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7/26/2019 004 Pentahetarenes With Two Heteroatoms in Positions 1,2
1/15
PENTAHETARENES WITH TWO HETEROATOMSIN POSITIONS 1, 2
1. General
2. Syntheses2.1. Isoxazoles and pyrazoles2.2. Isothiazoles
3. Functionalisation3.1. Functionalisation by electrophil ic substitution at C-4
a) Nitrationb) Sulfonationc) Halogenation
3.2. Functionalisation via metallation3.3. Skeleton rearrangements
Modifications (improvements, additions, cor rections, up to dates etc.) are subjected to no notice.
7/26/2019 004 Pentahetarenes With Two Heteroatoms in Positions 1,2
2/15
Mircea Darabantu MASTER D-1
HETARENE PENTAATOMICE CUDOI HETEROATOMI IN POZITIILE 1,2
1. Generalitati:a)reprezentanti tipici: 1,2 - azolii
H
1,2-tiazOL 1,2-diazOL 1,2-oxazOL izotiazol pirazol izoxazol
1
55
44 33
2215
4 3
2
1
ON
NN
SN
b) caracterul aromatic: caracter aromatic in general mai scazutdecat analogii 1,3 - azoli: S >NH > O.
XN
XN
+X
N
+
_
XN
XN
_
XN
+ +
_
_
1 2
34
5 N-2: aza-atom "pi ridinic" cu bazicitate si nucleofilicitate diminuate fata de analogul
N-3 din 1,3-azoli datorita vecinatatii
dezactivante (efect -IX) a heteroatomului X
(O > S > N)
c) caracterul acido bazic: bazicitate sensibil mai scazuta decat analogii 1,3-azoli
Echilibrul Marimea dedefinitie
NH S O
pKb
pKa
11.52.5
14.5- 0.5
17.0- 3.0
pKa
pKb
14.2- 0.2
--
--
pKb
pKa
7.07.0
11.52.5
13.20.8
pKa
pKb
14.4- 0.4
--
--
-H+ XNH
+
+
NHX
+H+
XN
XNH
+
_N
N_
H
-H+
NN+H
+ NN
_
NN
H+
N
X+
HN
X-H+
+H+N
X
N
X
+
H
N
N_
_
H
N
N
N
N
-H+
+H+
N
N
_
7/26/2019 004 Pentahetarenes With Two Heteroatoms in Positions 1,2
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Mircea Darabantu MASTER D-0
NN
H1
2
34
5
pyrazoleas neutral form
NN
H+ 0.300
- 0.269
+ 0.025- 0.107
+ 0.051
1H - NMR
NN
H
7.617.31
7.61
charges values (ppm)
13C - NMR
NN
H
134.6105.8
134.6
values (ppm)
NN
1
2
34
5
pyrazole
as anionic form
1H - NMR
NN
7.356.05
7.35
values (ppm)
_ _
H values (ppm)
8.57
6.87 8.57
HN
NH+1
H - NMR
pyrazolium
cation
5
4 3
2
1NNH+
OHH
+
1
2
34
5
izoxazolium
cation
1H - NMR
ONH
+
9.187.26
9.01
values (ppm)
ON
1
2
34
5
izoxazoleas neutral form
ON
+ 0.200
- 0.279
+ 0.075- 0.097
+ 0.101
1H - NMR
ON
8.146.28
8.39
charges
values (ppm)
values (ppm)
9.60
7.90 9.10
S
NH+
1H - NMR
izothiazolium
cation
5
4 3
2
1S
NH+
values (ppm)
147.8
123.4 157.0
SN
13C - NMR
values (ppm)
charges
8.72
7.26 8.54
SN
1H - NMR+ 0.052
- 0.095 + 0.047
- 0.283
+ 0.280SN
izothiazoleas neutral form
5
4 3
2
1SN
7/26/2019 004 Pentahetarenes With Two Heteroatoms in Positions 1,2
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Mircea Darabantu MASTER D-2
Nota 1: influenta - vecinatatii heteroatomilorse face resimtita mai mult asupra valorilor pKaalespeciilor protonate (1,3 - azoli iprotonatisunt specii mai slab acidedecat 1,2 - azolii protonati)
Consecinta 1: functionalizarea prin SE la CH=heterociclic se realizeaza in conditii mai putin dure
Nota 2: influenta - vecinatatii atomilor de azot este nesemnificativa asupra valorilor pKa ale
speciilor neutre (pirazol vs. imidazol).
