphthalene, and phenanthrene. The total yield of anilines toward hydrazoic acid was from 15 to 100%. As in the

NH2

NH2 NH2

+

NH2

NH2

HN3

AlCl3/HCl

HN3

AlCl3/HCl

HN3

AlCl3/HCl

HN3

AlCl3/HCl

R

NH2+ H

NH2

R R

Me3SiN3+ 2F3CSO3H H2NN2+ O3SCF3+ Me3SiOSO2CF3

+ H2NN2+ O3SCF3

O3SCF3CF3SO3H

7396%

N2


15/26



previous case, the excess of aromatic compound and low reaction regioselectivity are the drawbacks of themethod.

The amination of mesitylene can be accomplished without trifluoroacetic acid in a NaN3CF3SO3Hsystem [89]. The yield of mesidine in this case was 72%.

A method of direct electrophilic amination of arenes with a sodium azidePPA system was developedrecently in our laboratory. The method was found to be applicable for the amination of arenes containingelectrondonating substituents [90], including crown ethers with an aromatic ring [93], naphthalene [90],

and naphthalenes with electrondonating substituents such as pyrimidine [91, 92] (Scheme 45).

Scheme 45.

The advantage of this system consists in regioselectivity close to 100% at sufficiently high yields of 1886% [9093]. We assume that such a selectivity is explained by a reaction mechanism that includes theazo coupling stage shown in Scheme 46.

RO RO

NH2

MeO

MeO

MeO

MeO NH2

MeO

OMe NH2

OMe

OMe

NH2

N NH

R

HN N

R

N NH

R

NH2

HN N

R

NH2

O

O

O

O

O

O

NH2

O

O

O

O

O

O

1) NaN3/PPA

2) H2O7486%

R = H, Me;

1) NaN3/PPA

2) H2O

78%

74%

1) NaN3/PPA

2) H2O

1) NaN3/PPA

2) H2O31%

1) NaN3/PPA

2) H2O

6276%

6276%

NaN3

PPA

61%


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374


AKSENOV et al.

Scheme 46.

Another advantage of this system is the possibility to combine it with other reactions. An example ofcascade transformation shown in [95] includes the Schmidt reaction, electrophilic amination, and heterocyclization (Scheme 47).

Scheme 47.

The probable mechanism of this reaction according to [94] is shown in Scheme 48.

Scheme 48.

...O P

O

OH

N N+

N

...O P

O

HO

N N+

NH

...O P

O

HO

N N+

NH

...O P

O

HO

N N NH

Ar ...O P

O

HO

N N NH

Ar OH P

HO

O

N N NH2

ArHO

H

+

PPANaN3

ArH

PPA

PPA

A H2O

H+

H+N2

H3PO4

ArNH2

A

a b

N NH

R

R' O

N N

R

NN

R'

3144%

NaN3PPA

N NH

R

R' O

N NH

R

HN R'

O

N N

R

HNNH

NN

PPA

R'

O

N N

R

NN

R'

N NH

R

NHN

R'

N NH

R

NHN

R'HO

N

N

PPA

NaN3PPA

NaN3PPA

PPA

N2

O2


17/26



Other examples of tandem reactions with participation of sodium azide in PPA are given in [93, 95,96]. In these papers, the system of reagents is used cooperatively with 1,3,5triazines. The reaction with

benzene or naphthalene derivatives as initial arenes results in theorthoannelation of the pyrimidine ring[93, 95] (Scheme 49).

Scheme 49.

The mechanism of the transformation is shown in Scheme 50 and includes the stages of electrophilicaddition of phosphoryl azide to initial triazine, followed by the opening of the threemembered ring andclosure of the pyrimidine ring [93, 95] (Scheme 50).

Scheme 50.

In the reaction of pyrimidines, aperiannelation occurs [96] (Scheme 51).

RO

R'N N

NR'' R''

R''

RO

R' N

N

R''

R''

N N

NR R

R

N

N

R

R

1) NaN3/PPA

2)

R = H, Me;R' = H, OMe; R'' = H, Me, Ph;

4954%

1) NaN3/PPA

2)

R = H, Me, Ph;2226%

...O P

O

HO

N N N

H

Ar +

N

N

N

R

R

R

...O P

O

HO

N N NH

Ar

N

N

NH

R

R

R

+

N

N

N

R

R

N

R

Ar

H

H

X

N

N

R

R

X

Y NH

N

N

R

R NH2

N

N

R

RNH

N

N

R

R NH2

RN

NH2N

R

R

N ArH

+

R

N2

PPA

H+

HN=CHNH2

HN=CRNH2

H+

H+


18/26

376


AKSENOV et al.

