Highly enantioselective lewis base organocatalyzed hydrosilylation of γ-imino esters

Article abstract of DOI:10.1002/ejoc.201101268

Highly enantioselective hydrosilylation of γ-imino esters with trichlorosilane promoted by a chiral Lewis base proceeded smoothly to provide various optically active γ-amino esters in good yields (up to 96 %) with excellent enantioselectivities (up to 99%

Full text of DOI:10.1002/ejoc.201101268

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201101268
Highly Enantioselective Lewis Base Organocatalyzed Hydrosilylation of
γ-Imino Esters
Zhou-Yang Xue,[a,b] Li-Xin Liu,[a,b] Yan Jiang,[a,b] Wei-Cheng Yuan,[a] and
Xiao-Mei Zhang*[a]
Keywords: Amino acids / Enantioselectivity / Hydrosilylation / Lewis bases / Organocatalysis
Highly enantioselective hydrosilylation of γ-imino esters with
trichlorosilane promoted by a chiral Lewis base proceeded
smoothly to provide various optically active γ-amino esters in
good yields (up to 96%) with excellent enantioselectivities
(up to 99%ee) at –10 °C in Cl2CHCHCl2. The side reactions
were successfully reduced by rational modification of the
substrate. The absolute configuration of one of the γ-amino
esters was determined. Finally, two pharmaceutically inter-
esting enantioenriched γ-lactams were synthesized from two
γ-amino esters.
Introduction
Optically active γ-amino acids, γ-lactams, δ-amino
alcohols, and α-substituted pyrrolidines are very important
compounds because they are not only building blocks of γ-
peptides[1,2] but also key intermediates of some pharmaceu-
tical substances,[3–5] including compounds used in the treat-
ment of inflammatory disorders,[3g] cannabinoid receptor 1
activity inhibitors,[3d] and protein kinase B inhibitors.[4b]
Interestingly, all of these compounds mentioned above
can be readily prepared from γ-amino esters which also ex-
ist in some natural products (Scheme 1).[6] Therefore, asym-
metric synthesis of optically active γ-amino esters is of great
importance. However, there are very few reports on the syn-
thesis of chiral γ-amino esters.[7] Although some progress
has been achieved in the chiral transition metal complex
catalyzed hydrogenation of the corresponding ketimines,[7b]
the organocatalyzed asymmetric synthesis of γ-amino esters
is still a great challenge.
Recently, asymmetric reactions involving the Lewis base
activation of Lewis acids has attracted much attention.[8]
Among these reactions, the chiral Lewis base catalyzed
asymmetric hydrosilylation of C=N bonds is an important
approach used to prepare enantioenriched nitrogen-con-
taining compounds because of the mildness, economic ad-
vantage, and environmental benignancy of this transforma-
tion.[9] Several groups have achieved impressive progress in
Scheme 1. Compounds that could be derived from γ-amino esters.
this field.[10] Malkov et al. demonstrated the first chiral
Lewis base organocatalyzed enantioselective hydrosilylation
of γ-imino esters.[10d] Although the reaction provided the γ-
amino esters with high ee values, it suffered from poor
yields owing to the generation of byproducts arising from
ring closure of the desired γ-amino esters.
As an ongoing study on chiral Lewis base catalyzed
asymmetric hydrosilylation of C=N bonds,[10m–10q] herein
we would like to describe the chiral Lewis base organocata-
lyzed enantioselective hydrosilylation of γ-imino esters in
which the desired optically active γ-amino esters were pro-
duced not only with excellent enantioselectivities but also
in almost quantitative yield.
[a] Key Laboratory for Asymmetric Synthesis and
Chiral-Technology of Sichuan Province,
Chengdu Institute of Organic Chemistry,
Chinese Academy of Sciences,
Chengdu 610041, China
Fax: +86-28-85257883
Results and Discussion
E-mail: xmzhang@cioc.ac.cn
[b] Graduate School of Chinese Academy of Sciences,
Beijing 100049, China
First, chiral picolinamide catalysts 1a–g (Figure 1) were
evaluated for their ability to promote the hydrosilylation of
methyl 4-(4-methoxyphenylimino)-4-phenylbutanoate (2a)
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101268.
Eur. J. Org. Chem. 2012, 251–255
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
251

