A facile method for the synthesis of oxindole based quaternary α-aminonitriles via the Strecker reaction

Article abstract of DOI:10.1039/c0ob00174k

The direct α-cyanoamination of isatins using TMSCN has been developed, which is carried out in methanol without any catalyst. A new bifunctional cinchona alkaloid-based phosphinamide catalyst 7 could promote the Strecker reaction of isatins derived ketimine with TMSCN in up to 74% ee.

Full text of DOI:10.1039/c0ob00174k

Organic &
Biomolecular
Chemistry
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Volume 8 | Number 17 | 7 September 2010 | Pages 3809–4028
ISSN 1477-0520
COMMUNICATION
Jian Zhou et al.
A facile method for the synthesis
of oxindole based quaternary
␣-aminonitriles via the Strecker
reaction
EMERGING AREA
Maurizio Benaglia and Sergio Rossi
Chiral phosphine oxides in
present-day organocatalysis
1477-0520(2010)8:17;1-X

COMMUNICATION
www.rsc.org/obc | Organic & Biomolecular Chemistry
A facile method for the synthesis of oxindole based quaternary a-aminonitriles
via the Strecker reaction†
Yun-Lin Liu, Feng Zhou, Jun-Jie Cao, Cong-Bin Ji, Miao Ding and Jian Zhou*
Received 26th May 2010, Accepted 24th June 2010
First published as an Advance Article on the web 8th July 2010
DOI: 10.1039/c0ob00174k
The direct a-cyanoamination of isatins using TMSCN has
been developed, which is carried out in methanol without
any catalyst. A new bifunctional cinchona alkaloid-based
phosphinamide catalyst 7 could promote the Strecker reaction
of isatins derived ketimine with TMSCN in up to 74% ee.
(TMSCN) turned out to be a widely used cyanide source, less
harmful and easily manageable.9 Since several reactions involving
silicon based nucleophiles could be promoted by the reaction
solvent without any catalyst,12 we first evaluated if the reaction
of ketimine 1a with TMSCN could be accelerated by the reaction
solvent. The solvent effects were summarized in Table 1.
To be strict, all the reactions shown in Table 1 were carried out
at 50 ◦C under an atmosphere of nitrogen. In 1,2-dichloroethane
(DCE) or toluene, no reaction took place after 48 h, partly because
the ketimine 1a dissolved poorly in the two solvents. In THF,
DMSO or DMF, the ketimine 1a dissolved well, but still only trace
amount of the desired product was detected. It was unexpected
that almost no reaction took place in CH3CN or DMF, because
it was reported that CH3CN as the solvent could accelerate the
three component Strecker reaction of aldehydes,12d and DMF as
the solvent could promote the cyanosilylation of aldehydes.12c
All these solvents that might serve as neutral coordinate-
organocatalysts12h failed to promote the desired reaction, suggest-
ing that ketimine 1a was not reactive enough. Recently, Rueping
et al. found that chiral diols such as TADDOL were effective
catalysts for the enantioselective Strecker reaction of aldimines
and HCN.13 Although the use of alcohol for the hydrogen-bond
activation of ketimines was not reported in their work, we tried the
alcohol as the solvent,14,15 with the anticipation that the alcohol
solvent might serve as both a hydrogen-bonding donor promoter
to activate the ketimine and a Lewis base to facilitate the cyanide
transfer.16 To our delight, the use of MeOH as solvent greatly
accelerated the reaction, and almost full conversion could be
achieved within 15 h at 50 ◦C, affording product 3a in 84% yield
(entry 2). In EtOH and i-PrOH (entries 3–4), the reaction still
worked well but were obviously slower than that in methanol.
To investigate the role of alcoholic solvent in this reaction,
several control experiments were conducted using DCE as the
solvent (entries 5–12), since no reaction took place in DCE. First,
to check if the HCN produced from methanol and TMSCN16
served as a Brønsted acid15b–f to promote the reaction, MeOH
was used as an additive for the reaction. Although the ketimine 1a
dissolved poorly in DCE, the use of one equiv. of MeOH relative to
TMSCN afforded product 3a in 13% yield, suggesting that HCN
alone was inefficient to promote the reaction (entry 5). With the
increase of the usage of MeOH additive, the yield of product 3a
was improved (entries 5–10). Obvious improvement in the yield
of product 3a was observed when six equiv. of MeOH relative
to TMSCN was added (entry 8), and the use of ten equivs of
MeOH improved the yield to 79% (entry 10). Based on these
results, the role of MeOH to promote this reaction was believed to
activate ketimine 1a through hydrogen-bonding and to facilitate
the cyanide transfer. The efficiency of the MeOH as the solvent
to promote the reaction was further demonstrated by the control
The 3,3-disubstituted oxindole structure motif is a privileged het-
erocyclic subunit in a large number of bioactive natural products
and drugs.1 Much attention has been devoted to the development
of catalytic asymmetric method for the synthesis of sufficient
amount of these compounds and their analogues for biological
evaluation and studies on the structure–activity relationship,
aiming at the development of new therapeutic agents or impor-
tant biological tools.2 Among these structures, 3-substituted-3-
aminooxindole is very useful because of its presence in natural
products and several pharmaceutical candidates,3 including the
potent gastrin/CCK–B receptor antagonist AG-041R3a and the
vasopressin VIb receptor antagonist SSR-149415.3b–c Although
several methods have already been developed to prepare this
important structure motif, including imine addition reaction,4
intramolecular arylation,5 alkylation of 3-aminooxindole,6 Man-
nich reaction7 and direct amination of 3-substituted oxindoles,8
catalytic asymmetric methods are very limited.5a,8 Surprisingly,
the catalytic asymmetric addition of nucleophiles to ketimines
derived from isatins has not been reported yet, despite it is a very
straightforward method for the synthesis of this subunit.
As part of a program directed at the synthesis of 3,3-
disubstituted oxindole derivatives for biological evaluation, we
have already developed a highly enantioselective amination of
unprotected 3-substituted oxindoles8b and the Michael addition
of unprotected 3-substituted oxindoles to nitroolefins.2r We are
also interested in the synthesis of 3-substituted-3-aminooxindoles
via the Strecker reaction9 which is largely unexplored. Only
one example was reported using isatin, KCN and methylamine
hydrochloride or aniline in large excess of acetic acid.10 In
this communication, we wish to report a catalyst-free direct
a-cyanoamination11 of isatins using TMSCN, and the initial
results in the catalytic asymmetric Strecker reaction of isatins
derived ketimines with TMSCN using a new bifunctional cinchona
alkaloid-based chiral phosphinic amide catalyst.
For convenience and safety, other cyanating agents were intro-
duced to avoid the use of toxic HCN, and trimethyl silyl cyanide
Shanghai Key Laboratory of Green Chemistry and Chemical Processes,
Department of Chemistry, East China Normal University, 3663 N.
Zhongshan Road, Shanghai 200062, (P. R. China). E-mail: jzhou@
chem.ecnu.edu.cn; Fax: +86-21-62234560; Tel: +86-21-62234560
† Electronic supplementary information (ESI) available: General ex-
perimental procedures and NMR spectra of products. See DOI:
10.1039/c0ob00174k
This journal is
The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 3847–3850 | 3847
©

