Highly enantioselective conjugate addition of thioglycolate to chalcones catalyzed by lanthanum: Low catalyst loading and remarkable chiral amplification

Article abstract of DOI:10.1002/anie.201000105

(Figure Presented) Ramping it up: The titled reaction has been developed using a chiral N, N′-dioxide-La^III complex as the catalyst. The reaction proceeded with high yield and enantioselectivity, Remarkably, a high asymmetric amplification was observed, as 98% ee was achieved using 1 mol% L/ La(OTf)₃ in which the ee value of L was 2% ee. Tf= trifluoromethanesulfonyl.

Full text of DOI:10.1002/anie.201000105

Communications
DOI: 10.1002/anie.201000105
Asymmetric Synthesis
Highly Enantioselective Conjugate Addition of Thioglycolate to
Chalcones Catalyzed by Lanthanum: Low Catalyst Loading and
Remarkable Chiral Amplification**
Yonghai Hui, Jun Jiang, Wentao Wang, Weiliang Chen, Yunfei Cai, Lili Lin, Xiaohua Liu, and
Xiaoming Feng*
The enantioselective conjugate addition of thiol nucleophiles
to electron-deficient olefins is one of the most valuable
methods for the synthesis of optically active chiral sulfur
compounds, which are widely used in organic and medicinal
chemistry.[1,2] Since the pioneering works of Hiemstra and
Wynberg, and Mukaiyama and co-workers,[3] great endeavors
have been devoted to the enantioselective sulfa-Michael
reaction.[4,5] However, the substrate scope was limited to a,b-
unsaturated imides,[6] aldehydes,[7] and cyclic enones.[3a,8] As
for the simple chalcone derivatives, only few examples were
documented with restricted substrate scope.[9] Additionally
thioglycolate, a good alternative sulfur source which is
relatively cheap and less toxic[10] compared to the other
thiol nucleophiles, has not yet been applied in catalytic
asymmetric sulfa-Michael reactions. Herein we report a
highly efficient catalytic enantioselective conjugate addition
of thioglycolate to chalcones using N,N’-dioxide/La(OTf)3
complexes as the catalyst. Excellent yields (up to 99%) and
enantioselectivities (up to 99% ee) were achieved for a wide
range of chalcones using a 1 mol% catalyst loading. A
remarkably high asymmetric amplification was observed for
the reaction, which makes it possible to obtain excellent
enantioselectivity of the reaction using a very low enantio-
meric excess of the ligand (only 2% ee). The reaction could be
additionally performed using a 0.1 mol% to 0.01 mol%
catalyst loading, wherein the enantioselectivity of the reaction
was maintained.
in CH2Cl2 at 258C (Table 1). Different metal complexes
promoted the reaction smoothly in 1 hour, but the ee values
were very low (less than 15% ee, Table 1, entries 1–5).[13]
A
comparatively better metal was La(OTf)3, which gave the
Table 1: Enantioselective conjugate addition of thioglycolate (2a,b) to
chalcone 1a catalyzed by N,N’-dioxide–metal complexes.[a]
Entry
Ligand
Metal
Solvent
Yield [%][b]
ee [%][c]
N,N’-dioxides, which have a sterically and electronically
tunable chiral environment, have been applied to many
asymmetric syntheses both in our group and the groups of
others.[11,12] We first examined N,N’-dioxide L1 which was
complexed in situ with various rare-metal salts to catalyze the
reaction between chalcone 1a and methyl thioglycolate (2a)
1
2
3
4
5
6
7
8
L1
L1
L1
L1
L1
L2
L3
L4
L5
L6
L7
L5
L5
L5
L5
L5
L5
Yb(OTf)3
In(OTf)3
Sc(OTf)3
Y(OTf)3
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
CHCl2CHCl2
CH2ClCH2Cl
CH2ClCH2Cl
CH2ClCH2Cl
CH2ClCH2Cl
88
58
73
90
96
69
56
94
95
89
95
90
89
96
98
99
96
4
0
12
10
14
41
13
87
92
91
75
93
94
95
La(OTf)3
La(OTf)3
La(OTf)3
La(OTf)3
La(OTf)3
La(OTf)3
La(OTf)3
La(OTf)3
La(OTf)3
La(OTf)3
La(OTf)3
La(OTf)3
La(OTf)3
9
[*] Y. H. Hui, J. Jiang, W. T. Wang, W. L. Chen, Y. F. Cai, Dr. L. L. Lin,
Dr. X. H. Liu, Prof. Dr. X. M. Feng
10
11
12
13
14
15
16
17
Key Laboratory of Green Chemistry & Technology
Ministry of Education, College of Chemistry, Sichuan University
Chengdu 610064 (China)
Fax: (+86)28-8541-8249
E-mail: xmfeng@scu.edu.cn
97[d]
97[d,e]
94[d,e,f]
[**] We acknowledge the National Natural Science Foundation of China
(No. 20732003), PCSIRT (No. IRT0846), and National Basic
Research Program of China (973 Program)(No. 2010CB833300) for
financial support, and Sichuan University Analytical & Testing
Centre for NMR analyses.
