Rhodium catalyzed asymmetric 1,4-addition of arylboronic acids to β,γ-unsaturated α-keto ester using chiral tert- butanesulfinylphosphines ligands

Article abstract of DOI:10.1016/j.tetlet.2014.04.074

Rh-Catalyzed asymmetric 1,4-selective addition of arylboronic acids to β,γ-unsaturated α-keto ester was developed using chiral tert-butanesulfinylphosphine as ligand, good yields (up to 87%), good 1,4-regioselectivities (up to 96:4), and high enantioselec

Full text of DOI:10.1016/j.tetlet.2014.04.074

Tetrahedron Letters xxx (2014) xxx–xxx
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Tetrahedron Letters
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Rhodium catalyzed asymmetric 1,4-addition of arylboronic
acids to b,c-unsaturated a-keto ester using chiral
tert-butanesulfinylphosphines ligands
Juanjuan Wang a,b,c, Bing Wang b,c, Peng Cao b,c, Jian Liao b,c,
⇑
a Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, China
b Natural Products Research Center, Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, China
c Graduate School of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Rh-Catalyzed asymmetric 1,4-selective addition of arylboronic acids to b,c-unsaturated a-keto ester was
Received 18 March 2014
Revised 16 April 2014
Accepted 22 April 2014
Available online xxxx
developed using chiral tert-butanesulfinylphosphine as ligand, good yields (up to 87%), good 1,4-regiose-
lectivities (up to 96:4), and high enantioselectivities (up to 94% ee) were achieved.
Ó 2014 Published by Elsevier Ltd.
Keywords:
Rh-Catalyzed
b,c-Unsaturated a-keto ester
1,4-Selective addition
Asymmetric catalysis
tert-Butanesulfinylphosphine
Since developed by Hayashi and miyaura, Rh-catalyzed asym-
metric conjugated addition (ACA) of arylboronic acids to elec-
tron-deficient olefins has become an important strategy for
enantioselective carbon–carbon bond formation.1 During the past
decade, a broad scope of electron-deficient substrates, such as
b,
c-unsaturated a-ketoester by using tert-butanesulfinylphosphine
as ligand.
The initial reaction was carried out with ethyl 2-oxo-4-phen-
ylbutyrate (1a), 2 equiv of PhB(OH)2 (2a), and 50 mol % KOH
(1.0 M in H2O) in the presence of (1.0 mol %) [Rh(CH2CH2)2Cl]2/
(3 mol %)sulfinylphosphine L1. The reaction proceeded smoothly
in dichloromethane (DCM) at 40 °C for 4 h, to our delight, the
1,4-adduct was obtained as major product and with good isolated
yield and modest enantioselectivity (73% yield and 60% ee, Table 1,
entry 1). Encouraged by this result, we next systematically
screen the reaction condition. The solvent dramatically affected
the 1,4/1,2-selectivity and alcohols were proved to be the best
solvent for the regioselectivity (Table 1, entries 2–5). Although
ethyl alcohol can provide up to 95:5 ratio of 1,4/1,2-adduct, the
yield was poor due to the hydrolysis of substrate and product
under the strong basic condition (Table 1, entry 4). Trifluoroethanol
(CF3CH2OH) was identified as ideal solvent to give the 1,4-adduct
in good yield with moderate enantiomeric excess (86% yield and
52% ee, Table 1, entry 5). In addition, the excellent regioselectivity
was obtained in the presence of KOH (Table 1, entries 5–8).
[Rh(COD)Cl]2 was used and the result did not have great change.
