Organocatalytic asymmetric Michael reaction with acylsilane donors

Article abstract of DOI:10.1039/c2ob26950c

We have developed an organocatalytic asymmetric Michael reaction of acylsilane through the selection of acylsilane substrates and organocatalysts, thus creating a rare example of acylsilane α-alkylation with a chiral guanidine catalyst, which afforded pro

Full text of DOI:10.1039/c2ob26950c

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ARTICLE TYPE
Organocatalytic Asymmetric Michael Reaction with Acylsilanes Donors†
Lei Wu a, Guangxun Li a, Qingquan Fu a, Luoting Yu* b, and Zhuo Tang*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
5
We have developed an organocatalytic asymmetric Michael reaction of acylsilane through the selection of
acylsilane substrates and organocatalyst, thus creating a rare example of acylsilanes α-alkylation with
chiral guanidine catalyst, which afforded products in good yields and high stereoselectivity. The
corresponding adducts described here have also been demonstrated to be useful in synthesis of unnatural
amino acids and biologically active compounds.
have described the Palladium-catalyzed allylic alkylation of
acylsilanes with monosubstituted allyl Substrates, which can
10 Introduction
afford high ee and middle dr ratio29. Despite these creative efforts,
the direct α-alkylation of acylsilanes induced by the
35 deprotonation of it’s alpha position by chiral base remains a
considerable challenge. For this reason, acylsilanes with
hydrogen atoms on the alpha position was applied in this study as
nucleophile to obtain chiral α-alkylated product (Fig. 1).
Since their discovery by Brook in 19571-3, acylsilanes have been
the one of the most powerful and efficient compounds that have
the silicon directly attached to the carbonyl group, exhibiting
particular physical and chemical properties4-8, so that they could
15 be easily transformed into many different derivatives in one spot,
such as acid9-12 , ketone13-15, alcohol16, 17, aldehyde11,
,
18, 19
cyanogens20, amide12, 20, 21 and ester 20, 22. Beside of these radical
reaction, a great deal of effort has been devoted to the
development of various kinds of reactions acylsilanes participated,
20 Involving
stereocontrolled
nucleophilic
additions23,
stereocontrolled aldol reactions24, cyclization25, coupling
reaction26, α-halogenations3, Enantioselective reduction27.
40
Fig. 1. Structure of nucleophile acylsilanes
Fig. 2 Chiral guanidines applied in this study.
As we know, Michael reactions are among the most powerful
and efficient methods for carbon-carbon bond formation.
Particularly, the development of organocatalytic asymmetric
45 Michael reactions of carbonyl compounds with nitroalkenes has
garnered great interest in recent years30-34. However, asymmetric
Michael reactions using monoester or acid directly as
prenucleophiles have never reported. The use of pyrazole amide
as special Michael donors to react with nitro olefins, reported by
50 Barbas III and co-workers, and the use of quinine derived urea
catalysts gave the desired product with high dr and ee value35.
However, most of good results for these reactions are limited to
Michael donors substrates with electron-withdrawing aromatic.
Herein, we describe a new organocatalyzed asymmetric Michael
55 reaction by using acylsilane as donor to afford diverse and
structurally complex α-alkyl acylsilanes with high diastereo- and
25
However, due to the slightly higher pKa values (the values
being approximately 16)28 compared with ketones, aldehydes,
and 1,3-dicarbonyl substrates, the α-alkylation of acylsilanes is
more difficult. In addition, more challenges still remain regarding
substrate scope and reaction selectivity, including diastereo- and
30 enantioselectivity. More recently, Xue-Long Hou and co-workers
aNatural Products Research Center, Chengdu Institution of Biology,
Chinese Academy of Science, Chengdu 610041, P. R. China. E-mail:
tangzhuo@cib.ac.cn; Fax: +86-28-85243250; Tel: +86-28-85243250;
b State Key Laboratory of Biotherapy, West China Hospital, Sichuan
University, Chengdu 610041, China
†Electronic supplementary information (ESI) available: Experimental
procedures, spectroscopic and analytical data for all compounds,
Mechanism evidence and the determination of absolute configuration for
Michael adducts. See DOI: 10.1039/XXXXXXXXX
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Organic & Biomolecular Chemistry
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enantioselectivity.
optical yield (Table 1, entry 4-6). Further examinations were
Due to the versatile property of acylsilanes, the use of
acylsilanes as promising Michael donors is meaningful. However,
this poses a distinct and formidable challenge: the slightly lower
pKa values of acylsilanes limited the catalyst selection28. Chiral
guanidines, because of the characteristics of high pKa values and
hydrogen-bonding activation, have been shown to be efficient
catalysts for enantioselective reactions36-39. Recently, Xiaoming
Feng group has described serveral asymmetric reactions which
focused on the sterically hindered amide subunit backbones.
Table 2. Generality of reaction demonstrated with a variety of
nitroolefins electrophilesa
View Article Online
5
10 was catalyzed by bifunctional guanidine catalysts40-44. Intrigued
by these challenges and advantages, we have pursued a chiral
guanidine catalyzed enantioselective Michael reaction, using
acylsilanes and nitro olefins as substrates.
