Organocatalytic, highly enantioselective vinylogous mukaiyama-michael reaction of acyclic dienol silyl ethers

Article abstract of DOI:10.1002/anie.201207058

Now also acyclic: The first catalytic, enantioselective, vinylogous Michael reaction of linear, acyclic dienol silyl ethers was achieved. The reaction, based upon the principle of iminium ion catalysis, delivered 1,7-dioxo compounds in one step with good

Full text of DOI:10.1002/anie.201207058

Angewandte
Chemie
DOI: 10.1002/anie.201207058
Asymmetric Catalysis
Organocatalytic, Highly Enantioselective Vinylogous Mukaiyama–
Michael Reaction of Acyclic Dienol Silyl Ethers**
Vaishali Gupta, Sai Sudhir V., Tanmay Mandal, and Christoph Schneider*
Dedicated to Prof. Herbert Mayr on the occasion of his 65th birthday
Vinylogous carbon–carbon bond forming processes of
extended dienol derivatives, ideally proceeding with high
regio- as well as enantioselectivity in the presence of a chiral
catalyst, are among the most valuable transformations in
synthetic organic chemistry on the basis of their carbon-
backbone-generating character and the establishment of new
stereogenic centers and functional groups. For the prototype
of this class, the vinylogous Mukaiyama–aldol reaction,[1]
a broad range of highly effective and selective methods
exist since the ground-breaking work of Carreira, Evans,
Denmark, Campagne, Kiyooka, Kalesse, and List. In the last
few years, some very effective procedures for the execution of
the vinylogous Mannich reaction have also been developed
that rely either on chiral Lewis acids or on Brønsted acids, and
these deliver the corresponding Mannich bases in very high
optical purity.[2]
tionally be formed in various stereoisomeric forms. Therefore,
most of the previously reported processes are limited to
substrates which a priori have a strong preference for the g-
position in the nucleophile, such as silyloxyfurans,[4] the
corresponding unsaturated butyrolactones[5] and -lactams,[6]
and a,a-dicyanoalkenes.[7] Furthermore, two organocatalytic
procedures were recently disclosed in reactions with nitro-
alkenes, which for the first time were suitable for other
substrates as well. Thus, Melchiorre and co-workers revealed
that b-alkyl-substituted cyclohexenones selectively engaged
b-nitrostyrenes und alkylidene cyanoesters at the g-position
by way of dienamine catalysis and delivered products with
excellent enantioselectivity.[8] More recently, Curti, Casiraghi,
et al. reported thiourea-catalyzed reactions of 3-alkylidene
oxindols with bifunctional cinchona alkaloid-based catalysts,
which gave exceptional regio- and enantioselectivity in
reactions with nitroalkenes.[9] A conceptually different and
diastereoselective access to vinylogous Michael products has
been devised by Johnson and co-workers based upon their
previously developed sequential vinylation-[1.2]-Brook rear-
rangement of silyl glyoxylates.[10]
We report herein the first catalytic, enantioselective,
vinylogous Mukaiyama–Michael reactions of simple acyclic
dienol silyl ethers 1 with a,b-unsaturated aldehydes, furnish-
ing synthetically valuable, chiral 1,7-dioxo compounds as
products. Regio- as well as enantioselectivity in these
reactions are uniformly excellent, and in the case of g-
substituted dienol silyl ethers, products with two new
stereogenic centers are obtained, which are furthermore
formed with good diastereoselectivity. As the catalytic
principle we utilize the iminium ion activation mode first
established by MacMillan, which reversibly converts conju-
gated aldehydes into reactive, chiral a,b-unsaturated iminium
salts.[11]
Vinylogous Michael reactions of dienol derivatives, how-
ever, proceeding both with high regio- and enantioselectivity,
have been reported much less frequently (Scheme 1).[3]
A
major cause for this deficiency may be the existence of two
reactive sites in both reaction partners principally giving rise
to four regioisomeric products altogether, which may addi-
Scheme 1. Previously employed substrates in vinylogous Michael reac-
tions in comparison to dienol silyl ether 1.