(Table 1, entry 9). We then evaluated different ligands in the
optimized conditions, bifunctional sulfinylphosphine ligand
(L9),13 bissulfoxide (L10),12d sulfoxide-olefin (L11),12j and (R)-BINAP
(L12) demonstrated no reactivity. Other sulfinylphosphines
a
,b-unsaturated aldhyde,2 ketones,1a,3 esters,4 amides,5 sulfones,6
phosphonates,7 nitro alkenes8 and so on, have been extensively
exploited. Albeit b,c-unsaturated a-ketoesters have been widely
applied in many reactions owing to their dense functionalizations,9
it is still a class of challenging substrates for Rh-catalyzed ACA. In
2008, Zhou et al. realized an 1,2-addition of arylboronic acid to b,
c-
unsaturate -ketoesters by using a Rh/spirophosphite catalytic
a
system, moderate to excellent yield (up to 93%) and excellent
enantionselectivity (up to 93%) were achieved.10 Very recently,
by using sulfinamide–olefin ligand, Xu et al. reported an example
of 1,4-adduct as the major product for the Rh-catalyzed 1,2-/1,4-
selective addition of arylboronic acid to b,c-unsaturated a-ketoes-
ter, however, only 5% ee for 1,4-adduct was observed.11 Our recent
interest was focused on designing chiral sulfoxide ligands and
developing their application in asymmetric transition-metal cata-
lyzed reactions.12 In this work, we would like to present a rhodium
catalyzed asymmetric 1,4-selective addition of arylboronic acids to
⇑
Corresponding author. Tel./fax: +86 28 82890822.
E-mail address: jliao@cib.ac.cn (J. Liao).
http://dx.doi.org/10.1016/j.tetlet.2014.04.074
0040-4039/Ó 2014 Published by Elsevier Ltd.
Please cite this article in press as: Wang, J.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.04.074

2
J. Wang et al. / Tetrahedron Letters xxx (2014) xxx–xxx
Table 1
Reaction conditions screening
O
OEt
O
O
O
3aa
1,4 - addition (
)
O
[Rh(CH2CH2)2Cl]2 (1 mol%)
B(OH)2
OEt
+
+
L
O
(3 mol%),base(50 mol%),
solvent,40 oC for 4h
O
OEt
1a
2a
HO
O
4aa
1,2 - addition (
)
Ligands:
S
S
P
O
O
S
S
O
PPh2
PPh2
R2
L1
PPh2
S
S
S
O
O
Ph
O
R1
PPh2
O
R
R
1 = OMOM, R2 = H
O
1 = OMOM, R2 =OMe
1 = OMOE, R2 =OMe
S
L2
L3
Ph
(R)-BINAP
R
L4 R1 = OMe, R2 = OMe
L10
L12
L9
L11
L8
L5 R1 = OMe, R2 = H
L6
L7
R
R
1 = H, R2 = OMe
1 = H, R2 = H
Entry
Ligand
Solvent
Base
1,4-/1,2-adductb
Yield (3aa)c (%)
ee (3aa)d (%)
1
2
3
4
5
6
7
8
L1
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
L6
L7
L8
L9
L10
L11
L12
DCM
DCE
THF
EtOH
KOH
KOH
KOH
KOH
KOH
K2CO3
Cs2CO3
tBuOK
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
84:16
82:18
86:14
95:5
92:8
71:29
83:17
85:15
86:14
95:5
91:9
92:8
93:7
93:7
96:4
75:25
nd
73
69
53
46
86
56
53
68
65
81
88
80
72
80
83
47
nr
60
53
49
62
52
66
64
60
56
72
70
64
57
50
55
76
nd
nd
nd
nd
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
9e
10
11
12
13
14
15
16
17
18
19
20
nd
nd
nd
Trace
Trace
nr
a Reaction conditions: 0.3 mmol of 1a, 2.0 equiv of 2a, 1 mol % of [Rh(CH2CH2)2Cl]2, 3 mol % of ligand, 0.5 equiv of base (1.0 M in H2O) in 1 mL of solvent at 40 °C for 4 h.
b
Determined by 1H NMR of the crude product.
Isolated yield.
Determined by chiral HPLC analysis.
[Rh(COD)Cl]2 was used. nr = no reaction, nd = no determined.
c
d
e
(L2–L7) and bissulfinylphosphine (L8)14 could also promote this
reaction (Table 1, entries 10–16) and L2 was the best ligand (81%
yield and 72% ee). The electron-donating substituted (L2–L4)
groups are good for the enantioselectivity.
