Chiral Frustrated Lewis Pairs Catalyzed Highly Enantioselective Hydrosilylations of 1,2-Dicarbonyl Compounds

Article abstract of DOI:10.1021/jacs.5b13104

A highly enantioselective hydrosilylation of 1,2-dicarbonyl compounds was successfully realized for the first time utilizing the combination of tricyclohexylphosphine and chiral alkenylborane derived in situ from diyne as a frustrated Lewis pair catalyst. A variety of optically active α-hydroxy ketones and esters were obtained in 52-98% yields with 86-99% ee's.

Full text of DOI:10.1021/jacs.5b13104

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Communication
Chiral Frustrated Lewis Pairs Catalyzed Highly Enantioselective
Hydrosilylations of 1,2-Dicarbonyl Compounds
Xiaoyu Ren, and Haifeng Du
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b13104 • Publication Date (Web): 11 Jan 2016
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Chiral Frustrated Lewis Pairs Catalyzed Highly Enantiose-
lective Hydrosilylations of 1,2-Dicarbonyl Compounds
Xiaoyu Ren and Haifeng Du*
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Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute
of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Supporting Information Placeholder
for the reduction of imines gave racemic products.14
ABSTRACT: A highly enantioselective hydrosilylation of
With the use of a binaphthyl-based chiral borane, up to
1,2-dicarbonyl compounds was successfully realized for
62% ee was obtained for imine substrates by the same
the first time utilizing the combination of tricyclohex-
group.15 Recently, Klankermayer and co-workers utilized
ylphosphine and chiral alkenylborane derived in situ
camphor derived chiral borane and tri-tert-
from diyne as a frustrated Lewis pair catalyst. A variety
butylphosphine as a FLP catalyst for the same reaction to
of optically active α-hydroxy ketones and esters were
give up 87% ee.16 Mechanism studies suggest that the
heterolytic cleavage of the Si-H bond by the FLP catalyst
results in the formation of silylium and hydridoborate
ionic complex.16 To the best of our knowledge, this is the
highest enantioselectivity for this type hydrosilylation till
now. Particularly, it is noteworthy that there still lack
highly enantioselective catalysts for the Piers type hy-
drosilylation of ketones and other unsaturated com-
pounds, and FLP catalysts might be a good choice to
solve this problem.
obtained in 52-98% yields with 86->99% ee’s.
Optically active α-hydroxy carbonyl compounds are
very important structural motifs in numerous biological-
ly interesting compounds and are also utilized as versa-
tile chiral building blocks for asymmetric synthesis.1 Ac-
cordingly, various methods have been well established
for the synthesis of these compounds. Among them, the
asymmetric reduction of vicinal dicarbonyl compounds
is a particularly powerful and straightforward approach.
α-Keto esters are the most often studied substrates for
the enantioselective reduction and great progress has
been achieved.1c,2 But, in sharp contrast, the reduction of
1,2-diketone to α-hydroxy ketones has been less devel-
oped. In 1985, Ohta and co-workers reported a highly
enantioselective reduction of benzils to benzoins cata-
lyzed by the enzyme system of a microorganism.3 Later
on, several other biocatalysis systems were also employed
for this reaction.4 Besides the enzyme catalysis, only very
few examples on the transition-metal catalyzed asym-
metric hydrogenation were reported by Ohgo and co-
workers using a chiral cobalt complex as catalyst to give
up to 78% ee.5 Despite these advances, the catalytic high-
ly enantioselective reduction of 1,2-diketones still re-
mains an unsolved problem.6
Catalytic hydrosilylation of unsaturated compounds is
a very useful reaction in organic chemistry.7 In 1996,
Piers and co-workers disclosed an interesting B(C6F5)3-
catalyzed hydrosilylation of carbonyl compounds.8 The
borane Lewis acid activated the Si-H bond instead of the
carbonyl group to form silylium and hydridoborate ions,
which is very similar to the H-H bond activation mecha-
nism in the chemistry of frustrated Lewis pairs (FLPs).9
This type of activation attracts both synthetic and mech-
anistic interest for chemists, and this area has witnessed
a very rapid growth.10,11 However, the asymmetric ver-
sion of Piers type hydrosilylation is still in the start-up
stage.12 In 2008, Oestreich and co-workers reported a
B(C6F5)3-catalyzed hydrosilylation of acetophenone us-
ing a chiral silane to afford the chiral alcohol with 38%
ee.13 But the combination of B(C6F5)3 with chiral silanes
In our previous work, we developed an in situ catalyst
generation strategy via a hydroboration of alkenes with
Piers’ borane HB(C6F5)2, which has been successfully
applied to the FLP-catalyzed hydrogenation.17-19 Signifi-
cantly, a chiral borane catalyst derived from chiral diene
was found to be effective for the hydrosilylation of imin-
es to give up to 82 % ee.20 Very recently, we developed
chiral alkenylboranes derived from chiral diynes for the
metal-free asymmetric hydrogenation of silyl enol
ethers.21 With these diverse and efficient chiral boranes
in hand, we further devote our efforts to the challenging
hydrosilylation of 1,2-diketones and α-keto esters. Here-
in, we wish to report our preliminary results on this sub-
ject.