In orienting experiments, we had found that silyl dien-
olates of a,b-unsaturated ketones and thioesters proved to be
the most suitable nucleophiles for this reaction. Thus, chiral
imidazolidinone 3a[12] in combination with 2,4-dinitrobenze-
nesulfonic acid catalyzed the vinylogous g-1,4-regioselective
Michael reaction of dienol silyl ethers 1a and 1b with
crotonaldehyde in dichloromethane/water (10:1) and deliv-
ered the corresponding Michael products with 76% and 45%
yield, respectively (Table 1, entries 1 and 2). The enantiose-
lectivity, however, was unsatisfactory (40% ee and 37% ee,
respectively). Likewise, vinylketene-S,O-silylacetals 1c und
1d were converted into products 4c and 4d, respectively, with
only moderate yields and selectivities. As side products the g-
1,2-coupled regioisomers were formed additionally as vinyl-
[*] V. Gupta, Dr. S. Sudhir V., Dr. T. Mandal, Prof. Dr. C. Schneider
Institut fꢀr Organische Chemie, Universitꢁt Leipzig
Johannisallee 29, 04103 Leipzig (Germany)
E-mail: schneider@chemie.uni-leipzig.de
[**] We gratefully acknowledge generous financial support from the
Deutsche Forschungsgemeinschaft (SCHN 441/10-1), the
Deutscher Akademischer Austauschdienst for a predoctoral fel-
lowship (V.G.), the Alexander von Humboldt Foundation for
postdoctoral fellowships (S.S. und T.M.), and Evonik and Chemetall
for chemicals.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207058.
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Table 1: Optimization of the vinylogous Michael reaction.[a]
entry 7).[14] The absolute configuration of product 5a was
established after derivatization (see the Supporting Informa-
tion) by comparison with literature data and is consistent with
the transition-state model put forth for these reactions.
With this optimized procedure in hand, we successfully
performed vinylogous Michael reactions of dienol silyl ether
1g and a broad range of a,b-unsaturated aldehydes 2a–k,
delivering the corresponding vinylogous Michael products 5
as single regioisomers throughout and with typically 70–80%
yield and 99% ee (Table 2). Of particular significance was the
reaction of b-silyl-substituted aldehyde 2j, which delivered
No.
Dienolate 1
R1
Cat.
Product
Yield [%][b]
ee [%][c]
(R2, SiR3)
Table 2: Organocatalytic, vinylogous Michael reactions.[a]
1
1a
(Ph, TBS)
1b
Me
Me
Me
Me
Ph
3a
3a
3a
3a
3b
3b
3b
4a
4b
4c
4d
4e
4 f
76
40
37
54
50
95
98
99
2
45
(tBu, TBS)
3
1c[d]
55[e]
56[e]
40[g]
40[g]
74
No.
Aldehyde 2 (R)
t [h]
Product
Yield [%][b]
ee [%][c]
(StBu, TBS)
1d[d]
(SPh, TBS)
1e
4
1
2
3
4
5
6
7
8
2a (Ph)
48
33
38
36
36
48
36
38
34
28
48
5a
5b
5c
5d
5e
5 f
5g
5h
5i
74
77
76
73
70
72
72
76
72
90
50
99
99
98
99
99
99
99
99
99
>99
71
2b (4-ClC6H4)
2c (4-BrC6H4)
2d (4-NO2C6H4]
2e (4-MeOC6H4)
2 f (2-MeC6H4)
2g (2-NO2C6H4)
2h (3-MeOC6H4)
2i (2-Thienyl)
2j (PhMe2Si)
2k (Me)
5[f]
6[f]
7[h]
(Ph, TMS)
1 f[d]
Ph
(StBu, TMS)
1g[i]
Ph
5a
(Ms, TMS)
9
[a] Standard conditions: 0.50 mmol (2.0 equiv) dienol silyl ether 1,
0.25 mmol (1.0 equiv) aldehyde 2, 20 mol% 3 and 2,4-dinitro-benzene-
sulfonic acid (DNBSA), 1 mL CH2Cl2/H2O (10:1), 08C, 12–36 h. [b] Yield
of chromatographically purified product. [c] Determined by HPLC on
a chiral stationary phase. [d] 1E/1Z=3:1. [e] 10% of the g-1,2-product
were isolated additionally. [f] para-Nitrobenzoic acid (PNBA) as cocata-
lyst in CH2Cl2/H2O (10:1), RT. [g] 35% of the a-1,4-product were isolated
additionally. [h] PNBA as cocatalyst in toluene/H2O (10:1), RT. [i] 1E/
1Z=99:1. TMS=trimethylsilyl, TBS=tert-butyldimethylsilyl, Ms=me-
sityl.
