N-Heterocyclic-Carbene-Catalyzed Umpolung of Imines

Article abstract of DOI:10.1002/anie.201611268

N-Heterocyclic carbene (NHC) catalysis has been widely used for the umpolung of aldehydes, and recently for the umpolung of Michael acceptors. Described herein is the umpolung of aldimines catalyzed by NHCs, and the reaction likely proceeds via aza-Breslo

Full text of DOI:10.1002/anie.201611268

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Communications
Chemie
International Edition: DOI: 10.1002/anie.201611268
German Edition: DOI: 10.1002/ange.201611268
Carbenes
N-Heterocyclic-Carbene-Catalyzed Umpolung of Imines
Atanu Patra, Subrata Mukherjee, Tamal Kanti Das, Shailja Jain, Rajesh G. Gonnade, and
Akkattu T. Biju*
Dedicated to Professor Gilbert Stork on the occasion of his 95th birthday
Abstract: N-Heterocyclic carbene (NHC) catalysis has been
widely used for the umpolung of aldehydes, and recently for the
umpolung of Michael acceptors. Described herein is the
umpolung of aldimines catalyzed by NHCs, and the reaction
likely proceeds via aza-Breslow intermediates. The NHC-
catalyzed intramolecular cyclization of aldimines bearing
a Michael acceptor resulted in the formation of biologically
important 2-(hetero)aryl indole 3-acetic-acid derivatives in
moderate to good yields. The carbene generated from the
bicyclic triazolium salt was found to be efficient for this
transformation.
T
he organocatalytic transformations catalyzed by N-hetero-
cyclic carbenes (NHCs) have been widely used for the access
to various carbocycles, heterocycles, and acyclic compounds.[1]
The underlying principle behind many of the NHC organo-
catalytic reactions is the umpolung of aldehydes.[2] The
nucleophilic acyl anion equivalents (Breslow intermediate)[3]
generated by the addition of NHCs to aldehydes can add to
a variety of carbonyl compounds (benzoin reaction),[4]
Michael acceptors (Stetter reaction),[5] and even to unpolar-
ized, electron-neutral carbon–carbon multiple bonds (hydro-
acylation reactions; Scheme 1a).[6] Moreover, NHCs are
employed for the conjugate umpolung of enals for the
generation of homoenolate equivalents.[7] In the majority of
cases, the umpolung using NHCs is limited to aldehydes. In
2006, Fu and co-workers extended the umpolung concept to
Michael acceptors for the synthesis of cyclic a,b-unsaturated
Scheme 1. NHC-catalyzed umpolung.
esters by an intramolecular b-alkylation (Heck-type reaction;
Scheme 1b).[8a] Moreover, efficient tail-to-tail dimerization of
methacrylates by the NHC-catalyzed umpolung of Michael
acceptors were demonstrated independently by the groups of
Matsuoka[9] and Glorius[10] (Scheme 1c).[11]
The addition of NHCs to alkyl halides leading to the
generation of the deoxy-Breslow intermediates was demon-
strated by the groups of Jacobi von Wangelin[12] and Mayr.[13]
Moreover, the reaction of NHCs with iminium salts, resulting
in the isolation of aza-Breslow intermediates, was uncovered
by Rovis and co-workers.[14] However, catalysis using these
intermediates, to the best of our knowledge, is unknown. We
envisioned that the NHC catalysis, using aldimines as
substrates, proceeding by the aza-Breslow intermediates will
considerably expand the scope of carbene-catalyzed trans-
formations. Herein, we report the NHC-catalyzed umpolung
of aldimines[15] for intramolecular reactions proceeding by
aza-Breslow intermediates (Scheme 1d). Specifically, NHC-
catalyzed cyclization of aldimines bearing a Michael acceptor
afforded biologically important 2-aryl indole 3-acetic acid
derivatives in moderate to good yields. Notably, the cyanide-
catalyzed umpolung of imines has recently been reported by
Cheon and co-workers.[16] It is noteworthy that 2-arylindole 3-
acetic acid derivatives are endowed with interesting biological
properties and they are subunits in several natural products.[17]
Our present studies were initiated by treating the aldimine
1a, bearing a Michael acceptor moiety, under NHC catalysis
[*] A. Patra, S. Mukherjee, T. K. Das, Dr. A. T. Biju
Organic Chemistry Division
CSIR-National Chemical Laboratory
Dr. Homi Bhabha Road, Pune-411008 (India)
and
Academy of Scientific and Innovative Research (AcSIR)
New Delhi 110020 (India)
E-mail: at.biju@ncl.res.in
Homepage: http://academic.ncl.res.in/at.biju
S. Jain
Physical/Materials Chemistry Division
CSIR-National Chemical Laboratory
Dr. Homi Bhabha Road, Pune-411008 (India)
Dr. R. G. Gonnade
Centre for Materials Characterization
CSIR-National Chemical Laboratory
Dr. Homi Bhabha Road, Pune-411008 (India)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201611268.
