Enantioselective sulfa-Michael addition of thioacids to α,β-unsaturated ketones with bifunctional organocatalyst

Article abstract of DOI:10.1016/j.tetlet.2012.02.052

Organocatalytic conjugate addition of thioacids to α,β- unsaturated ketones has been studied in the presence of cinchona alkaloid derived urea catalyst. Both the enantiomers of products are accessible with the same level of enantioselectivity using pseudo

Full text of DOI:10.1016/j.tetlet.2012.02.052

Tetrahedron Letters 53 (2012) 2121–2124
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Tetrahedron Letters
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Enantioselective sulfa-Michael addition of thioacids to
ketones with bifunctional organocatalyst
a,b-unsaturated
Nirmal K. Rana a, Rajshekhar Unhale b, Vinod K. Singh a,b,
⇑
a Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India
b Indian Institute of Science Education and Research Bhopal, Bhopal 460 023, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Organocatalytic conjugate addition of thioacids to a,b-unsaturated ketones has been studied in the pres-
Received 28 December 2011
Revised 7 February 2012
Accepted 10 February 2012
Available online 17 February 2012
ence of cinchona alkaloid derived urea catalyst. Both the enantiomers of products are accessible with the
same level of enantioselectivity using pseudoenantiomeric quinine/quinidine derived catalysts. The cata-
lytic process provides optically active thioesters with high chemical yields (up to 99%) and useful enanti-
oselectivity (up to 83% ee). The reaction was performed with 1 mol % of catalyst in toluene at room
temperature. A transition state model has been proposed to explain the stereochemical outcome of the
reaction.
Keywords:
Organocatalyst
Thiourea
Ó 2012 Published by Elsevier Ltd.
Sulfa-Michael
Thioacetic acid
a,b-Unsaturated ketones
Sulfur containing chiral frameworks are common in valuable
thioacids to a,b-unsaturated carbonyls, which can show substrate
biologically active natural products and pharmaceutical agents.1
The asymmetric reaction of sulfur-centered nucleophiles with elec-
tron-deficient olefins, viz. sulfa-Michael addition (SMA), offers a
convenient and practical method for the preparation of enantiome-
rically enriched sulfur containing molecules.2 In recent years, a
great deal of efforts have been directed toward the development
of enantioselective sulfa-Michael addition. Asymmetric proton-
generality and high enantioselectivity. In recent years, the asym-
metric Michael addition reactions promoted by chiral bifunctional
thiourea derivatives derived from cinchona alkaloids have been rec-
ognized as an effective method for asymmetric carbon–carbon and
carbon–hetero bond formation.10 We have recently reported an effi-
Table 1
Effect of catalyst loading and temperature on enantioselective sulfa-Michael addition
ation in sulfa-Michael addition to a-substituted acrylates enabled
of thiobenzoic acid to 2-cyclohexenonea
access to stereogenic center in the addition/protonation product b
to sulfur atom.3 Tandem sulfa-Michael/aldol and sulfa-Michael/
Michael have also been investigated.4 Unfortunately, enantioselective
sulfa-Michael addition promoted by both chiral metal-complexes
and organocatalysts have been limited to the use of aromatic5 and
aliphatic thiols.6 The optically active thioethers, obtained by the
asymmetric 1,4-addition of aromatic thiols to enones, could not
be easily cleaved selectively or transformed into versatile thiol
groups. After the pioneer report of Gawronski et al.,7a enantioselec-
O
O
O
O
1a,
toluene
+
Ph
SH
S
Ph
3a
4a
2a
Entry
1a (mol %)
Temp (°C)
Time (h)
Yield (%)
eeb (%)
1
2
10
5
2
rt
rt
rt
rt
rt
rt
10
0
ꢀ15
ꢀ30
ꢀ60
2
5
99
99
99
99
99
90
99
97
94
90
81
29
56
59
64
64
49
60
60
59
49
44
tive Michael addition of thioacids to
a,b-unsaturated ketones has
3
10
18
36
48
24
24
36
48
48
4c
5
1
become a useful method for the preparation of optically active
keto-thioesters,7 which could easily be converted into versatile
thiol groups under mild reaction conditions.8 Organocatalytic
asymmetric conjugate addition of thioacetic acid to b-nitroalkenes
has also been reported.9 Most of the reported methods have limited
substrate scope and low stereoselectivity. Thus, there is still a need
to develop catalyst systems for the Michael addition reaction of
0.5
0.1
1
1
1
6
7
8
9
10
11
1
1
a
Reactions were carried out with 0.2 mmol of 2a and 0.24 mmol of 3a in 1 mL of
toluene, unless noted otherwise.
b
⇑
Corresponding author. Fax: +91 512 2597436.
E-mail address: vinodks@iitk.ac.in (V.K. Singh).
Determined by HPLC using Chiralcel OD-H column.
c
Absolute stereochemistry of the product was determined to be (S).12
0040-4039/$ - see front matter Ó 2012 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2012.02.052