45
Entry
R1
Yield[%]b
4b, 86
4c, 78
4d, 84
4e, 81
4f, 77
Syn/antic
16:1
ee[%]c
91
H
1
2
Results and discussion
15 Table 1. Optimization of the reaction catalystsa
27:1
92
3
80:1
93
H3CO
4
>99:1
52:1
91
OCH3
5
93
OCH3
Cl
6
4g, 85
4h, 75
4i, 61
17:1
90
Entry
1
2
3
4
5
6
7
8
Cat.
3a
3b
3c
3d
3e
3f
3g
3h
3i
Acylsilane
Yield[%]b
47
Syn/antic
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
16:1
ee[%]c
76
81
64
73
73
68
91
76
89
37
62
78
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
7
19:1
93
46
37
35
42
55
43
34
52
39
42
47
81
Cl
Br
8
82:1
91
9
4j, 78
49:1
93
9
Br
10
4k, 77
>99:1
96
10
11
12
13
3j
3k
3l
OCH3
OCH3
11
12
4l, 83
45:1d
44:1
94
93
3g
91
a Unless otherwise noted, the reaction was carried out with 0.10 mmol
20 acylsilane, 0.25 mmol nitroolefin and 20 mol% guanidine catalyst in
toluene (1.5 mL) at 0℃ for 20 h. b Yield of isolated product. c Determined
by chiral HPLC.
CF3
4m, 76
OCH3
OCH3
OCH3
13
4n, 79
>99:1
93
F
14
15
4o, 86
4p, 84
99:1
91
93
We initiated our studies by evaluating template Michael
25 reaction with acylsilane 1a and nitroolefin 2a in the presence of
different kinds of catalysts. As we expected, most of chiral
organobases, such as cinchonine or quinine, couldn’t catalyzed
this reaction (See ESI†, Table S1). However, the reaction
catalyzed by chiral guanidine 3a derived from prolinamide (Fig. 2)
30 proceeded smoothly and afforded the desired product in 40%
yield with excellent diastereoselectivity (99:1 dr.) and moderate
enantioselectivity (76% ee) (Table 1, entry 1). Therefore, more
chiral guanidines have been synthesized and evaluated (3b-3l, Fig.
2).
>99:1d
O
16
17
4q, 75
4r, 85
25:1
12:1
89
90
a Unless otherwise noted, the reaction was carried out with 0.1 mmol
acylsilane, 15mol % 3g and 60 mg 4A molecular sieve in toluene (1.5
mL) at 0℃ for 12 h, and 0.25 mmol nitroolefin dissolved in toluene was
50 added to the mixture partially in 4 time over 8 h (See ESI†, Scheme S4). b
Yield of isolated product; c Determined by chiral HPLC. d Determined by
1H NMR.
35
As a result, catalysts containing electron donating groups on
the phenyl group of the amide domain gave higher
enantioselectivity (Table 1, entry 1-3). However, attempts to
optimize the reaction by then conducting it with other catalysts
bearing electron donating groups at different position of phenyl
40 group failed to provide the desired improvements in chemical and
2
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Table 3. Generality of the reaction with respect to the acylsiliane
Catalyst 3g with diphenylmethanamine backbone gives the best
result with 99:1 dr and 91 % ee (Table 1, entry 7-9). We have
tried to improve the catalytic result through introducing more
derivative a
15 hydrogen bond donors into chiral guanidine catalysts (3k, 3l).
View Article Online
Unfortunately, expected improvement of reaction result can’t be
found (Table 1, entry 11-12). The substitution of trimethylsilyl
group with dimethylphenyl silyl group on acylsilane gaves higher
yield (81%) with slightly lower dr (16:1) and similar ee value
20 (91%) (Table 1, entry 13,). In addition, it is noteworthy that 1a
was more difficult to synthesize comparing with 1b (See ESI†,
Scheme S1). Therefore, we explored the scope of the chiral
guanidine catalyst 3g with dimethylphenyl acylsilane as the
substrate.
Entry
1
R1
Yield[%]b
Syn/antic
>99:1
ee[%]c
92
5a, 81
25
Under the optimized conditions (See ESI†, Table 2), various
nitroolefin substrates were investigated to afford a wide range of
products 4 containing two chiral centers with good diastereomeric
ratios (>10:1 dr) and high ee values (89-96% ee), which was
illustrated in Table 2 . It is interesting to note that for the use of β-
2
5b, 78
24:1
95
3
4
5c, 75
5d, 82
25:1
98:1
92
92
30 aryl nitroolefins, the position and the electronic properties of the
substituents on the aromatic ring appeared to have a very limited
effect on stereoselectivity (Table 2, entry 1-14,).
Regardless of the type of substituents on the aromatic rings, be
them electron-withdrawing (Table 2, entry 7-10, 12 and 14),
35 electron -donating (Table 2, entry 2-6, 11, 13), or the neutral
(Table 2, entry 1) or the substitution pattern (para, meta, or ortho;
Table 2, entry 2-10), the reactions of these nitro olefins with
acylsilane gave the desired product in good yield and selectivity.