1. chiral diyne 2 (5 mol %)
HB(C6F5)2 (10 mol %)
phosphine 3 (10 mol %)
O
HO
Ph
H
O
Ph
Ph
Ph
PhMe2SiH (1.1 equiv)
toluene, 50 °C, 5 h
O
1a
4a
2. Py•HF, hexane, rt, 0.5 h
2a: Ar = 3,5-tBu2C6H3
2b: Ar = 4-MeC6H4
2c: Ar = 4-MeOC6H4
2d: Ar = 4-tBuC6H4
2e: Ar = 4-FC6H4
Ar
Ar
Me
Me
2f: Ar = 4-CF3C6H4
Ar
Ar
2g: Ar = 4-PhC6H4
2h: Ar = 2-MeO-5-tBuC6H3 2k: Ar = 3,5-tBu2C6H3
2i: Ar = 2,5-(MeO)2C6H3 2l: Ar = 4-MeOC6H4
2j: Ar = 3,5-(CF3)2C6H3
Scheme 1. Chiral FLP-catalyzed asymmetric hydrosilyla-
tion of benzil 1a
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Attempts were started by using chiral alkenylborane
Page 2 of 5
ited an obvious advantage over chiral diene 5 on both
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2
3
4
5
6
7
8
generated from chiral diyne 2a and Piers’ borane for the
hydrosilylation of benzil (1a) with 1.1 equivalent of
PhMe2SiH (Scheme 1). The desired product benzoin (4a)
can be obtained in 25% conversion and 22% ee (Table 1,
entry 1). When tri-tert-butylphosphine (3a) was used as
an additional Lewis base, 77% ee was obtained (Table 1,
entry 2). The obvious improvement on the enantioselec-
tivity using additional phosphine Lewis base is also ob-
served in the asymmetric hydrosilyalion of imines re-
ported by Klankermayer and co-workers.16 This promis-
ing result encouraged us to test commercial available
phosphines 3b-d for this reaction (Table 1, entries 3-5).
To our pleasure, up to 76% conversion and 98% ee were
obtained when tricyclohexylphosphine (3d) was used as
a Lewis base (Table 1, entry 5). Under this condition, a
variety of chiral diynes 2b-l were next examined. Termi-
nal diynes 2b-j afforded low to moderate reactivities and
good to excellent enantioselectivities (Table 1, entries 6-
14). But, internal diynes 2k,l gave extremely low conver-
sions and ee’s (Table1, entries 15 and 16). Overall, the
combination of chiral diyne 2a and tricyclohex-
ylphosphine (3d) gave the optimal results.
reactivity and enantioselectivity, which is likely attribut-
ed to the electronic and/or steric difference of alkenyl-
borane and alkylborane. The clear reason still awaits fur-
ther efforts.
Table 2. Optimization of reaction conditionsa
entry PhMe2SiH solvent
(equiv)
temp
(°C)
conv. ee
(%)b
76
74
82
80
30
20
55
62
81
85
94
78
(%)c
1
2
3
4
5
6
7
8
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
2.0
3.0
4.0
toluene
toluene
toluene
toluene
CH2Cl2
Et2O
dioxane
hexane
mesitylene 60
50
30
60
80
60
60
60
60
98
98
98
98
83
80
94
96
97
97
99
97
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10
11
12
13
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16
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20
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48
49
50
51
52
53
54
55
56
57
58
59
60
9
10
11
12
toluene
toluene
toluene
60
60
60
Table 1. Evaluation of phosphines and diynesa
a
All reactions were carried out with benzil (1a) (0.20
entry diyne 2 phosphine 3
conv. (%)b ee (%)c
mmol), chiral diyne 2a (0.01 mmol), HB(C6F5)2 (0.02
mmol), and phosphine 3d (0.02 mmol) in solvent (0.4 mL)
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
--
25
12
27
22
77
74
83
98
78
69
89
71
80
71
92
71
88
9
for 5 h. Determined by crude H NMR. c The ee was de-
b
1
tBu3P (3a)
Mes3P (3b)
termined by chiral HPLC.
Ph2(C6F5)P (3c) 39
Cy3P (3d)
3d
3d
3d
3d
3d
3d
3d
3d
3d
3d
3d
76
21
15
28
19
21
16
42
15
39
7
tBu
tBu
tBu
tBu
tBu
tBu
vs
9
10
11
12
13
14
15
16
a
5
2a
tBu
11% conv, 56% ee
tBu
94% conv, 99% ee
Figure 1. Comparison of chiral diyne 2a with chiral diene 5
26
0
A variety of 1,2-diketones 1a-l were then subjected to
the hydrosilylation. As shown in Table 3, both electron-
withdrawing or donating substituents were well tolerant
to give optically active α-hydroxy ketones 4a-l in 52-98%
yields with 86->99% ee’s (entries 1-12). It is noteworthy
that the diol byproducts were not obtained in all cases
although three equivalents of PhMe2SiH were used.