2122
N. K. Rana et al. / Tetrahedron Letters 53 (2012) 2121–2124
Table 2
of aromatic thiols to
standing of activation modes of 1a, we selected it as pivot point
for the enantioselective addition of thioacid to ,b-unsaturated
a
,b-unsaturated ketones.11 With the under-
Effect of solvent on enantioselective sulfa-Michael addition of thiobenzoic acid to 2-
cyclohexenonea
a
O
O
carbonyl compound. Treatment of 2-cyclohexenone with thioben-
zoic acid in the presence of 10 mol % of the catalyst 1a in toluene at
room temperature furnished the Michael adduct 2a in a 29% ee and
quantitative yield (Table 1, entry 1). The initial results encouraged
us to carry out detailed investigation to improve the selectivity. To
our delight, decrease in catalyst loading from 10 to 1 mol % led to
increase in the enantioselectivity, without affecting the chemical
yield of the reaction. When the reaction was carried out with
0.5 mol % of catalyst loading, the enantioselectivity did not im-
prove and a longer reaction time was required for the completion
of the reaction. However, further lowering the catalyst loading to
0.1 mol % resulted in the decrease of enantioselectivity and yield
of the reaction.
Thus, with optimized catalyst amount, the effect of temperature
was investigated. Lowering the reaction temperature did not show
much effect on the chemical yield, but the enantioselectivity
decreased gradually (Table 1, entries 7–11). The lower enantiose-
lectivity at lower temperature may be due to the different arrange-
ment of transition state. Although a drop in enantioselectivity with
lowering temperature is generally uncommon, very similar trend
of the enantioselectivity dependence on reaction temperature has
been observed for thiourea catalyzed several Michael addition
reactions.3g,7c,11,13 Subsequently, the effect of solvent was studied
(Table 2). Among various solvents used for the reaction, toluene
was found to be the best in terms of selectivity.
O
O
1a
(1 mol %), rt
+
Ph
SH
S
Ph
eeb (%)
3a
4a
2a
Entry
Solvent
Toluene
m-Xylene
n-Hexane
CH2Cl2
CHCl3
DCE
THF
Et2O
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
18
18
18
14
14
14
18
18
18
12
99
99
99
99
99
99
99
99
99
99
64
46
44
41
46
34
30
44
42
12
1,4-Dioxane
Acetonitrile
10
a
Reactions were carried out with 0.2 mmol of 2a and 0.24 mmol of 3a in 1 mL of
solvent at rt, unless noted otherwise.
b
Determined by HPLC using Chiralcel OD-H column.
cient catalytic asymmetric sulfa-Michael addition reaction of aro-
matic thiols to
In our continuing efforts, we chose to explore the unique reactivity
profile of thioacids by its addition to ,b-unsaturated ketones. Here,
we report catalytic asymmetric sulfa-Michael addition of thioacids
to ,b-unsaturated ketones catalyzed by a bifunctional epi-quinine
amine urea.
In our previous report, quinine derived urea 1a was found to be
an efficient organocatalyst in the enantioselective Michael addition
a
,b-unsaturated ketones.11
a
a
After initial optimization of the reaction conditions with cata-
lyst 1a, various cinchona alkaloid derived thiourea catalysts
(Fig. 1) were screened in the above reaction, and the results are
F3C
CF3
R'
R'
F3C
CF3
HN
HN
S
HN
HN
X
HN
HN
S
H
H
H
N
N
N
MeO
MeO
R
N
N
N
1d
1e:
R = OMe
1a: R' = CF3, X = O
1f: R = H
1b:
R' = H, X = O
1c: R' = H, X = S
F3C
CF3
F3C
Ph
N
H
H
N
NH
O
S
NH
N
H
X
F3C
R
HN
N
N
1g:
1h:
1i:
R = OMe, X = S
1j
R = H, X = S
R = OMe, X = O
Ph
NH
O
HN
S
H
HN
S
N
HN
HN
S
N
H
H
HN
HN
N
N
N
MeO
MeO
MeO
N
1m
1k
1l
Figure 1. Chiral bifunctional urea/thiourea catalysts.

