Enantioselective Strecker-type reaction between azomethine imines and trimethylsilyl cyanide catalyzed by a cinchona alkaloid-derived thiourea bearing multiple hydrogen-bonding donors

Article abstract of DOI:10.1039/c3ra41640b

The asymmetric Strecker-type reaction between azomethine imines and TMSCN was catalyzed by 1 mol% of a cinchona alkaloid-derived thiourea bearing multiple hydrogen-bonding donors to afford optically active amino nitriles in good to excellent yields and enantioselectivities.

Full text of DOI:10.1039/c3ra41640b

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Enantioselective Strecker-type reaction between
azomethine imines and trimethylsilyl cyanide catalyzed
by a cinchona alkaloid-derived thiourea bearing
multiple hydrogen-bonding donors3
Cite this: RSC Advances, 2013, 3, 9154
Received 27th February 2013,
Accepted 23rd April 2013
DOI: 10.1039/c3ra41640b
Nai-Kai Li, Zhao-Min Liu, Xiao-Fei Huang, Jin-Xin Zhang, Xiang Chen, Yong Wang*
and Xing-Wang Wang*
www.rsc.org/advances
The asymmetric Strecker-type reaction between azomethine
imines and TMSCN was catalyzed by 1 mol% of a cinchona
alkaloid-derived thiourea bearing multiple hydrogen-bonding
donors to afford optically active amino nitriles in good to
excellent yields and enantioselectivities.
respectively reported by Shibata8a and Hayashi8b groups from
2009. Thus, we envisaged that azomethine imines could also be
employed as effective electrophiles in the catalytic asymmetric
Strecker-type reaction. In fact, earlier in 2007, the Svete group
explored the reaction of 3-pyrazolidinone-1-azomethine imines
with potassium cyanide in the presence of acetic acid.9
Unexpectedly, the formed amino nitriles proceeded to the b-
eliminative N–N bond cleavage through ring transformation
reaction to give N-(3-amino-2-benzamido-3-oxo-1-phenylpropyl)ar-
ylimidoyl cyanides in most cases. Herein, the asymmetric Strecker-
type reaction between azomethine imines and TMSCN was
developed by using a cinchona alkaloid-derived thiourea catalyst
with multiple hydrogen-bonding donors, which furnished opti-
cally active amino nitriles in good to excellent yields and
enantioselectivities.
Introduction
The catalytic enantioselective Strecker reaction is one of the most
direct and atom economic carbon–carbon bond formation
reactions, which provides enantiomerically pure a-amino nitriles.1
The resultant a-amino nitriles can be facilely converted to natural
or non-natural chiral a-amino acids, chiral diamines as well as
biologically and pharmaceutically active compounds.2 Since the
first asymmetric catalytic version was reported in 1996,3 the
catalytic enantioselective Strecker reaction has been extensively
studied, and great achievements have been made during the last
decades.1 Thereafter, the organocatalytic enantioselective Strecker
reaction has been developed extremely quickly.1f,4 Except for the
classic aldimine and ketimine substrates, the Strecker reactions of
fluoro substituted ketimines and cyclic aldimines or ketimines
have been separately developed by several groups, which catalyzed
by Takemoto’s thiourea or cinchona alkaloid-based (thio)urea
catalysts to afford a-amino nitriles in good to excellent yields and
enantioselectivities.5
Results and discussion
The reaction of azomethine imine 2a with 1.2 equiv of TMSCN was
initially tested in the absence of a catalyst in toluene at room
temperature, which proceeded smoothly to afford the adduct 4a in
78% yield within 5 h (Table 1, entry 1). Then, several types of
Azomethine imines, well-known partners of asymmetric
cycloaddition reactions with olefins or alkynes,6 are scarcely
reported as an acceptor of the nucleophilic addition. Actually,
though non-asymmetric addition of Grignard reagents to azo-
methine imines was established in 1976,7 enantioselective
trifluoromethylation and arylation of azomethine imines were
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P.
R. China. E-mail: yowang@suda.edu.cn; wangxw@suda.edu.cn
Electronic supplementary information (ESI) available: Experimental procedures,
3
characterization and spectral data of the Strecker-type reaction products. CCDC
902498 and 902501. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3ra41640b
Scheme 1 Catalyst candidates.