Z.-Y. Xue, L.-X. Liu, Y. Jiang, W.-C. Yuan, X.-M. Zhang
SHORT COMMUNICATION
in dichloromethane at –10 °C. The results are summarized catalyst 1f both exhibited excellent enantioselectivity of
in Table 1.
95% (Table 1, Entries 3 and 6). Replacement of the two
phenyl groups in 1c with more sterically hindered p-tolyl
groups resulted in a slight increase in the ee value (97%ee;
Table 1, Entry 7). Thus, 1g was determined as the most ap-
propriate catalyst in this study.
It is worth noting that the reactions always provided
poor yields of the desired product while accompanied by
some side products. Cyclized γ-lactam 4 was the main side
product, which was mixed with some unidentified impuri-
ties and could not be isolated in pure form. To our surprise,
besides expected γ-lactam 4, about 6% of unexpected α,β-
unsaturated ketimine 5 was also detected (Figure 2). We
reasoned that oxygen might cause the oxidation of substrate
2a to generate 5. Therefore, we tried to conduct the reaction
under an argon atmosphere. However, compound 5 still
arose (Table 1, Entry 8). It suggested that a small amount of
2a underwent self-dehydrogenation in the reaction system.
Then, we tried to perform the reaction in a more dilute
solution. This attempt only led to a small increase in the
yield of 3a (Table 1, Entries 9 and 10). The problem of low
yield still remained unresolved.
Figure 1. Chiral Lewis base organocatalysts evaluated in this study.
Table 1. Screening of chiral Lewis base catalysts in the enantioselec-
tive hydrosilylation of γ-imino ester 2a.[a]
Figure 2. Main side products of the asymmetric hydrosilylation of
2a.
Entry
Cat*
Conversion[b] [%] Yield [%][c]
ee [%][d]
1
2
3
4
5
6
7
1a
1b
1c
1d
1e
1f
1g
1g
1g
1g
100
100
100
100
100
100
100
100
100
100
67
62
56
51
53
64
59
60
65
67
46
78
95
92
90
95
97
95
96
94
To restrict side reactions, substrates 2b–d bearing bulkier
ester groups were subjected to the reaction. We were very
gratified to find that this strategy worked much more ef-
ficiently. The yields increased as the size of the ester in-
creased (Table 2, Entries 2–4). When tert-butyl substrate 2d
was subjected to the reaction, the yield of corresponding γ-
amino ester 3d reached 85% and almost no γ-lactam 4 and
dehydrogenated product were produced (Table 2, Entry 4).
It could be easily understood why no cyclized γ-lactam 4
was detected in the reaction of 2d, although it was some-
what puzzling that no dehydrogenated product was ob-
served. This was a promising result, although the enantio-
selectivity did drop significantly (Table 2, Entry 4). Accord-
ingly, the tert-butyl group was determined as the most fa-
vorable ester group in the substrates and 2d was employed
8[e]
9[f]
10[g]
[a] Unless specified otherwise, the reactions were carried out with
the catalyst (10 mol-%) and HSiCl3 (2.0 equiv.) on a 0.5-mmol scale
in dichloromethane (2.2 mL) at –10 °C for 18 h. [b] Monitored by
TLC. [c] Isolated yield. [d] The ee values were determined by chiral
HPLC (Chiralcel OD). [e] The reaction was carried out under an
argon atmosphere. [f] The reaction was conducted on a 0.2-mmol
scale in 2.2 mL of dichloromethane. [g] The reaction was conducted
on a 0.1-mmol scale in 2.2 mL of dichloromethane.
In the presence of 10 mol-% of these catalysts, although in the following investigations. Subsequently, several chlor-
all of the reactions gave full conversions within 18 h, only inated solvents were examined. All of these solvents pro-
moderate yields of desired product 3a were obtained vided the product in high yields (Table 2, Entries 4–9). To
(Table 1). Ephedrine-derived catalyst 1a[10o,10q] delivered our delight, we found that the reaction performed in
poor enantioselectivity (Table 1, Entry 1). Proline-derived 1,1,2,2-tetrachloroethane proceeded smoothly to give γ-
catalyst 1b[10i,10n] displayed much higher but still unsatisfac- amino ester 3d in high yield with an excellent ee value
tory enantioselection (Table 1, Entry 2). Afterwards, cata- (Table 2, Entry 7). In addition, the reaction was complete
lysts 1c–f[10p] bearing bulky substituents at the C4 position within only 12 h in this solvent. Thus, 1,1,2,2-tetrachloro-
of the pyrrolidine ring were tested. To our delight, remark- ethane was selected as the optimal solvent. Further varia-
able increases in the ee values were observed (Table 1, En- tion of the reaction temperature caused little change in the
tries 3–6), in which O-pivoloyl catalyst 1c and O-benzoyl yield and enantioselection (Table 2, Entries 8 and 9).
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Eur. J. Org. Chem. 2012, 251–255