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Table 1: Optimization of reaction conditions.[a]
Entry
Variation of the standard conditions
Yield [%][b]
1
2
3
4
5
none
4 instead of 3
5 instead of 3
6 instead of 3
82 (82)
<5
<5
<5
52
7 instead of 3
6
7
8
9
10
11
12
13
14
K2CO3 instead of KOtBu
DBU instead of KOtBu
Et3N instead of KOtBu
DMSO instead of DMF
1,4-dioxane instead of DMF
toluene instead of DMF
808C instead of 1008C
6 h instead of 12 h
10 mol% of 3 instead of 20 mol%
73
78
29
72
41
<5
38
75
71
[a] Standard conditions: 1a (0.25 mmol), 3 (0.05 mmol), KOtBu
(0.05 mmol), DMF (2.0 mL), 4 ꢀ M.S. (50 mg), 1008C, 12 h. [b] The
yields were determined by 1H NMR analysis (CDCl3) of the crude
reaction mixture using CH2Br2 as the internal standard. Yield of isolated
product given within parentheses. DMF=N,N-dimethylformamide,
DMSO=dimethylsulfoxide, Mes=2,4,6-trimethylphenyl, M.S.=molec-
ular sieves.
Scheme 2. Scope with respect to the NHC-catalyzed umpolung of
imines for the synthesis of indoles. General conditions: 1 (0.5 mmol),
3 (0.1 mmol), KOtBu (0.1 mmol), DMF (4.0 mL), 4 ꢀ MS (100 mg),
1008C, 12 h. Yields of the isolated products are given. [a] Reaction run
for 36 h. [b] Run for 48 h. [c] Reaction run on 0.25 mmol scale.
[d] Structure confirmed by X-ray analysis.
reaction conditions (Table 1). Gratifyingly, treatment of 1a
with a carbene generated from the triazolium salt 3, using
KOtBu as the base in DMF, afforded the 2-aryl indole 3-acetic
acid derivative 2a in 82% yield (entry 1).[18] The indole 2a
was likely formed by the umpolung of 1a using NHC, and
subsequent conjugate addition. Compared to the NHC
generated from 3, other carbenes generated from the
precursors 4–6 are less effective (entries 2–4), and the carbene
generated from 7 furnished 2a in 52% yield (entry 5). The
screening of bases revealed that K2CO3 resulted in 73% of 2a
and DBU produced 78% of 2a, whereas Et3N was not
efficient for this transformation (entries 6–8). The reaction
afforded 72% of 2a when carried out in DMSO (entry 9), and
the performed reactions in 1,4-dioxane and toluene led to
inferior results (entries 10 and 11). The reaction temperature
of 1008C and stirring for 12 hours was needed for good
conversion into 2a (entries 12 and 13). Moreover, reducing
the amount of 3 resulted in a lower yield of 2a (entry 14).
With the optimized reaction conditions in hand, we then
examined the substrate scope of this annulation reaction
which proceeds by the umpolung of imines (Scheme 2). A
series of aldimines synthesized by the condensation of
aromatic amines with the aromatic aldehydes having elec-
tron-releasing, electron-neutral, and moderately electron-
poor groups at the 4-position of the ring are well tolerated in
the cyclization reaction to furnish the functionalized indoles
in 75–95% yields (2a–g). Disappointingly, imines formed
from aldehydes having electron-withdrawing groups, such as
CN, at the 4-position furnished a reduced yield of the indole
product (2h). Moreover, substrates having 2-substitution and
3-substitution, as well as disubstitution on the aryl ring
attached to the imine carbon center underwent smooth
umpolung using NHCs, thus leading to the formation of the
desired product in good yields (2i–m). Additionally, various
heterocyclic-aldehyde-derived aldimines furnished the
desired 2-heteroaryl indole derivatives in good yields (2n–
r). Interestingly, the a,b-unsaturated aldimine 1s afforded the
expected indole 2s in 42% yield. Besides, sterically different
alkoxycarbonyl groups at the Michael acceptor moiety were
tolerated, and in all cases the target product was formed in
good yield (2t–v). Furthermore, this reaction is not limited to
a,b-unsaturated esters as the Michael acceptor, but instead
a,b-unsaturated ketones can also be used as the Michael
acceptors, thus providing the indole functionalized ketones in
high yields (2w–x). In the case of indole derivative 2w, the
structure was confirmed using single-crystal X-ray analysis.[19]
Finally, a series of aldimines with electron-releasing and
2
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electron-withdrawing groups at the N-aryl ring furnished the
Based on these experiments, a proposed mechanism is
indoles in high yields (2y–cc).
shown in Scheme 4. The free carbene generated from the
precursor 3 undergoes a nucleophilic attack on the imine
moiety of 1, thus generating the tetrahedral intermediate C,
which upon proton transfer forms the key aza-Breslow
To gain insight into the mechanism of this transformation,
we performed several mechanistic experiments. The reaction
likely proceeds by the umpolung of the imine, thus generating
the aza-Breslow intermediate A (Scheme 3a). However, the
indole product 2 can also be envisioned by the umpolung of
Scheme 4. Tentative mechanism of the reaction.