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Table 1 Screening of catalystsa
Table 2 Further optimization of multiple hydrogen-bonding catalyst 1h
catalyzed asymmetric Strecker reactiona
Entry Catalyst Solvent
Temp. (uC) Time (h) Yieldb (%) eec (%)
Entry Catalyst loading (mol%) Time (h) Yieldb (%) eec (%)
1
2
3
4
5
6
7
8
—
1a
1b
1c
1d
1e
1d
1d
1d
1f
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
rt
rt
rt
rt
rt
rt
5
78
82
70
77
89
75
95
94
94
90
92
90
92
—
0.8
0.6
0.7
0.7
3
0.2
3.5
27
8
23
233
220
59
25
67
79
86
86
87
1
2
3
5
2.5
1
0.5
1
1
10
15
18
35
20
60
60
8
90
95
97
94
92
93
94
96
90
91
95
92
90
92
93
92
4
5d
6e
7f
8g
Mesitylene rt
Mesitylene 215
Mesitylene 215
Mesitylene 215
Mesitylene 215
Mesitylene 215
Mesitylene 215
9d
10
11
12
13
1
1
1g
1h
1i
1.5
0.75
3
a
Unless otherwise noted, reactions were carried out with 2a (0.2
mmol), 3 (0.24 mmol, 1.2 equiv) in mesitylene (0.5 mL) at 215 uC.
90
63
b
e
c
d
Isolated yields. Determined by chiral HPLC. 1.0 mL mesitylene.
f
g
a
20 mg 3 Å MS. 20 mg 4 Å MS. 10 mmol scale reaction.
Unless otherwise noted, reactions were carried out with 2a (0.2
mmol), 3 (0.24 mmol, 1.2 equiv) and 1 (0.02 mmol, 10 mol%) in
b
c
corresponding solvents (0.5 mL). Isolated yields. Determined by
chiral HPLC. 10 mol% (S)-BINOL was added as the additive.
d
be somewhat necessary for high enantiocontrol in this asymmetric
Strecker reaction (Table 1, entries 10–12 vs. Table S3, ESI, entries
3
1–9). In addition, when 1i bearing (1S, 2R)-2-amino-1,2-dipheny-
lethanol moiety was used for this transformation, much lowered
enantioselectivity was observed (63% ee, Table 1, entry 13), which
implied that mismatched chiral stereogenic centers between
cinchona alkaloid backbone and 2-amino-1,2-diphenylethanol
moiety were disadvantaged for induction of the enantioselectivity.
Finally, quinine-derived catalysts 1h with multiple hydrogen-
bonding donors turned out to be the potent catalyst for this
reaction in terms of both activity and enantioselectivity.
The feasibility to reduce the catalyst loading to a practical level
was also examined (Table 2). Gratefully, when the catalyst loading
was further decreased from 5 mol% to 1 mol%, 97% yield and
95% ee were obtained, albeit the reaction time was prolonged
(entries 1–3). Notably, even though the catalyst loading of 1h was
reduced to 0.5 mol%, the reaction still afforded the desired
product 4a in 94% yield with 92% ee (Table 2, entry 4).
Furthermore, the other parameters, including substrate concen-
tration and molecular sieves, were also examined for this reaction,
but no better results were observed (Table 2, entries 5–7). To
demonstrate the value of our catalytic asymmetric synthetic
method in the scalable synthesis of chiral amino nitriles, a
gram-scale synthesis was performed with 1 mol% catalyst loading.