Nota 3: in ambele cazuri se manifesta direct efectul Ial heteroatomului Xasupra azotului piridinc(paralelism accentuatintre intensitatea efectului Isi valorile pKbale speciilor neutre)
c) tautomeria pirazolilor :-fenomen dinamic rapid (tautomerie prototropica) in scala de timp spectrala- consecinta: echivalenta pozitiilor C-3 si C-5 in cazul derivatilor mono-, di- sau tri- C-
substituiti
44
2
1
2
1
3
5
5
3
N
N
H
H
N
N
tautomer 2Htautomer 1H
21
21
3
55
3
NN
baza tare
acid slabHbaza tare
acid slab
HN
N
R1
R2 R3
R1
R2 R3
2. Sinteze:2.1. Izoxazoli si pirazoli :a) deconectare hidro li tica: (1-5)-(2-3)
X
NR1
R2 R3
1
2
34
5
HX
N
OH
R3R2
R1
OXH
N
R1
R2
R3
R1
R2
R3
O
ONH2
HX
1,3-dicetonaX = O, hidroxil aminaX = NH, NR hidrazina (optiona l) substitui ta
-aspect particular: r e g i o s e l e c t i v i t a t e a cic lizarii daca R1 R
2 R
3si X NH
R2
R3
O
ONH2
HXR1
X
NR1
R2 R3
1
2
34
5
-2H2O R2
R3
O
OXH
H2NR1
R1
R2 R3
1
2
3
4 5-2H2O
X
N
7/26/2019 004 Pentahetarenes With Two Heteroatoms in Positions 1,2
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Mircea Darabantu MASTER D-3
- se formeaza cu larga majoritate regioizomerul provenit din atacul nucleofil al amineiasupra carbonilului cel mai electrofil
5-(4-Methoxyphenyl)-3-
-(3-nitrophenyl)-izoxazole
O
O
CH3O
O2N
p+= - 0.78
m= + 0.73
HO
NH2
-H2O
O
N
CH3O
OH
O2N
O
NCH3O
NO2
1
2
34
5
1
2
34
5
O
Nm-O2N-C6H4
C6H4-OCH3-p
p= + 0.24
p-Cl-C 6H4
O
N
PhH3C
5
4 3
2
1
5
4 3
2
1
O
N
Cl
H3C
-2H2O
HO
NH2
O
O
Cl
H3C
3-(4-Chlorophenyl)-4-methyl--5-phenylizoxazole
N
N
COOEt
PhO
O
O OEt
N
NEtOOC
Ph
5
4 3
2
1
5
4 3
2
1
-2H2O
HN
NH2
Ph
5-Cyclopropyl- -3-ethoxycarbonyl- -1-phenylpyrazole
!
Rolul tautomeriei substratului:
HO
O
A=B
D
O
A=B
D
O
X NH2
efect
HO
CH3
O
89.2% ca enol
A=B
D
O
O
H
A=B: e lectronoatragatoriD: electronodonori
7/26/2019 004 Pentahetarenes With Two Heteroatoms in Positions 1,2
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Mircea Darabantu MASTER D-3a
Procese model de cicloaditie [4 + 2] 1,3 - dipolara
1.Aditia anionului alil la etena
_
_
_
_
Dipol Dipolarofi l
ST
Stare de Tranzitie
#
4e 2e
+
Diagrama energetica a ocuparii orbitalilor din partenerii de reactie:
E (< 0 !)