intermediate A. Disappointingly, all attempts to isolate the
NHC-imine adduct failed in our hands.[20] The nucleophilic
intermediate A then undergoes an intramolecular 1,4-addi-
tion (aza version of Stetter reaction) to form the ester/keto
enolate D. The intermediate D upon proton transfer and
elimination of NHC forms the desired product 2. Preliminary
computational results with density functional theory (DFT) at
the TZVP/PBE level of theory indicate that the reaction
proceeds by the umpolung of imines, as the more stable
intermediate is found to be the one that leads to the imine
umpolung.[18]
We also performed the functionalization of the indole 3-
acetic acid derivative 2a (Scheme 5). Reduction of 2a using
LiAlH4 afforded the alcohol 10 in 75% yield. Copper-
catalyzed arylation of 10 by dearomatization furnished the
furoindoline 11 having two quaternary carbon centers in 75%
Scheme 3. Studies towards the mechanism of this reaction.
a Michael acceptor, thus generating the deoxy-Breslow
intermediate B followed by its interception with the imine
and subsequent release of carbene. To get information on the
mode of action in the present case, the NHC-catalyzed
reaction was performed with imines (8a–c) derived from
aromatic aldehydes having strongly electron-withdrawing
groups. These reactions did not afford the expected indole
derivative. It is reasonable to believe that the generated aza-
Breslow intermediate in these cases will be less nucleophilic
for the conjugate addition. Given the precedence of umpo-
lung of a,b-unsaturated esters bearing a b-substituent,[8a] the
absence of indole formation indicates that the umpolung of
Michael acceptors is not operating in this case. This reaction
mode was again confirmed by the NHC-catalyzed reactions of
various a,b-unsaturated amides (9a–d). In this case also, the
desired indole was not formed. Naturally, the generated aza-
Breslow intermediate cannot add to the weakly electrophilic
a,b-unsaturated amides. It is noteworthy that umpolung of
a,b-unsaturated amides were reported by Fu and co-workers
as early as 2006[8a] and the absence of indole formation
indicates that umpolung of Michael acceptors is not operative
in the present case.
Moreover, the NHC-catalyzed reaction of the imine [D]-
1a, having deuterium at the a-position of the ester, underwent
smooth cyclization reaction leading to the formation of the
indole product [D]-2a in 81% yield and with 100% deute-
rium incorporation in the product (Scheme 3b). This experi-
ment also sheds light on the umpolung of imines operating in
the present case.
Scheme 5. Functionalization of indole 3-acetic acid derivatives. Tf=tri-
fluoromethanesulfonyl.
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yield.[21] Moreover, basic hydrolysis of the ester moiety in 2a
allowed the synthesis of the acid 12 in 95% yield. Amidation
of 12 using piperidine gave the amide 13 in 87% yield. The
reduction of 13 under LiAlH4 conditions afforded the trypt-
amine 14, which is a selective high-affinity h5-HT2A antago-
nist.[22]
In conclusion, we have developed the NHC-catalyzed
umpolung of imines for intramolecular reactions, and the
reaction proceeds via aza-Breslow intermediates. The reac-
tion of aldimines, having a Michael acceptor moiety, under
NHC catalysis resulted in the formation of 2-(hetero)aryl
indole 3-acetic acid derivatives in an aza-Stetter reaction.
Further studies on exploring the scope of this imine umpolung
triggered by NHCs are ongoing in our laboratory.
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Acknowledgments
This activity was supported by Board of Research in Nuclear
Sciences (BRNS), Government of India (Grant No. 37(2)/14/
49/2014-BRNS/). A.P. and T.K.D. thank CSIR for fellowships
and S.M. thanks UGC for a fellowship. We thank Dr. Anup
Bhunia for discussions, Dr. Kumar Vanka for the computa-
tional studies, Dr. P. R. Rajamohanan for the excellent NMR
support, and Dr. B. Santhakumari for the HRMS data.
Conflict of interest
The authors declare no conflict of interest.
Keywords: imines · indoles · N-heterocyclic carbenes ·
organocatalysis · umpolung
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Manuscript received: November 17, 2016
Revised: January 4, 2017
Final Article published: && &&, &&&&
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Communications
Carbenes
A. Patra, S. Mukherjee, T. K. Das, S. Jain,
R. G. Gonnade, A. T. Biju*
&&& — &&&
N-Heterocyclic-Carbene-Catalyzed
Umpolung of Imines
The other way round: The NHC-catalyzed
intramolecular cyclization of aldimines
bearing a Michael acceptor resulted in the
formation of biologically important 2-
(hetero)aryl indole 3-acetic acid deriva-
tives in moderate to good yields. The
carbene generated from the bicyclic tri-
azolium salt was found to be key for the
success of the reaction.
6
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