10
5j
5k
11[d]
[a] Standard Conditions: 0.50 mmol (2.0 equiv) dienol silyl ether 1g,
0.25 mmol (1.0 equiv) aldehyde 2, 20 mol% of 3b und para-nitrobenzoic
acid (PNBA) each, 1 mL toluene/H2O (10:1), RT, 24–48 h. [b] Yield of
chromatographically purified product. [c] Determined by HPLC on
a chiral stationary phase (see the Supporting Information). [d] This
reaction was conducted with 20 mol% benzoic acid as cocatalyst in
ethanol. TMS=trimethylsilyl, Ms=mesityl.
ogous Mannich products, which contained the imidazolidi-
none in the product (Table 1, entries 3 and 4).
vinylogous Michael product 5j with 90% yield and > 99% ee
(Table 2, entry 10). In general, aromatic and heteroaromatic
aldehydes proved to be suitable reaction partners in this
reaction. Aliphatic and hence enolizable conjugated alde-
hydes, however, such as crotonaldehyde (2k), furnished
products with diminished yields and enantioselectivities
(Table 2, entry 11).
Furthermore, we wondered whether g-substituted dienol
silyl ethers may be employed in this process, as they would
furnish Michael products with two new stereogenic centers.
At the outset it was unclear whether the exceptional
regioselectivity observed above would be deteriorated
through the additional g-substitution and with which degree
of diastereoselectivity the products would form. To study the
influence of the double-bond geometry, we prepared the g-
methyl-substituted dienol silyl ethers 6a and 6b as stereo-
chemically pure 3Z- and 3E-isomers[15] and subjected them to
reactions with various a,b-unsaturated aldehydes under the
previously optimized conditions.
A breakthrough to a highly enantioselective reaction was
realized by switching to the Jørgensen–Hayashi catalyst 3b.[13]
Using 3b in combination with para-nitrobenzoic acid (PNBA)
as cocatalyst (20 mol% each) converted dienol silyl ether 1e
and cinnamaldehyde (2a) into the desired vinylogous Michael
product 4e with 40% yield and 95% ee along with the
undesired a-1,4-coupled regioisomer, which was formed in
35% yield (Table 1, entry 5). A very similar result was
obtained in the reaction of thioester-based silyl dienolate
1 f, which again gave rise to significant amounts of the a-1,4-
regioisomer, but the desired vinylogous Michael product 4 f
was formed with excellent enantioselectivity (Table 1,
entry 6). Eventually, the formation of the undesired a-1,4-
regioisomer was completely suppressed by changing the
phenyl group into a sterically more demanding mesityl
group, which also shielded the a-position within the dienol
moiety. Thus, the reaction of dienol silyl ether 1g and
cinnamaldehyde (2a) catalyzed by 3b and PNBA (20 mol%
each) delivered the desired vinylogous Michael product 5a as
a single regioisomer with 74% yield and 99% ee (Table 1,
The 3Z-diastereomer 6a reacted with cinnamaldehyde
(2a) to yield a 92:8-diastereomeric mixture of vinylogous
Michael product 7a in 77% combined yield from which pure
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anti-diastereomer anti-7a was separated chromatographically
in 70% yield and with 99% ee (Table 3, entry 1).[16] Despite
the additional steric hindrance at the g-position imposed by
the methyl group, the reaction remained highly regioselective,
and no a-regioisomer was observed whatsoever.
Table 3: Vinylogous Michael reactions of the 3Z-dienol silyl ether 6a.[a]
Scheme 2. Vinylogous Michael reaction of 3E-dienol silyl ether 6b.
In conclusion, we have established the first catalytic,
enantioselective vinylogous Michael reaction of acyclic dienol
silyl ethers, which furnished valuable chiral 1,7-dioxo com-
pounds with good yields and excellent regio- as well as
enantioselectivity. With g-substituted nucleophiles, a second
stereogenic center was formed additionally with typically
good diastereoselectivity. In particular, reactions of the 3Z-
configured dienol silyl ether 6a delivered vinylogous Michael
products anti-7 as practically pure diastereo- and enantiomers
in good yields after chromatographic purification. Studies to
further improve this process and extend its scope are
currently ongoing in our laboratory.