OMA*
OMN
OML
_D i p o l
+ 0.577
+ 0.577+ 0.577
+ 0.577
+ 0.577
- 0.577
+ 0.707
- 0.707
LUMO: - 1.414
HOMO: 0.000
NHOMO: + 1.414
LUMO: - 1.000
HOMO: + 1.000
+ 0.707+ 0.707
+ 0.707 - 0.707
D i p o l a r o f i l
E min. (LUMO - HOMO) orb itali s t e r e o d i r e c t o r i: - 1.000
E min. = -1.000
E max. = -2.414
HOMO
DIPOLD O N O R
LUMO
DIPOLAROFILA C C E P T O R
7/26/2019 004 Pentahetarenes With Two Heteroatoms in Positions 1,2
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Mircea Darabantu MASTER D-3b
2. Aditia anionului alil la acetilena
+
2e4e
#
STStare de Tranzitie
DipolarofilDipol
_
_
_
_
_
anion ciclopentadienil
OMF impl icati LUMO HOMO
Anion al i l - 1.414 0.000
Aceti lena - 1.300 + 1.500
Emi n(LUMO - HOMO) = - 1.300
3. Aditia anionului azaalil la acetilena
+
2e4e
#
ST
Stare de Tranzitie
DipolarofilDipol
N
_N
_N
_N
_
N_
anion pirolat
OMF impl icati LUMO HOMO
Anion azaal i l - 1.117 0.000
Aceti lena - 1.300 + 1.500
Emi n(LUMO - HOMO) = - 1.300
N
LUMO dipolarofil : acetilena
HOMO dipol : anionul azaali l
Emin(LUMO - HOMO) = - 1.300
+ 0.707
- 0.707
+ 0.707
- 0.707
0.000
Donor
AcceptorSTEREO
DIRECTORI
Limitele teoriei:in pofida faptului ca diferentele de energie minime calculate(LUMO HOMO)sunt aceleasi in ambele cazuri, numai aditia anionilor azaalil (substituiti) se cunoaste laacetilena
7/26/2019 004 Pentahetarenes With Two Heteroatoms in Positions 1,2
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Mircea Darabantu MASTER D-4
-b) izoxazoli; deconectarea ca proces retro Diels - Alder: (1-5)-(3-4)
ONR1(2)
R2(1) R3
1
2
34
5
C
C
C
N
O
R2(1)
R1(2)
R3
+
_
C
N
O
R3
+
_
D i p o lD i p o l a r o f i l nitriloxid
Exemplul 1:
CH3O
N+
O-
H3C
ON
CH3
CH3O
1
2
34
5
N+
O-
H3CCH3O
E X C L U S I V5-Methoxy-3-methylizoxazole
ON
CH3CH3O
1
4
5 2
3
L I P S A
ELUMO= - 1.361
EHOMO= + 1.173
CH3O Dipolarofil
LUMO
HOMO
EHOMO
= + 1.403
HOMO
ELUMO= - 0.245LUMO
CH3 N+
O-
Dipol
E(< 0 !)
E = - 2.764E = - 1.418
min. E[ ELUMO- EHOMOadica - 1.361 (+1.403) si - 0.245 (+ 1.173)] = 1.418
o r b i t a l i s t e r e o d i r e c t o r i
HOMO Dipolarofil D O N O R
LUMO DipolA C C E P T O R
+ 0.483
+ 0.631
+ 0.238
+ 0.772
N+
O-
H3C
CH3O
CH3O
H
N
CH3
O STstarea de tranzitie
ON
CH3
CH3O
7/26/2019 004 Pentahetarenes With Two Heteroatoms in Positions 1,2
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Mircea Darabantu MASTER D-5
Nota 1:tipul reactiei cicloaditie 1,3-dipolara [4 + 2]
Nota 2: tipul interactiei donor (HOMO Dipolarofil) acceptor (LUMO Dipol) intre orbitaliistereodirectoricare asigura cu cea mai mare probabil itatestarea de tranzitie ST.
Nota 3: celalalt regioizomer, in acest caz, este total absent datorita diferentei energetice
discrepante intre cele doua interactii perturbationale (HOMO LUMO) posibile (-1.418 vs. -2.764); el corespunde celeilalte perechi de orbitali (LUMO Dipolarofil HOMO Dipol).
Exemplul 2:- dipolarofilul ca alchena functionalizata
_
+
C
N
O
CH3
H3CO
LG: MeO, Me2N, NO22-izoxazolina
5
4 3
2
1
R3R2(1)
R1(2)
ON
- LGH
LG
HOMO Dipolarofil D O N O R
LUMO DipolA C C E P T O R
+ 0.574
+ 0.709
+ 0.238
+ 0.772
5
43
2
1
ON
CH3
CH3O
N
+
O-
H3C
D i p o l a r o f i l
D i p o l
_
+
R3
R1(2)
R2(1)
C
N
O
5
4 3
2
1
R3R2(1)
R1(2)
ON
LG
majoritar
3-Methyl-5-methoxy-2-izoxazoline
- MeOH 5
43
2
1
ON
CH3
CH3O
EHOMO(+ 0.809 ELUMO(- 0.245 E(LUMO - HOMO) = - 1.054 orbitalii stereodirectori
ELUMO (- 1.068) EHOMO(+ 1.403) E(LUMO - HOMO) = - 2.471
H
CH3O N
CH3
O STstarea de tranzitie
ON
CH3
CH3O
- generarea in situ a nitriloxizilor (R = aril, alchil)
R-CH=O R-CH=N-OH R-C N +-O--2H (ox.)