Scheme 51.

The mechanism of the transformation is close to that shown in Scheme 50. It is confirmed by the factthat the reaction leads to corresponding amides if the reaction mixture is kept for 2 h at 7080Candtreated with water after the addition of triazines.

Alkyl and arylamination of aromatic compounds with the use of alkyl and aryl azides is described in anumber of papers.

One of the first such works dealt with the decomposition of aryl azides in the presence of phenol andaniline [97, 98]. The authors reported on the formation of phydroxy(amino)diphenylamines in thesereactions. The series of aryl azides and arenes involved in the reactions was extended in later work [99].

Later, the reaction of aryl azides with arenes was carried out in the presence of trifluoroacetic acid(TFA) [100, 101] and trifluoromethanesulfonic acid, which provided an opportunity to increase substantially the yield of amination products (Scheme 52). The reaction was found to result in a complex mixtureof amination and arylation products.

Scheme 52.

Several papers reported on the introduction of a sulfonamido group in arenes upon decomposition ofarylsulfonyl azides in the presence of an aromatic compound [103105]. The reaction of substituted benzenes leads to a mixture of sulfonamidation products at the ortho, meta, andparapositions (Scheme 53).Considerable amounts of the metasubstitution product form only when an electronwithdrawing substituent is present in the substrate molecule. The reactions of intramolecular amination with azides are themost interesting; these lead, as a rule, to different heterocyclic compounds in a good yield.

NHN

R

N N

NR1

R1

R1

NHN

R

NHN

N NH

R1

R1

R1

N

N

NH

R1

R

NHN

R1

NH

R

O

1) NaN3/PPA

2)

t

H2O

7279%

3847%

N3 R

+

HN

RHN

+

R

+

NH2

+ NH2

R

+

N3 NHPh

R

TFA

or TFSA

73%

8% 8%

TFA

or TFSA

82%


19/26



Scheme 53.

A large number of works is dedicated to the synthesis of indoles. There is a report on the synthesis ofimides of indole2,3dicarboxylic acids [106] (Scheme 54).

Scheme 54.

A large number of reports is devoted to the synthesis of indoles from the products of condensation ofbenzaldehydes with azidoacetic acid ester [107115] (Scheme 55).

Scheme 55.

This is associated with the high availability of the initial azides and the biological importance of hardtoaccess 4substituted indoles.

In addition to indoles, electrophilic amination with azides provides the preparation of other heterocyclic compounds. Thus, the conversion of 2azidobiphenyl into carbazole in quantitative yield wasdescribed in [116] (Scheme 56).

Scheme 56.

The synthesis of phenanthridines is described in [117]. It is likely that cations 9form at the first stageand then react with hydrazoic acid to give intermediate compound 10. The latter eliminates a nitrogenmolecule and undergoes ring expansion to yield phenanthridines (Scheme 57).

R

R

NH

SO2Ar

+

R

NH

SO2Ar

+

NH

SO2Ar

R

ArSO2N3

655%

N

OR

O

Cl

N

OR

O

N3

NH

N

O

O

R

NaN3 t

40%

R

O+

O

O

N3R'

O

N3

OR

NH

O

O

R

KOH t

40%

R' R'

NHN3

decalin

reflux

97%


20/26

378


AKSENOV et al.

Scheme 57.

4.4. Other Reagents for the Electrophilic Amination of Arenes

Electrophilic species can be generated upon decomposition of Naminoazines. This approachwas used in the development of a series of methods for direct electrophilic amination [118123]. The elec

trophilic species was produced by the photolysis or thermolysis of salts. The reaction with monosubstituted arenes results in a mixture of all isomeric substituted anilines (Scheme 58). This transformation hastheoretical rather than practical significance.

Scheme 58.

A method of direct electrophilic amination of arenes with the use of aXeF2Me3SiNCOCF3SO3Hsystem was suggested recently [124] (Scheme 59). The yield of amination products is 3545%, and monosubstituted benzenes form a mixture of orthoandparaisomers in approximately equal amounts.

Scheme 59.

There are several reports on electrochemical methods of electrophilic amination, for example, [125,126] (Scheme 60). A method of electrochemical amination of arenes with hydroxylamine in sulfuric acidin the presence of transition metal ions was described in [126]. The authors believe that the reaction occurs

with the participation of cation radicals by a chain mechanism. The current efficiency in the case of benzene is up to 147%.