Finally, we examined effects of the ester group for substrate 1
on the reactivity, regioselectivity and enantioselectivity of the
reaction.9j Several esters, such as methyl-1b, isopropyl-1c, tert-
butyl-1d, benzyl-1e, and cyclohexyl-1f were evaluated and the
results are summarized in Scheme 1. Isopropyl-1c and cyclo-
hexyl-1f, afforded both good regioselectivity and enantioselectivi-
ty. The reaction (for substrate 1f) proceeded smoothly at 20 °C and
1,4-adduct 3fa was obtained with high regioselectivity (93:7), high
enantioselectivity (84% ee), and high yield (86%).
With optimal condition in hand,15 we investigated the scope of
-unsaturated -keto esters and arylboronic acids (Table 2). A
variety of b, -unsaturated -keto esters were subjected to the
b,c
a
c
a
reaction with phenylboronic acid 2a and gave the desired 1,4-addi-
tion products 3fa–3qa in good yields (70–87%) and with moderate
to good ee values (60–84%). Electron-rich groups of para-substitu-
ents afforded better enantioselectivity than electron-poor ones and
the steric hindrance showed little influence on the reactivity, reg-
ioselectivity and enantioselectivity (Table 2, entries 1–7). 3,4-
Dichlorosubstituted 1m, meta-chloro/fluoro-substituted 1n or 1o
reacted with phenylboronic acid 2a to afford the corresponding
products 3ma, 3na, and 3oa with good results (Table 2, entries
8–10). ortho-Chlorosubstituted 1p also proceeded smoothly
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J. Wang et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
O
OR
O
O
B(OH)2
O
OR
3
[Rh(CH2CH2)2Cl]2 (1 mol%)
+
O
+
L2
(3 mol%),KOH (50 mol%)
TFE, 40 oC for 4h
O
1
2a
OR
HO
O
O
1b 3ba 4ba
R= Me (
)
/
= 79/28, 86% yield, 68% ee
4
R= Et(1a)
3aa/4aa=95:5, 81%yield, 72% ee
R= i-Pr (1c) 3ca/4ca= 94/6, 88% yield, 74% ee
R= t-Bu (1d) 3da/4da= 83/17, 80% yield, 69% ee
R= Bn (1e) 3ea/4ea= 90/10, 74% yield, 62% ee
R= Cy (1f) 3fa/4fa= 91/9,
R=Cy (1f) 3fa/4fa= 93/7,
87% yield, 77% ee
86% yield, 84% ee(20oC)
Scheme 1. Screening ester group of substrates.
Table 2
Substrate scope of asymmetric 1,4-addition to (E)-2-oxo-4-arylbut-3-enoate
R1
O
[Rh(CH2CH2)2Cl]2 (1 mol%)
B(OH)2
OR
R1
O
+
L2 (3 mol%),KOH (50 mol%),
Ar
CF3CH2OH,20 oC for 24h
O
OR
1
2
Ar
O
R = cyclohexyl
3
Entry
Ar (1)
R1 (2)
1,4-/1,2-adductb
Yield (3)c (%)
ee (3)d (%)
1
2
3
4
p-MeOC6H4 (1f)
p-iprC6H4 (1g)
p-tBu C6H4 (1h)
p-MeC6H4 (1i)
p-FC6H4 (1j)
p-Cl C6H4 (1k)
p-CN C6H4 (1l)
3,4-Cl C6H4 (1m)
m-Cl C6H4 (1n)
m-F C6H4 (1o)
o-Cl C6H4 (1p)
2-thiophenyl (1q)
C6H5 (1r)
p-MeOC6H4 (1f)
p-MeOC6H4 (1f)
p-MeOC6H4 (1f)
p-MeOC6H4 (1f)
p-MeOC6H4 (1f)
p-MeOC6H4 (1f)
p-MeOC6H4 (1f)
p-MeOC6H4 (1f)
p-MeOC6H4 (1f)
p-MeOC6H4 (1f)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
p-MeO (2b)
p-tBu (2c)
p-ipr (2d)
p-OTBS (2e)
p-OBn (2f)
p-Me (2g)
p-OMOM (2h)
p-CF3 (2i)
m-OMe (2j)
m-Cl (2k)
m-Naphthyl (2l)
93:7
94:6
96:4
94:6
92:8
91:9
90:10
92:8
94:6
86
87
86
81
83
79
70
82
85
78
77
75
75
73
72
66
74
78
67
72
87
71
64
84 (3fa)
83 (3ga)
84 (3ha)
80 (3ia)
77 (3ja)
73 (3ka)
60 (3la)
60 (3ma)
74 (3na)
77 (3oa)
71 (3pa)
64 (3qa)
71 (3rb)
92 (3fc)
94 (3fd)
88 (3fe)
80 (3ff)
80 (3fg)
77 (3fh)
54 (3fi)