R CH N O H
O HCl
-H2OR C N O
H
Cl
H+_
NR
Cl
O- Cl
-_
oxidanti: Cl2/NaOH
Pb(OAc)2 NBS
7/26/2019 004 Pentahetarenes With Two Heteroatoms in Positions 1,2
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Mircea Darabantu MASTER D-6
c) pirazoli; retrosinteza: deconectarea ca reactie retro Diels - A lder: (1 - 5) - (3 - 4)
- sinteza: cic loadit ie 1,3-dipolara [4 + 2] intre alchine (functionalizate) si diazo componentealifatice (diazometan, ester diazoacetic, etc); acestea din urma se prepara, uzual, in situ
diazo componente alifaticeD i p o l a r o f i l D i p o l
R3
_
+
R3
R1(2)
R2(1)
CH
N
N
C
C5
4 3
2
1
R3R2(1)
R1(2)
NN
H +
N
N
CH _
Exemplul 1:- sinteza pirazolului ca atare:
LUMO - acceptor HOMO - donor
- 0.483- 0.707
+ 0.707 + 0.672
N
N+
HN
N
E = -1.83
Orbitali stereodirectori() EHOMO ELUMO
Acetilena + 1.500 - 1.300Diazometan + 0.536 - 0.924
_CH2
N
N+
Exemplul 2:- regioselectivitate practic totalala prepararea 4-cianopirazolului
u r m e
H
NNNC
CN
N
LUMO - acceptor HOMO - donor
- 0.483- 0.378
+ 0.575 + 0.672
N
N+
HN
N
NC
E = -1.45
Orbitali stereodirectori() EHOMO ELUMO
Nitrilul acidului propiolic + 1.132 - 0.920Diazometan + 0.536 - 0.924
_
C
CH2
N
N+
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Mircea Darabantu MASTER D-6a
IMPORTANTA CICLOADITIILOR 1,3 - DIPOLARE [4 + 2] IN SINTEZA1,2 AZOLILOR INALT FUNCTIONALIZATI
O
Smith Kline Beecham Pharmaceuticals
N
N
HO
Br
H
1
2
34
5
rac - 3-Bromo-5-(2- tert -butylamino-1-hydroxyethane-1-yl)-izoxazole
1
2
Preparat utilizat la tratarea astmului bronsic
Problema:introducerea atomului de brom la C-3nu este posibilape heterociclul preformat; eltrebuie sa se gaseasca pe un precursor convenabil.
N
BrBr
O
H
K2CO3-HBr N
Br
O_
+
_
+
O
O
Br
N[4 + 2]Br2
-HBr
O N
Br
O
BrNaBH4
H:-
O
Br
NO
O N
Br
Br
HO
+ NaH
- H2, -NaBr ON
Br
O
H2N
H
ON
N
HO
Br
total
regioselectiva regioizomer unic
generata in situ
deschidere totalregioselectiva
prin control steric al t-Bu
7/26/2019 004 Pentahetarenes With Two Heteroatoms in Positions 1,2
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Mircea Darabantu MASTER D-7
2.2. Izotiazoli:a) cicl izarea redox a - iminotioamidelor :
5-Amino-3-methyl izothiazole
5
4
3
2 1N S
Me NH2
N S
Me NH-HCl
Cl-+
H3C
N
NH
SH
- NH3
oxidare
blanda
H3C
N
NH
SClH
Cl NH2
oxidare blanda
H3C
NH S
NH H
tionare
Grupare +E
H
H3C NH2
ONH
exces NH3
H3C OEt
O O
Nota: tiolamina ca H2N-SHeste prea instabila legatura N - S Nu este p r e f o r m a t a
b) cicl izarea redox a acizilor - imino - S - sulfonici:
SO3H- + H
+
(tiosulfatul alchenei)
2+
S
N
R
R
O-
S+
OHHO
R S
N
R
SO3H
H
NH3
acid S-sulfonic
R S
O
R
SO3H
H+
2Na+
O-
S O-
O-
-S
R
RO - H+
Nota: R (identici sau diferiti, alchil, aril)
3. Functionalizarea:- delimitarea si discriminarea centrelor reactive din 1,2-azoli:
orientarea data decatre heteroatom
orientarea data decatre N-piridinic
XN
1
XN
1
2
3
5
+_
NX
_+E+
E+( pozitie "orto" )
( pozitie "para" )
XN
1
XN
1
4
+
_
XNH
1
25
H+R-Li
i) in C- SE
ii) in metalare
7/26/2019 004 Pentahetarenes With Two Heteroatoms in Positions 1,2