OH

X N

X

+

HNN

+N

N +

H

X

HN3

H2SO4

XX

94%H+

H2O

HN3 N2

H+

9

1011

NH2+

N

R'

R R''

NH2

+

N

R'

R

NH2

+NR R'

NH2

+

R R

NH2

+

R

NH2

+

R

NH2

A or B or hvor t

A

1720% 810% 414%

B C

R

R'

R

NH2R'

+

R

R'

NH2

XeF2/Me3SiNCO

CF3SO3H

3445%


21/26



Scheme 60.

An interesting method of electrophilic amination of arenes under the action of azidodicarboxylic estersand related azo compounds was published recently [127, 128]. The reaction proceeds in the presence ofInCl3on SiO2as a catalyst under conditions of thermal or microwave activation [127] (Scheme 61). Benzene and naphthalene derivatives with electrondonating substituents were involved in the reaction. The

yield was 6080% with thermal activation and 6892% with microwave activation.

Scheme 61.

The reaction of halosubstituted phenols with a halogen in theparaposition results in the migration ofthe halogen to a free orthoposition on account of the ipsoattack at theparaposition [128] (Scheme 62).

Scheme 62.

Nitrenes produced in situ from nitro [129] and nitroso compounds [130] and triethyl phosphite wereused as electrophilic aminating reagents (Scheme 63).

O R O R

NH

O

+

O R

HN

O

NH2 NH2

NH2

+

NH2

H2N

Pt/MeCN

H2O

1854%1124%

72%27%

NH2OH

Ti(IV)

[125]

[126]

EtO2C NN CO2Et

+

NNH

CO2Et

CO2Et

R R

O O

NEtO2C

HN CO2Et

InCl3/SiO2

t or mw

InCl3/SiO2

t or mw

90%

6092%

OH

Hal

Hal'

+ N

NR

O

R

O

OH

Hal

Hal'

NNH

O R

R

O

+

Hal

Hal'

NNH

O R

R

O

HO

ZrCl4CH2Cl2

583% 086%


22/26

380


AKSENOV et al.

Scheme 63.

The last method considered in this review was developed recently in our laboratory. The method is based onthe acetamination of aromatic compounds with nitroalkanes in PPA [131, 132] (Scheme 64). The reaction can

be carried out with benzene and its derivatives with electrondonating substituents. Monosubstituted benzenesproduce only amination products at theparaposition, which is explained by the thermodynamic control of theacylation stage. The yield of acetanilides is from 60 to 92%. The corresponding amines can be obtained if thereaction mixture is refluxed after treatment with water [131]. Dibenzo18crown6 under these conditions produces a mixture of mono and diacetamination products [132].

Scheme 64.

N

NO2

Me

NH

N

Me

N

Me

O2N

N

HN

Me

O2N H

N

P(OEt)3

P(OEt)3

P(OEt)3

76%

5070%

5070%

[129]

[129]

[130]

R

HN R'

O

O

O

O

O

O

O

O

O

O

O

O

O

NHO

O

O

O

O

O

O

NHOH

N

O

O

O

O

O

O

O

HN

O

NH

O

R'CH2NO2

PPA[131]

6392%

CH3CH2NO2

PPA

54%

[132]

+

+ 33%

R


23/26



The reaction of pyrimidines with nitroethane in PPA leads to corresponding acetamides [133] or, at ahigher temperature, the products of theperiannelation of the pyrrole ring [134] (Scheme 65).

Scheme 65.

5. CONCLUSIONS

Thus, the literature currently comprises both multistep methods for the introduction of an aminogroup into the aromatic ring and the direct amination (amidation) of aromatic compounds. The multistepmethods that include nitration and reduction, azo coupling and reduction, acylation and the Schmidtreaction, and acylation and the Beckmann rearrangement are well studied from the theoretical and practical viewpoint. One can expect that further progress in this field will be associated with the introductionof new nitrating agents into practice.

The methods of direct amination (amidation) of aromatic compounds are relatively less studied boththeoretically and preparatively. A substantial disadvantage of these methods is their low regioselectivity.One can expect that further progress in this area will be associated with the introduction of novel aminating (amidating) electrophilic reagents into practice.

ACKNOWLEDGMENTS

This work was supported by the Federal Targeted Program Scientific and Academic Brainpower ofInnovative Russia for 20092013 (Grant no. 16.740.11.0162).

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