The substrates with condensed-ring (Table 3, entry 15) or hetero
40 aromatic groups (Table 2, entry 16) furnished the desired product
with high diastereoselectivity and enatioselectivity. The reaction
also worked well for the alkyl-nitroolefin, affording an 85% yield
of product with a 12:1 syn/anti ratio and a 90% ee value under the
optimized reaction conditions (Table 2, entry 17).
5
6
5e, 87
5f, 85
>99:1d
99:1
90
93
Cl
O
Si
NO2
7
8
5g, 51
5h, 71
>99:1d
>99:1
94
97
45
Further explorations were focused on the generality of the
reaction with regards to variation of the dimethylphenyl
acylsilane substrates under the same optimized condition. It is
noteworthy that the electronic properties and steric effects of the
aromatic ring substituents on acylsilane had no obvious
9
5i, 77
5j, 82
35:1
95
94
50 influences on the reactivities and stereoselectivity of guanidine 3g
catalyzed Michael reaction (Table 3). All the aryl acylsilanes with
electron- withdrawing (Table 3, entry 1, 2, 5, 6), electron -
donating groups (Table 3, entry 3, 4, 9-12) or condensed-ring
(Table 3, entry 7, 8) gave the desired products with high
55 diastereoselectivity and enatioselectivity. However, derivatives of
acylsilane with alkyl groups in place of an aromatic ring was
virtually unreactive under the model reaction conditions (data not
shown), thus suggesting that an aromatic functionality is required
to bring the pKa value into a functional range for these
60 organocatalytic conditions.
10
>99:1
11
12
5k, 76
5l, 81
>99:1d
44:1
93
96
In addition, Michael addition of acylsilanes to nitroolefins is an
efficient synthetic tool for the construction of other functionalized
compounds. For example, γ amino acids could be obtained in two
steps with high yield without changes in diastereo- and
65 enantioselectivity (See ESI†, Scheme S5). On account of the high
efficiency in this organocatalysis potential of the Michael adducts,
the reaction was carried out on a 2 g scale in the presence of 2g
(15 mol% ) with the substrates (1b and 2n) and gave the desired
product 4o in 90% yield and with 99:1 dr and 91% ee. 4o was
5
a The reaction was carried out with 0.1 mmol acylsilane, 15mol % 3g and
60 mg 4A molecular sieve in toluene (1.5 mL) at 0℃ for 12 h, and 0.25
mmol nitroolefin dissolved in toluene was added to the mixture partially
in 4 time over 8 h (See ESI†, Scheme S4). b Yield of isolated product; c
Determined by chiral HPLC. d Determined by 1H NMR.
10
The results suggested that the amide subunit in the guanidine
had a significant impact on the enantioselectivity of the reaction45.
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Organic & Biomolecular Chemistry
Page 4 of 5
successfully converted exclusively into the corresponding 40 example of acylsilanes α-alkylation with chiral guanidine catalyst,
carboxylic acid in good yield under oxidization condition10.
Further reaction of the 6a with SOCl2 in methanol to afforded
ester 7a, which was then reduced in the presence of raney Ni and
H2 to obtain 8a. Cyclization of 8a was carried out in toluene with
which afforded products in good yields and high stereoselectivity,
regardless of the type of substituents on the aromatic rings of
acylsilane or nitro olefin substrates. The described
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5
straightforward process to afford the corresponding adducts under
Et3N to furnish target 9a in 51% total yield for 4 steps of 45 mild reaction conditions has also been demonstrated to be useful
reactions without any loss of stereoselectivity (Scheme 1). The
final chiral product, 2-pyrolidinone 9a, possesses great potential
in pharmaceutical application46 which is active on the central
10 nervous system, in particular it has valuable anxiolytic and
antidepressive properties.
in synthesis of unnatural amino acids and biologically active
compounds. This class of novel acylsilane substrate as a
pronucleophile should facilite the development of a wide range of
asymmetric reactions that can be catalyzed by organic and metal
50 catalysts; some of these reactions have already been realized in
our group and will be reported in the near future.
O
O
H3COOC
SOCl2
H2O2, TBAF
THF, r.t.
Si
HO
Acknowledgements
MeOH, r.t.
NO2
NO2
NO2
F
F
F
We thank Prof. Zhenlei Song of Sichuan University for providing
the acylsilane for pilot experiment and his helpful suggestion.
55 This work was supported by Chinese Academy of Science
(Hundreds of Talents Program), National Sciences Foundation of
China (225013) and Opening Foundation of Zhejiang Provincial
Top Key Discipline.
4o
7a
6a
90% yield, 99:1 d.r., 91% ee
O
Et3N, 100oC
Toluene
H3COOC
Raney Ni
H2, r.t.
HN
NH2
8a
F
F
9a
total 51% yield in 4 steps,
99:1 d.r., 91% ee
Notes and references
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