Table 1 Reaction optimization
Entrya
Solvent
Additive
Time/h
Yieldb (%)
1
2
3
4
5
6
7
8
CH2ClCH2Cl
MeOH
EtOH
no
no
no
no
48
15
15
15
15
15
15
15
15
15
15
no
84
64
52
13
15
21
64
70
79
36
i-PrOH
CH2ClCH2Cl
CH2ClCH2Cl
CH2ClCH2Cl
CH2ClCH2Cl
CH2ClCH2Cl
CH2ClCH2Cl
CH2ClCH2Cl
MeOH (2.0 eq)
MeOH (4.0 eq)
MeOH (8.0 eq)
MeOH (12.0 eq)
MeOH (16.0 eq)
MeOH (20.0 eq)
9
10
11
MeOH (2.0 eq)
a 0.5 mmol in 4 mL of solvent under N2, b Isolated yield.
experiment using 10 mol% of Schreiner’s thiourea 415e with MeOH
as additive, which afforded product 3a in only 36% yield (entry 11)
On the basis of the aforementioned optimization, we further
tried to develop a one-pot sequential reaction of isatins 5, anilines
6 and TMSCN, since the direct a-cyanoamination of ketones11
still remained a big challenge and only one example of catalyst-
free version was reported,17 using solvent-free condition at 100 ◦C,
but limited to several cyclic ketones.
Table 2 Substrate scope of direct a-cyanoamination of isatins
Entrya Isatin 5
R3
3
Time/h Yieldb (%)
It was found that acceptable yields could be obtained if TMSCN
was added after the isatins 5 reacted with ani◦lines 6 in methanol for
1
2
3
4
5
6
7
8
R1 = Cl, R2 = H
p-OMe
p-OMe
p-OMe
p-OMe
p-OMe
p-OMe
p-OMe
p-OMe
p-OMe
m-OMe
p-Me
3a
3b
3c
3d
3e
3f
15
15
15
15
15
24
24
24
24
24
69
81
84
63
58
85
83
73
76
68
83
55
R1 = NO2, R2 = H
R1 = F, R2 = H
˚
one hour, in the presence of MS 5 A at 50 C. This reaction was
R1 = Br, R2 = Br
R1 = Br, R2 = H
R1 = OMe, R2 = H
R1 = Me, R2 = H
R1 = H, R2 = H
R1 = Me, R2 = Me
R1 = H, R2 = H
R1 = H, R2 = H
R1 = H, R2 = H
carried out in air, operationally very simple. Under the optimal
reaction condition, we next examined the substrate scope, and a
variety of different substituted isatins and anilines were subjected
to the standard condition (Table 2). Generally, the reaction worked
well whatever the substituents on the isatins. In the case of isatins
with electron-withdrawing groups, longer reaction time would lead
to the formation of side products, so the reactions were stopped
after 15 h, affording the desired product in good to high yield
(entries 1–5). While in the case of other isatins, the reaction time
was prolonged to 24 h and the desired product was obtained in
slightly higher yield (entries 6–9). The substituents of anilines 6
also influenced the reactivity, and electron-donating substituents
on the 4-position had a beneficial effect on the reaction outcome.
In the case of m-methoxyaniline and p-chloroaniline, the yields
were obvious lower (entries 10–12).
3g
3h
3i
9
10
11
12
3j
3k 24
p-Cl
3l
24
a Reaction scale: 0.5 mmol, b Isolated yield.
ESI†).19 With catalyst 7, we further examined the solvent effects,
and 1,1,2,2-tetrachloroethane (TTCE) as the solvent resulted in
acceptable yield and enantioselectivity (see ESI†). Under this
condition, a variety of ketimines were examined, and the results
were summarized in Table 3. Since some of the ketimines dissolves
poorly in TTCE, the conversion was not high even the reaction
time was nearly five days, and the corresponding products were
obtained in only moderate yield (entries 1–3 and 9) and up to 74%
ee. In the case of ketimines with a better solubility in TTCE, the
yield was higher, but the enantioselectivity was lower. Although
the yield and the enantioselectivity of this reaction had much
Based on our analysis of the role of alcohol in this reaction, we
next tried to develop an asymmetrical version of the Strecker reac-
tion of ketimines 1 with TMSCN,18 especially focusing on the use
of bifunctional catalyst. Of all the catalysts we screened, our newly
developed bifunctional cinchona alkaloid-based phosphinamide
catalyst 7 turned out to be the most enantioselective one (see
3848 | Org. Biomol. Chem., 2010, 8, 3847–3850
This journal is
The Royal Society of Chemistry 2010
©