N. K. Rana et al. / Tetrahedron Letters 53 (2012) 2121–2124
2123
Table 3
Table 4
Screening of different chiral catalystsa
Enantioselective sulfa-Michael addition between thioacids and
a,b-unsaturated
ketonesa
O
O
O
1
(1 mol %),
O
O
O
O
toluene, rt, 18 h
+
1a
(1 mol %)
O
Ph
SH
R
toluene, rt
S
Ph
eeb (%)
SH
3a
4a
2a
S
R
3a: R = Ph
4-13
2
3b:
R = CH3
Entry
Catalyst
Yield (%)
1
2
3
4
5
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1a
99
99
99
99
99
99
99
99
99
99
99
99
99
99
64
56
49
05
59
58
58
56
62
10
42
54
25
82
Entry Enone 2
3
Time
(h)
Yield (%)/
product
eeb
(%)
1
3a 18
3b 24
3b 24
99 4a
99 4b-(S)
99 4b-(R)
64
82
79
O
2c
3d
6
2a
7c
8c
9c
10
11
12
13
14d
4
5
3a 30
3b 36
3b 36
99 5a
99 5b-(R)
99 5b-(S)
63
81
80
O
6d
2b
7
8
3a 18
3b 24
3b 24
99 6a
99 6b-(S)
99 6b-(R)
60
83
80
O
a
9d
Reactions were carried out with 0.2 mmol of 2a and 0.24 mmol of 3a in 1 mL of
toluene at rt, unless noted otherwise.
2c
b
Determined by HPLC using Chiralcel OD-H column.
Opposite enantiomer was obtained as major.
Thioacetic acid was used as a nucleophile and reaction was continued for 24 h.
c
10
11
3a 18
3b 24
99 7a
99 7b
35
50
O
d
2d
12
13
3a 24
3b 30
99 8a
96 8b
32
42
O
summarized in Table 3. Intensive screening of several chiral cata-
lysts disclosed the significant impact of the substituent and cata-
lyst’s chiral scaffold on the enantioselectivity. Slightly lower
enantioselectivity with catalysts 1b and 1c compared to 1a indi-
cates that CF3 substituent on the aromatic ring of the catalyst is crucial
(Table 3, entries 2 and 3). A very low enantioselectivity with epi-qui-
nine derived catalyst 1d emphasizes the importance of the correct
relative orientation of thiourea and quinuclidine functional groups
in the catalyst’s chiral scaffold (Table 3, entry 4). Thioureas 1e and
1f also were tested in the reaction, however urea 1a was found to
be superior over the corresponding thiourea 1e catalyst. Sulfa-Mi-
chael addition product 4a, enriched in the opposite enantiomer,
was obtained with catalysts 1g–i (Table 3, entries 7–9). Thus, ac-
cess to both enantiomers was found to be possible with the same
level of enantioselectivity. When 60-cinchona thiourea 1j was used
for the reaction, poor enantioselectivity was observed (Table 3, en-
try 10). The result indicates that the appropriate distance between
acidic and basic groups is important for high enantioselectivity.
Catalysts 1k–m having additional chiral centers were also em-
ployed in the above reaction, but poor enantioselectivities were
observed. Finally, the enantioselectivity increased to a great extent
by changing the nucleophile from thiobenzoic acid to thioacetic acid
(Table 3, entry 14).
Ph
tBu
O
2e
14
15
3a 24
3b 30
95 9a
92 9b
27
40
tBu
tBu
2f
Br
Cl
F
16
17
3a 24
3b 30
99 10a
98 10b
40
46
O
2g
18
19
3a 24
3b 30
99 11a
99 11b
25
33
O
tBu
2h
20
21
3a 28
3b 36
92 12a
90 12b
5
24
O
tBu
MeO
2i
22
23
3a 24
3b 30
99 13a
95 13b
24
42
O
tBu
2j
With the optimized reaction conditions in hand, a variety of a,b-
unsaturated ketones were tested with both thiobenzoic and thio-
acetic acids as Michael donors and the results are summarized in
Table 4. Moderate to good enantioselectivities were obtained in
both the cases of 2-cyclohexenone and substituted cyclohexenone
(Table 4, entries 1–6). For a few typical cases, both the enantiomers
have been achieved with the same level of enantioselectivity by
using two pseudoenantiomeric catalysts 1a and 1i. Interestingly,
useful ee’s have been achieved with 2-cycloheptenone (Table 4,
entries 7–9). However, low enantioselectivity was obtained with
a
Reactions were carried out with 0.2 mmol of 2, 0.24 mmol of 3, and 0.002 mmol
of 1a in 1 mL of toluene at rt, unless noted otherwise.
b
Determined by HPLC using chiral column.
Absolute stereochemistry of the product was determined to be (S).12
c
d
Catalyst 1i was used and opposite enantiomer was obtained as major.
the aromatic ring of the acyclic enone resulted in poor enantioselec-
tivities (Table 4, entries 20 and 21).
cyclopentenone (Table 4, entries 10 and 11). Acyclic
a,b-unsatu-
Finally, the synthetic utility of the catalytic process have been
demonstrated in Scheme 1. The sulfa-Michael addition product
4b-(S) was successfully transformed into the corresponding
3-hydroxyl thioester 14 in a high yield almost without any loss
of enantioselectivity. Additionally, 14 could easily be converted
rated ketones offered lower enantioselectivities in most of the cases
as compared to cyclic enones. Acyclic enones with 4-halogenated
aromatics provided enantioselectivities in the range of 25–46%
(Table 4, entries 14–19). Having electron donating substituent on

2124
N. K. Rana et al. / Tetrahedron Letters 53 (2012) 2121–2124
O
References and notes
OH
O
NaBH4, MeOH,
0 °C, 2 h
O
O
ref. 3c
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N
CF3
O
O
OMe
O
H
N
N
H
N
H
F3C
O
H
(S)
S
R
S
O
Addition to Si face of enone
R
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Acknowledgments
V.K.S. thanks the Department of Science and Technology, India
for a research grant through J. C. Bose fellowship. N.K.R. thanks
the Council of Scientific and Industrial Research (CSIR), New Delhi
for research fellowship. R.U. thanks CSIR for a S. P. Mukherjee fel-
lowship. We thank Drs. Alakesh Bisai and Vishal Rai (IISER Bhopal)
for their help.
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2012.02.052.