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Mircea Darabantu MASTER D-8
Nota 1: la functionalizarea prinSE laCH= se manifesta E (N - piridinic) dominant fata de+ EX
(caS > NH > O) orientare la C - 4 (Meta Directing Group stronger than the Ortho DirectingGroup)
Nota 2: la functionalizarea viametalare se manifesta IX orientare la C - 5
Nota 3: atacul electrofil la N - piridinicse manifesta mai ales prin protonareca o consecintaa diminuarii nucleofilicitatii acestui centru prin - vecinatatea efectului IX (ca O > S > NH)
3.1. Funct ionalizarea pr in SE la C-4:a) Sulfonarea si nitrarea:
la f i e r b e r e aamestecului n i t r a n t
H
N
N
O2N
HN
N
O2N
100oCHNO3/ H2SO4
HN
N
pKa = + 2.5pKa = - 0.5
H+
N
S
-O3S
250oCH
+N
SSN
HO3S
+N
SH
150oCSO3/ H2SO4
SN
pKa = + 6.9pKa = + 2.5
H+
H
N
N
-O3S
160oCH
+N
NHH
NN
HO3S
+N
NH
H
100oCSO3/ H2SO4
HN
N
Nota: se mentin conditiile dure, similare 1,3-azolilor
b) Halogenarea:- numai bromurarea prezinta interes preparativ (vezi interschimbul halogen metal)
X = O; Br2(l)
X = NH; Br2 / AcONaX
N
Br
-HBr
Br-
Br Br
+X
N
HBr
XN
- facilitarea substitutiei de catre un presubstituent activant:
SNH2N
SN
H
Br
H2N+
Br Br
Br-
i) -HBr
ii) HO- SN
Br
H2N
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Mircea Darabantu MASTER D-9
3.2. Funct ionalizarea prin metalare:a) are loc la C - 5si este valabila numai pentru pirazoli N - substituiti siizotiazoli deoarece
SN
SNLi
EtO
O
OEt
O
- EtOLi
SN
EtO
O
O
NN
Ph
n-BuLi (-78 oC)
i) n-BuLi (-78 oC)
ii) + MeI, - LiI NN
Ph
H3C
5-Ethoxyoxalylizothiazole
b) in cazul particularal izoxazolilor, litioderivatul se descompune, de exemplu:
ON
Ph Ph
n-BuLi (-78 oC)
ON
Ph Ph
Li
Ph
OLi
+
Ph
CN
instabil
c) carbanioni stabil izati de catre heteroatomi apartinand pentaheterociclului 1,2 - azolic:
- numai izoxazolii (NU si pirazolii sau izotiazolii) stabilizeaza carbanionii rezultati prin
deprotonarea unei grupe metil de laC - 5 ( - stabilizare)
ON
CH3
H3Cn-BuLi (-78 oC)
ON
CH3
H2CLi
ON
CH3
H2C Li
PhCH2Br
-LiBr ON
CH3
Ph
O
Li
OLi
analogie cu deprotonareacompusilor crotonici
- crearea indirectade carbanioni la C - 4prin interschimb halogen (polarizabil) metal
ON
CH3
H3C Br2
ON
CH3
H3C
Br
n
-BuLi (-78oC)
- n-BuBr ON
CH3
H3C
Li
O C OA N H I D R U
H
+
ON
CH3
H3C
O
OH
1
2
34
5
4-Carboxy-3,5-dimethyl izoxazole
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Mircea Darabantu MASTER D-10
- uniipirazoli N - metilati pot stabiliza carbanioni prin deprotonarea selectivaa acestei
grupe(- stabilizare) in prezenta altora:
NN
CH3
H3C
CH3
-pozitie fata decarbonil mascat
-pozitie fata decarbonil mascat n-BuLi (-78 oC)
NN
CH3
H3C
Li
+ MeI
-LiI NN
Me
Me
Et
4. Reactii de rearanjare de schelet:- Nu auvaloare preparativa- sunt reactii electrociclice permise fotochimic
- sunt cunsocute doua mecanismeprincipale prin care unii 1,2-azolise transforma in1,3-azoli :
a) izoxazoli oxazoli (via 2H - azirine):
ON N
O 1
2
3
*N
O *
NH1
2
2H- aziri na1H- azirina
3
NH1
2
aziridina
3
h
b) izotiazoli tiazoli (mecanism Walk):
SN
Ph
N
S
Ph
NS
Ph
hS N
Ph