Table 3 Substrate scope of catalytic asymmetric Strecker reaction
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Entrya Ketimine 1
R3
3
Yieldb (%) Ee c (%)
1d
2d
3d
4
5
6
R1 = Cl, R2 = H
OMe
OMe
OMe
OMe
OMe
OMe
OMe
Me
3a
3c
3e
3f
3g
3h
3i
30
27
38
72
70
67
67
73
74
74
50
39
51
53
43
70
R1 = F, R2 = H
R1 = Br, R2 = H
R1 = OMe, R2 = H
R1 = Me, R2 = H
R1 = H, R2 = H
R1 = Me, R2 = Me
R1 = H, R2 = H
R1 = H, R2 = H
7
8
3k 45
3l 42
9d
Cl
(u) S. Wu¨rtz, C. Lohre, R. Fro¨hlich, K. Bergander and F. Glorius, J.
a Reaction scale: 0.25 mmol; b Isolated yield, c Determined by chiral HPLC
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room for further improvement, this represented the first example
of catalytic asymmetric addition of nucleophiles to isatin derived
ketimines to construct the desired quaternary 3-aminooxindoles.
In conclusion, we have developed the first example of catalyst
free a-cyanoamination of isatins to construct useful 3-substituted-
3-aminooxindole 3. This is also the first example of alcohol
solvent promoted one-pot sequential Strecker reaction of ketones
using TMSCN as a cyanide source. We also reported the first
example of catalytic asymmetric nucleophilic addition to isatin
derived ketimines using TMSCN. The new bifunctional cinchona
alkaloid-based phosphinamide catalyst 7 could promote the
Strecker reaction of ketimine 1 with TMSCN in moderate to
good yield and enantioselectivity. The simple and mild reaction
conditions including air-tolerance, together with the usefulness
of the product, make our method very useful. The development
of new bifunctional Brønsted acid-Lewis base catalyst is now
underway in our lab, aiming to develop a highly enantioselective
Strecker reaction of ketimines derived from isatins to prepare
chiral 3-substituted aminooxindoles.
Acknowledgements
The financial support from the Natural Science Foundation of
China (20902025) and East China Normal University are highly
appreciated.
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3850 | Org. Biomol. Chem., 2010, 8, 3847–3850
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