Enantioselective Lewis Base Organocatalyzed Hydrosilylation
Table 2. Enantioselective hydrosilylation of γ-imino esters 2a–d.[a]
Table 3. Enantioselective hydrosilylation of γ-imino esters 2d–p
promoted by 1g.[a]
Entry
R
Solvent
T
[°C]
t
[h]
Yield
[%][b]
ee
[%][c]
1
2
3
4
5
6
7
8
9
Me
Et
iPr
tBu
tBu
tBu
tBu
tBu
tBu
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
ClCH2CH2Cl –10
Cl3CCH3 –10
Cl2CHCHCl2 –10
Cl2CHCHCl2
Cl2CHCHCl2 –20
–10
–10
–10
–10
18
18
18
18
18
16
12
11
18
65
70
77
85
95
93
94
92
94
96
97
96
90
94
92
96
94
95
0
[a] Unless specified otherwise, the reactions were carried out with
catalyst 1g (10 mol-%) and HSiCl3 (2.0 equiv.) on a 0.2-mmol scale
in the corresponding solvent (2.2 mL). [b] Isolated yield. [c] The ee
values were determined by chiral HPLC.
[a] Unless specified otherwise, the reactions were carried out with
catalyst 1g (10 mol-%) and HSiCl3 (2.0 equiv.) on a 0.2-mmol scale
in 1,1,2,2-tetrachloroethane (2.2 mL) at –10 °C for 12 h. [b] Iso-
lated yield. [c] The ee values were determined by chiral HPLC.
[d] The absolute configuration of product 3l was determined by X-
ray crystallographic analysis.[11] [e] The reaction was conducted on
a gram-scale.
With the optimized conditions in hand, we examined the
scope of the reaction with a variety of γ-imino esters in the
presence of 10 mol-% of catalyst 1g in 1,1,2,2-tetrachloro-
ethane at –10 °C. The results are summarized in Table 3.
Not only almost quantitative yields but also excellent ee
values were observed with all of these γ-aryl-N-aryl-γ-imino
esters, including γ-heterocyclic aryl substrate 2o (Table 3,
Entries 1–14). The reaction system exhibited good tolerance
for para or meta substituents at the γ-phenyl group. The
electronic nature of the substituents had little effect on the
results (Table 3, Entries 2–12). The highest enantio-
selectivity of 99%ee was obtained with γ-imino ester 2n
(Table 3, Entry 12). We also tried to extend the reaction to
γ-ortho-substituted phenyl-γ-imino esters and γ-alkyl-γ-
imino esters. However, we failed to prepare these two kinds
of substrates.
Moreover, a gram-scale reaction was performed with γ-
imino ester 2l, and the reaction reached full completion in
12 h without a decrease in the yield or enantioselectivity
(Table 3, Entry 10). Consequently the absolute configura-
tion of γ-amino ester 3l was determined to be (S) by X-ray
crystallographic analysis.[11]
Scheme 2. Synthesis of chiral γ-lactam 6.
In order to further demonstrate the synthetic utility of
this transformation and the possibility of employing it in
diversity-oriented synthesis, we treated product 3p with po-
Furthermore, γ-amino ester 3m was subjected to the
tassium tert-butoxide in dry THF to generate γ-lactam 6 same cyclization with potassium tert-butoxide followed by
(Scheme 2), which can be employed to treat or prevent dis- deprotection of PMP with CAN (CAN = ceric ammonium
eases or disorders associated with the activity of canna- nitrate) to provide γ-lactam 7, which can be converted into
binoid receptor 1 (CB1).[3d] To date, the only way to prepare potent candidate 8 for the treatment of inflammatory disor-
optically active 6 is to separate the racemate by chiral ders[3g] or into pyrrolidine 9, which is a hydroxamate-based
HPLC.[3d] Now this compound can be synthesized in high inhibitor of deacetylases B,[5g] according to the literature
enantiomeric purity through easy procedures.
(Scheme 3).
Eur. J. Org. Chem. 2012, 251–255
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253

Z.-Y. Xue, L.-X. Liu, Y. Jiang, W.-C. Yuan, X.-M. Zhang
SHORT COMMUNICATION
Acknowledgments
We are grateful for financial support from the National Sciences
Foundation of China (20972155) and National Basic Research Pro-
gram of China (973 Program) (2010CB833300).
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Scheme 3. Synthesis of chiral γ-lactam 7 and its synthetic potential
in the construction of pharmaceutically active agents.
Conclusions
In conclusion, we have developed a general, highly enan-
tioselective hydrosilylation of γ-imino esters promoted by
chiral Lewis base organocatalysts. This transformation en-
ables the straightforward, mild, and highly effective synthe-
sis of various chiral γ-amino esters in high yield (96%) with
excellent enantioselectivities (99%). The problem of low
yield was successfully resolved by rational modification of
the substrate. The absolute configuration of γ-amino ester
3l was determined to be (S) by X-ray crystallographic
analysis. Finally, γ-amino esters 3p and 3m were employed
in the synthesis of two optically active γ-lactams that are
important in the construction of pharmaceutically active
agents.
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Experimental Section
General Procedure for the Asymmetric Hydrosilylation of γ-Imino
Esters: A solution of trichlorosilane (41 μL, 0.4 mmol, 2.0 equiv.)
in dry Cl2CHCHCl2 (0.2 mL) was added to a stirred solution of
the corresponding γ-imino ester (0.2 mmol) and the catalyst
(0.02 mmol) in dry Cl2CHCHCl2 (2 mL) at –10 °C. The mixture
was stirred at –10 °C for 12 h. Then, the reaction was quenched
with a saturated aqueous solution of NaHCO3 and extracted with
EtOAc. The combined organic layer was washed with brine and
dried with anhydrous Na2SO4; the solvents were evaporated. Purifi-
cation by column chromatography (silica gel, petroleum ether/
EtOAc) afforded the pure product. The ee values were determined
by using established HPLC techniques with chiral stationary
phases.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures and spectral and analytical data for
γ-imino esters and γ-amino esters, HPLC chromatograms for γ-
amino esters, X-ray crystal structure of 3l·HCl.
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Received: August 31, 2011
Published Online: November 29, 2011
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