No. Aldehyde 2 (R)
t [h] Prod. Yield [%][b,c] anti/syn ee [%][e]
[d]
1
2
3
4
5
6
7
8
9
10
2a (Ph)
2b (4-ClC6H4)
2c (4-BrC6H4)
2d (4-NO2C6H4) 38
2e (4-OMeC6H4) 38
2 f (2-MeC6H4)
2g (2-NO2C6H4) 40
2h (3-OMeC6H4) 42
2i (2-Thienyl)
2l (3-MeC6H4)
48
40
40
7a
7b
7c
7d
7e
7 f
7g
7h
7i
77 (70)
73 (63)
74 (66)
80 (66)
69 (61)
73 (64)
78 (64)
74 (61)
70 (62)
75 (64)
92:8
99
99
99
96
99
99
97
98
99
99
86:14
89:11
82:18
88:12
88:12
82:18
84:16
89:11
87:13
48
40
48
7j
[a] Standard Conditions: 0.50 mmol (2.0 equiv) dienol silyl ether 6a (1E/
1Z=6:1), 0.25 mmol (1.0 equiv) aldehyde 2, 20 mol% of 3b and PNBA
each, 1 mL toluene/H2O (10:1), RT, 24–48 h [b] Combined yield of both
diastereomers. [c] Yield of chromatographically purified anti-diastereo-
mer in brackets. [d] anti:syn Ratio determined by 1H NMR-spectroscopy.
[e] Determined by HPLC on a chiral stationary phase (see the Supporting
Information). TMS=trimethylsilyl, Ms=mesityl, PNBA=para-nitro-
benzoic acid.
Experimental Section
General procedure (with 5a; (Table 1, entry 7): a,a-Diphenylproli-
nolsilylether (3b, 16.3 mg, 0.05 mmol), para-nitrobenzoic acid
(8.4 mg, 0.05 mmol), and cinnamaldehyde (2a, 31 mL, 0.25 mmol)
were dissolved in toluene/water (10:1, 1 mL). Subsequently dienol
silyl ether 1g (130 mg, 0.50 mmol) was added and the solution was
stirred at room temperature for 48 h. After removal of the solvent, the
reaction mixture was directly purified by flash chromatography over
silica gel (n-hexane/ethyl acetate 4:1), which furnished vinylogous
Michael product 5a (118 mg, 74%), the optical purity of which was
determined by HPLC on a chiral AD-H-phase to be 99% ee (see the
Supporting Information).
Various more a,b-unsaturated aldehydes were success-
fully treated with 6a delivering the desired vinylogous
Michael products anti-7b-j in good yields and excellent
enantioselectivity (Table 3). The diastereoselectivity in favor
of the anti-diastereomer varied between 82:18 and 92:8, and
in every case the major anti-diastereomer was obtained
isomerically pure through chromatography in 60–70% yield
and essentially complete optical purity.
In contrast, reactions of the 3E-stereoisomer 6b pro-
ceeded much more slowly in the solvent mixture toluene/
water and delivered products with only moderate yields. Thus,
the reaction of 6b and cinnamaldehyde (2a) furnished the
vinylogous Michael product 7a in only 45% combined yield
and with 99% ee as 86:14 mixture of diastereomers. To
accelerate the reaction, we switched to ethanol as solvent,
which proved to be a good balance between enhanced
reaction rates and slightly diminished stereoselectivity.[17]
Under these conditions, 7a was formed in 65% yield after
45 h at room temperature, and pure syn-7a was isolated by
chromatography in 50 yield and with 96% ee (Scheme 2).[18]
Apparently, reactions of g-substituted dienol silyl ethers
are for the most part stereoconservative processes in which
the configuration of the nucleophile mainly determines the
relative configuration of the major product diastereomer. We
currently assume that open transition states are involved in
which the diastereoselectivity of the reaction is governed by
minimization of gauche interactions.[19]
Received: August 31, 2012
Published online: November 6, 2012
Keywords: dienol silyl ethers · enantioselectivity ·
.
iminium ion catalysis · Michael reaction · organocatalysis
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