[a] Unless otherwise noted, reactions were carried out with 0.1 mmol
chalcone 1a and 1.5 equiv methyl thioglycolate (2a) in solvent (0.5 mL)
at 258C for 1 h. [b] Yield of isolated product. [c] Determined by HPLC
analysis. [d] Reaction at 08C. [e] Used 1 mol% catalyst. [f] Used 1.5 equiv
ethyl thioglycolate (2b). Tf=trifluoromethanesulfonyl.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000105.
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Table 2: Substrate scope for catalytic asymmetric conjugate addition of
methyl thioglycolate (2a) to chalcones 1.[a]
product in 96% yield and 14% ee (Table 1, entry 5). To
improve the enantioselectivity, other chiral N,N’-dioxide
ligands were surveyed (Table 1, entries 6–11). Fortunately,
when the ligand L5, having a bulkier isopropyl group at the
ortho position of aniline, was used a 92% ee was achieved
(Table 1, entry 9). As for the chiral backbone moiety of
ligand, l-proline-derived N,N’-dioxide L5 was superior to
both L6 (derived from l-pipecolic acid) and L7 (derived from
l-ramipril acid; Table 1, entry 9 versus entries 10 and 11).
The effects of solvent, reaction temperature, and catalyst
loading were also investigated. In changing the solvent from
CH2Cl2 to CH2ClCH2Cl, the reaction gave the desired
product in high yield and enantioselectivity (Table 1,
entries 11–14). Up to 97% ee and a 98% yield was attained
when the reaction was run at 08C for 1 hour (Table 1,
entry 15). Decreasing the catalyst loading from 10 mol% to
1 mol% did not result in loss in either the yield or
enantioselectivity (Table 1, entry 16). Furthermore, this pro-
cess could tolerate air and moisture. Additionally, compound
2b was also a suitable thiol reagent, as a high yield and
enantioselectivity of 3b could be achieved under the opti-
mized reaction conditions (Table 1, entry 17). The absolute
configuration of 3a was determined by X-ray crystallography
to be R (Figure 1).[14]
Entry
1
R2, R3
t [h] Prod.