Product 4a (1.93 g) was readily obtained in 96% yield with 92% ee
when 1.53 mL TMSCN in one portion was added to the reaction
vessel (Table 2, entry 8).
bifunctional catalysts 1a–e (Scheme 1) with varying stereogenic
structures were subsequently investigated to catalyze the enantio-
selective Strecker-type reaction of azomethine imine 2a under 10
mol% of catalyst loading. Gratefully, quinine-based thiourea
catalyst 1d was proven to be a promising catalyst for this
transformation, and the amino nitrile product 4a was obtained
in 89% yield with 59% ee (Table 1, entry 5). The catalytic
performance with catalysts 1a and 1e vs. 1d clearly indicated that
both a basic quinuclidine and an acidic thiourea moiety are
crucial partners for the stereo-chemical outcome of this transfor-
mation (25% ee and 23% ee vs. 59% ee, Table 1, entries 2 and 6
vs. 5). Further results on the optimization of the other reaction
parameters, including the reaction solvents, reaction temperature,
additives as well as more quinine-derived thiourea catalysts, are
summarized in Table S1–3, ESI. Finally, when 10 mol% of (S)-
3
BINOL as an additive was introduced to this reaction system, the
expected product 4a was obtained in 94% yield with 86% ee in
mesitylene at 215 uC with 0.4 mol L21 of a substrate concentra-
tion, albeit within a reaction time of 27 h (Table 1, entry 9).
Considering the great effect on the stereo-chemical outcome in
the presence of catalytic amounts of (S)-BINOL as additives, several
achiral or chiral amino alcohols were incorporated into quinine-
derived thiourea catalysts 1f–i (Scheme 1), which were subse-
quently employed to catalyze this reaction under otherwise
identical conditions.10 As expected, the 1f–h catalyzed reactions
proceeded smoothly to afford the desired product 4a in increased
enantioselectivities. In particular, 1h bearing (1R, 2S)-2-amino-1,2-
diphenylethanol moiety was found to be the most efficient catalyst
to provide the product in 90% ee (Table 1, entry 12). It further
indicated that the incorporation of hydroxyl group (OH) seems to
Under the optimized reaction conditions, the substrate scope
and limitations of the reaction were subsequently investigated for
this enantioselective Strecker-type reaction of azomethine imine 2
with TMSCN. As shown in Table 3, the azomethine imines 2b–j
bearing electron withdrawing substituents (-F, -Cl, -Br, -CF3) on the
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Table 3 Substrate scope examinationa
Entry Substrates (R)
Time Products Yieldb (%) eec (%)
1
2
3
4
5
6
7
8
Ph (2a)
18 h 4a
3 d 4b
23 h 4c
10 h 4d
10 h 4e
20 h 4f
97
98
95
98
98
99
99
99
97
90
95
97
98
93
95
92
95
89
97
93
87
95
97
90
90
83
90
85
2-FC6H4 (2b)
4-FC6H4 (2c)
2-ClC6H4 (2d)
3-ClC6H4 (2e)
4-ClC6H4 (2f)
2-BrC6H4 (2g)
3-BrC6H4 (2h)
4-BrC6H4 (2i)
4-CF3C6H4 (2j)
2-MeOC6H4(2k)
4-MeOC6H4 (2l)
Fig. 1 (a) X-ray crystal structure of 4h; (b) X-ray crystal structure of 4i.
3 d
3 d
3 d
4g
4h
4i
9
10
11
12
13
14
20 h 4j
To further investigate the mechanism of the Strecker-type
reaction between azomethine imines and TMSCN, B3LYP/6-
31G(d,p) calculations were performed with Gaussian 03.11 As
shown in Fig. 2, the most stable transition state (TS) was
characterized by multiple hydrogen bonds formed between acid–
base multifunctional catalyst 1h and the substrate 2h with a
relative lower activation barrier of 5.14 kcal mol21. The mechan-
ism was favored by the simultaneous activation of azomethine
imines through the stronger double-hydrogen-bonding activation
of 1h and the carbonyl group of 2h with bond lengths of 2.004 and
2.126 Å, involving a strongest triple hydrogen bond (1.837 Å)
between hydroxyl group (OH) and the carbonyl group of 2h. As a
Lewis base, the basic quinuclidine moiety activated the HCN
which was in situ generated from TMSCN. More interestingly, a
fourth hydrogen bond (2.175 Å) was located between thiourea
moiety and the protonated amine. As a result, the nucleophilic
cyanide attacks on the CLN double bond from the Re face, leading
5 d
6 d
4k
4l
4m
4n
3,5-(MeO)2C6H3 (2m) 6 d
5 d
(2n)
15
16
17
18
19
2-Naphthyl (2o)
2-Furanyl (2p)
3-NO2C6H4 (2q)
4-CNC6H4 (2r)
Cyclohexyl (2t)
6 d
20 h 4p
6 d
6 d
7 d
4o
99
99
90
87
95
87
74
d
4q
4r
4t
—
—
d
12
a
Unless otherwise noted, reactions were carried out with 2a (0.2
mmol), 3 (0.24 mmol, 1.2 equiv) and 1 (0.002 mmol, 1 mol%) in
mesitylene (0.5 mL) at 215 uC. Isolated yields. Determined by
chiral HPLC. The products could not be determined by HPLC.