71 (3fj)
74 (3fk)
69 (3fl)
5
6e
7e
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
92:8
86:14
85:15
81:19
85:15
86:14
77:23
81:19
87:13
80:20
91:9
92:8
88:12
83:17
a Reaction conditions: 0.3 mmol of (E)-2-oxo-4-arylbut-3-enoate 1, 2.0 equiv of arylboronic acid 2, 1 mol % of [Rh(CH2CH2)2Cl]2, 3 mol % of L2, 0.5 equiv of KOH (1.0 M in H2O)
in 1 mL of CF3CH2OH at 20 °C for 24 h.
b
Determined by 1H NMR of the crude product.
Isolated yield.
Determined by chiral HPLC analysis.
At 40 °C for 4 h.
c
d
e
(Table 2, entry 11). Heterocyclic, like thiophenyl, substrate 1q also
worked well with 2a to give 3qa in good yield (75%) and moderate
ee value (64%).
In the examination of arylboronic acids scope, we found that
stereoelectronic properties of aryl groups have great influence on
the outcome of the reaction. Electron-rich substituents were bene-
fited to the enantioselectivity (Table 2, entries 14–18). For
instance, (p-tBu)phenyl and (p-i-propyl)phenyl boronic acids pro-
vided adducts 3fc and 3fd with excellent enantiomerical excess
(92% and 94%). The type of m-substituents on arylboronic acids
have little effect on the reaction, (Table 2, entries 21–23) but o-
substituted arylboronic acids showed no reactivity, probably due
to the huge steric hindrance.
In summary, we developed a Rh-catalyzed asymmetric 1,4-
selective addition of arylboronic acids to b,c-unsaturated a-keto
ester using tert-butanesulfinylphosphines as ligand, moderate to
excellent 1,4-regioselectivities (up to 96:4), good yields (up to
87%), and good enantioselectivities (up to 94% ee) were achieved.
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J. Wang et al. / Tetrahedron Letters xxx (2014) xxx–xxx
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Acknowledgments
We are grateful to the support of National Science Foundation
of China (21272226 and 21202160) and the Major State Basic
Research Development Program (973 program, 2010CB833300).
Supplementary data
Supplementary data (experiment details, copies of HPLC, 1H and
13C NMR spectra for products) associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.tetlet.2014.04.074.
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15. Procedure for rhodium catalyzed asymmetric selectivity 1,4-addition of arylboronic
acids to b, c-unsaturated a-keto ester: Under an argon atmosphere and at room
temperature, to a 10 mL Schlenk tube with a teflon cap was added ligand L2
(5.5 mg. 3 mol %) and [Rh (C2H4)2Cl]2 (1.2 mg, 1 mol %) followed by 1.0 mL
DCM. The mixture was stirred for half hour, after removal of the solvent, (E)-2-
oxo-4-arylbut-3-enoate (0.3 mmol), arylboronic acid (0.6 mmol) and degassed
KOH (1.0 M in H2O, 0.15 mL, 0.15 mmol). The reaction mixture was stirred at
20 °C for 24 h. The reaction mixture was then directly charged on to a column
(silica gel) for flash chromatography with a mixture of petroleum ether/EtOAc
(15:1) to afford the product.
16. A proposed 1,4-/1,2-selective addition model was provided in Supporting
information.
Please cite this article in press as: Wang, J.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.04.074