Yield [%][b] ee [%][c]
1
2
3
4
5
6
7
8
1a
1c
1d
1e
1 f
1g
1h
1i
Ph, Ph
1
2
2
1
1
1
1
1
1
1
1
1
1
1
1
2
1
2
1
1
1
1
1
1
1
4
4
(R)-3a 99 97(R)
Ph, 4-MeC6H4
Ph, 2-MeOC6H4
Ph, 3-MeOC6H4
Ph, 4-MeOC6H4
Ph, 2-ClC6H4
Ph, 3-ClC6H4
Ph, 4-ClC6H4
Ph, 4-FC6H4
3c
3d
3e
3 f
99
99
99
95
99
99
99
94
92
99
95
99
99
97
99
99
96
97
99
99
95
97
99
96
85
86
97
98
97
97
97
97
97
97
96
99
96
99
95
80[d]
97
96
96
96
98
98
98
96
96
95
90[d]
72[e]
3g
3h
3i
3j
3k
3l
9
1j
1k
1l
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Ph, 4-BrC6H4
Ph, 3-NO2C6H4
1m Ph, 4-NO2C6H4
3m
3n
3o
3p
3q
3r
1n
1o
1p
1q
1r
Ph, 4-PhC6H4
Ph, 2-naphthyl
Ph, CH3
Ph, t-butyl
4-MeC6H4, Ph
4-MeOC6H4, Ph
2-MeOC6H4, Ph
4-FC6H4, Ph
4-ClC6H4, Ph
4-BrC6H4, Ph
4-NO2C6H4, Ph
2-naphthyl, Ph
2-furyl, Ph
1s
1t
3s
3t
1u
1v
1w
1x
1y
1z
3u
3v
3w
3x
3y
3z
3aa
3ab
1aa 2-furyl, 2-furyl
1ab CH3, Ph
[a] Unless otherwise noted, reactions were carried out with 1 mol% L5/
La(OTf)3 (1:1), 0.2 mmol chalcones 1, and 1.5 equiv methyl thioglycolate
(2a) in ClCH2CH2Cl (0.5 mL) at 08C for 1–4 h. [b] Yield of isolated
product. [c] Determined by HPLC analysis, and the absolute configu-
ration of 3a was determined by X-ray crystallographic analysis. [d] Used
10 mol% catalyst. [e] Used 2 mol% catalyst.
reaction (Table 2, entry 16 versus 15). In addition, the current
catalyst system was also efficient for the heteroaromatic
substrates, which provided the corresponding adducts in up to
95% ee and 96% yield (Table 2, entries 25 and 26). When
benzalacetone was employed, moderate results were also
obtained (Table 2, entry 27). Notably, the conjugate addition
of other thiols (such as tBuSH, EtSH, PhSH, and PhCH2SH)
to chalcone 1a was also tested under the optimized reaction
conditions, but only trace products could be detected. These
experimental phenomena indicated 2a might be activated by
coordination to the catalyst at the carbonyl group.[16] Con-
sequently, the rigid enones 4a and 4b were subjected to the
reaction (Scheme 1), and excellent results were maintained
with up to 95% ee and 93:7 d.r. To the best of our knowledge,
most of them represented the best results for sulfa-Michael
reaction using chalcone derivatives as the acceptor to date.
The relationship between the enantiomeric excess of the
ligand L5 and the product 3a was investigated. A remarkably
strong positive nonlinear effect[17] was observed for the sulfa-
Michael reaction (Figure 2). The reaction could even achieve
excellent enantioselectivity (94% ee) using L5 having only a
Figure 1. X-ray crystallographic structure of (R)-3a. Thermal ellipsoids
drawn at 30% probability.
Under the optimized reaction conditions (Table 1,
entry 16), the asymmetric conjugate addition of methyl
thioglycolate to various chalcones[15] were examined to
provide the corresponding chiral sulfur compounds 3 in
excellent yields and enantioselectivities (Table 2). Neither the
electronic nature nor the steric hindrance of the substitution
at the aromatic ring (R2 or R3) had any obvious influence
upon the enantioselectivity and reactivity (Table 2, entries 1–
13 and 17–23). The naphthyl-substituted derivatives 1o and
1y proceeded well with 2a, giving the products 3o and 3y,
respectively, with excellent enantioselectivity (Table 2,
entries 14 and 24). The substrate with the bulkier aliphatic
R3 substituent (such as tert-butyl) was more efficient for the
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cone derivates with thioglycolate catalyzed by a
chiral L5/La(OTf)3 complex. This method rep-
resents a rare example of the highly efficient
asymmetric synthesis of the optically active
chiral sulfur compounds in the presence of
1 mol% of [LaIII(L5)] (up to 99% ee). Addi-
tionally, excellent enantioselectivity (93% ee)
was maintained when using a 0.01 mol% cata-
lyst loading (on 4.165 g scale), which highlighted
the potential value of the catalyst system.
Scheme 1. Asymmetric sulfa-Michael addition to rigid enones 4a and 4b using methyl
thioglycolate (2a).