b
c
d
phenyl ring were well-tolerated and led to their desired products
4b–j in 90–99% yields with 87–97% enantioselectivities (Table 3,
entries 2–10). For substrates 2k–n bearing electron-donating
substituents (-OMe and -OCH2O-) on the phenyl ring, the desired
products 4k–n were obtained in 93–98% yields albeit with varying
levels of enantiomeric excesses from 83% ee to 90% ee (entries 11–
14). Substrates 2o–p bearing 2-naphthyl and 2-furyl groups were
also suitable for this transformation to give the desired products
4o–p in 99% yields with 87% and 74% enantioselectivities (entries
15 and 16). When azomethine imines 2q–r bearing stronger
electron withdrawing substituents (-NO2, -CN) on the phenyl ring
were employed for this transformation, 87% and 90% yields were
achieved. However, we found that the resulting products 4q and 4r
were rapidly converted into 5q and 5r through a b-eliminative N–N
bond cleavage, when the samples went through HPLC analyses
i
using PrOH–hexane as the eluent (Scheme S1, ESI ). In addition,
3
when azomethine imine 2s bearing a basic pyridine ring was
tested, no amino nitrile product was detected (Scheme S1, ESI ).
3
Unfortunately, under our optimized conditions, catalyst 1h was
found to be ineffective for enantiocontrol when 2t bearing alkyl
group (cyclohexyl) was used as substrate. Finally, we were fortunate
to obtain single crystals of compound 4h and 4i, for which the
absolute configurations were unambiguously established to be S
on the basis of the X-ray crystallographic analysis (Fig. 1).12
Fig. 2 B3LYP/6-31G(d,p)-optimized transition state (TS) for 1h catalyzed Strecker
reaction of 2h.
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high level of enantioselectivity, as observed in the investigation.
Conclusions
In summary, we have developed an efficient organocatalytic
enantioselective Strecker reaction between azomethine imines and
TMSCN in the presence of cinchona alkaloid-derived catalyst
bearing multiple hydrogen-bonding donors. The concerted multi-
ple hydrogen-bonding interaction of catalyst with substrates was
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Acknowledgements
We are grateful for financial support of the National Natural
Science Foundation of China (21072145, 21272166), the
Program for New Century Excellent Talents in University
(NCET-12-0743), Scientific Research Foundation for Returned
Scholars, Ministry of Education of China ([2010]1174). This
project was also funded by the Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD).
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3
12 CCDC 902498 (4h). C11H10BrN3O, a block crystal (0.276 6 0.211
6 0.143 mm), T = 293(2)K, l(Mo-Ka) = 0.71073 Å, monoclinic,
space group: P21, a = 5.6282(9) Å, b = 9.0457(14) Å, c = 22.768(4)
Å, V = 1159.2(3) Å3, 6749 total reflections, 2154 unique, Rint
=
0.0442, R1 = 0.0462 (I . 2s), wR2 = 0.1173; CCDC 902501 (4i).
C11H10BrN3O, a block crystal (0.265 6 0.145 6 0.112 mm), T =
293(2)K, l(Mo-Ka) = 0.71073 Å, monoclinic, space group: P1, a =
5.5927(10) Å, b = 9.9073(17) Å, c = 11.448(2) Å, a = 113.882(4)u, b
= 91.874(4)u, c = 99.063(4)u, V = 569.50(18) Å3, 3448 total
reflections, 2757 unique, Rint = 0.0186, R1 = 0.0355 (I . 2s), wR2
= 0.0814.
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