Table 3: Catalytic asymmetric conjugate addition of methyl thioglycolate
(2a) to chalcones 1 using 2% ee catalyst L5/La(OTf)3.[a]
Entry
1
R2, R3
Prod.
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
1a
1a
1 f
1i
Ph, Ph
Ph, Ph
(R)-3a
(S)-3a
3 f
3i
3l
3o
3r
3u
3v
95
93
83
99
88
99
89
99
99
92
94(R)
95(S)[d]
90
94
98
93
84
93
94
Ph, 4-MeOC6H4
Ph, 4-ClC6H4
Ph, 3-NO2C6H4
Ph, 2-naphthyl
4-MeC6H4, Ph
4-FC6H4, Ph
4-ClC6H4, Ph
2-furyl, Ph
1l
1o
1r
1u
1v
1y
10
3z
85
[a] Unless otherwise noted, reactions were carried out with 1 mol% non-
enantiopure L5 (2% ee)/La(OTf)3 (1:1), 0.2 mmol chalcones 1, and
1.5 equiv methyl thioglycolate 2a in ClCH2CH2Cl (0.5 mL) at 08C for 2 h.
[b] Yield of isolated product. [c] Determined by HPLC analysis and the
absolute configuration was determined by X-ray crystallographic analy-
sis. [d] Used 1 mol% non-enantiopure L5 (À2% ee)/La(OTf)3 (1:1).
Figure 2. Chiral amplification in the sulfa-Michael Reaction of 1a with
2a catalyzed by L5/La(OTf)3.
2% ee. The product (S)-3a was also obtained in 95% ee by
using a catalyst derived from a ligand at very low enantio-
meric excess (À2% ee). Such a phenomenon was universally
presented in various chalcones (Table 3). The remarkable
asymmetric amplification makes it possible to achieve high
enantioselectivity (up to 98% ee) in the reaction by using L5
having a low ee value. The remarkably strong positive non-
linear effect implied that the reaction occurred in the
presence of polymeric La species. The heterochiral La
complex was easily formed and showed extremely low or
even no catalytic activity for the reaction. Though the exact
catalytically active catalyst is unclear, it is believed that the
catalytically active intermediates had provided an excellent
chiral environment for the reaction.
Meanwhile, a strong positive nonlinear effect was observed
in this catalyst system, and the substrate scope was addition-
ally expanded to chalcones using a 2% ee catalyst at a
1 mol% catalyst loading. Additional investigations into the
mechanism of the asymmetric induction[19] and the extension
of the methodology to other types of additions are ongoing.
Experimental Section
General experimental procedure: The mixture of ligand L5 (1.2 mg,
0.002 mmol), La(OTf)3 (1.2 mg, 0.002 mmol), and chalcone 1a
(41.6 mg, 0.2 mmol) in CH2ClCH2Cl (0.5 mL) was stirred at 358C
for 0.5 h, cooled to 08C, and then methyl thiolglycolate 2a (0.3 mmol)
was added. After the reaction was completed as determined by TLC,
In light of the remarkable asymmetric amplification, we
additionally tested the application of the
current catalyst system to prepare (R)-3a on
large scale using low catalytic loading
(Scheme 2). By using only 0.1 mol% of L5/
La(OTf)3, product (R)-3a was obtained in
excellent yield (98%) and 94% ee. Notably,
the catalyst loading was additionally lowered
to 0.01 mol%, and the excellent enantioselec-
tivity (93% ee) was maintained.[18]
In summary, we have developed an effi-
cient asymmetric conjugate addition of chal- Scheme 2. Scaled-up reaction under low catalyst loading.
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the mixture was purified by flash chromatography on silica gel
(petroleum ether/ethyl acetate 6:1) to afford the desired product 3a in
99% yield with 97% ee.
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yield using L5–Y(OTf)3.
[14] CCDC 759697 (3a) contains the supplementary crystallographic
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were afforded with poor ee values (see the Supporting Informa-
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reaction using ESI-MS. The sample was prepared from thiogly-
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[19] A proposed working model is described in the Supporting
Information.
Angew. Chem. Int. Ed. 2010, 49, 4290 –4293
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 4293