Moreover, this current catalytic hydrosilylation system
was also effective for the reaction of α-keto esters without
any further optimization. As shown in Table 4, the
asymmetric hydrosilylation of α-keto esters 6a-h pro-
ceeded cleanly to furnish the desired optically active α-
hydroxy esters 7a-h in high levels of reactivities and en-
antioselectivities (entries 1-8). Acetophenone was also an
effective substrate for the asymmetric hydrosilylation,
under the current reaction conditions, 1-phenylethanol
was obtained in 95% yield with 42% ee.
All reactions were carried out with benzil (1a) (0.20
mmol), chiral diyne (0.01 mmol), HB(C6F5)2 (0.02 mmol),
phosphine (0.02 mmol), and PhMe2SiH (0.22 mmol) in
toluene (0.4 mL) at 50 °C for 5 h. b Determined by crude 1H
NMR. c The ee was determined by chiral HPLC.
The reaction conditions were optimized to further im-
prove the reactivity. It was found that higher tempera-
ture gave a slightly higher conversion without loss of
enantioselectivity (Table 2, entries 1-4). Solvents were
found to influence the hydrosilylation largely on reactivi-
ties and ee’s (Table 2, entries 5-9). Increasing the amount
of PhMe2SiH afforded a satisfactory 94% conversion with
99% ee (Table 2, entries 3, 10-12). A comparison of chi-
ral diyne and diene for the hydrosilylation of benzil (1a)
was subsequently conducted under the optimal reaction
conditions. As shown in Figure 1, chiral diyne 2a exhib-
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Table 3. Asymmetric hydrosilylation of 1,2-diketonesa
diol byproducts were observed although three equivalen-
1
2
3
4
5
6
7
8
ts of PhMe2SiH were used. Significantly, chiral diyne
exhibited an obvious advantage over chiral diene for
thehydrosilylation. Moreover, this catalytic hydrosilyla-
tion system can be further extended to α-keto esters
without any further optimization, and furnished the de-
sired α-hydroxy esters in excellent yields and ee’s. Fur-
ther efforts on expanding the substrate scope of Piers
type hydrosilylation and exploring chiral alkenylboranes
in the asymmetric hydrogenation are underway in our
laboratory.
1. chiral diyne 2a (5 mol %)
HB(C6F5)2 (10 mol %)
Cy3P (3d) (10 mol %)
O
HO
Ar
H
O
Ar
Ar
Ar
PhMe2SiH (3.0 equiv)
toluene, 60 °C, 5 h
O
1
4
2. Py•HF, hexane, rt, 0.5 h
entry
product (4)
yield (%)b
ee (%)c
1
2
3
4
4a: Ar = Ph
4b: Ar = 2-FC6H4
4c: Ar = 3-MeC6H4
91
89
86
>99
99
96
99
96
86
96
94
96
89
88
94
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10
11
12
13
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ASSOCIATED CONTENT
4d: Ar = 3-MeOC6H4 98
Supporting Information. Procedure for the metal-free cat-
5
4e: Ar = 3-iPrOC6H4
4f: Ar = 3-ClC6H4
4g: Ar = 4-MeC6H4
4h: Ar = 4-EtC6H4
4i: Ar = 4-tBuC6H4
4j: Ar = 4-CyC6H4
4k: Ar = 4-FC6H4
77
98
94
90
96
94
70
alytic asymmetric hydrosilylation of 1,2-dicarbonyl com-
pounds, characterization of products, and data for the de-
termination of enantiomeric excesses along with the NMR
spectra. This material is available free of charge via the In-
ternet at http://pubs.acs.org.
6 d
7
8
9
10
11e
12d
AUTHOR INFORMATION
Corresponding Author
haifengdu@iccas.ac.cn
4l: Ar = 3,5-Me2C6H3 52
a All reactions were carried out with 1,2-diketones 1 (0.40
mmol), HB(C6F5)2 (0.04 mmol), chiral diyne 2a (0.02
mmol), Cy3P (0.04 mmol), and PhMe2SiH (1.2 mmol) in
Funding Sources
No competing financial interests have been declared.
b
toluene (0.8 mL) at 60 °C for 5 h unless other noted. Iso-
ACKNOWLEDGMENT
c
d
lated yield. The ee was determined by chiral HPLC.
We are grateful for the generous financial support from the
National Natural Science Foundation of China (21222207
and 21572231).
HB(C6F5)2 (0.06 mmol), chiral diyne 2a (0.03 mmol), and
Cy3P (0.06 mmol) were used. e HB(C6F5)2 (0.08 mmol), chi-
ral diyne 2a (0.04 mmol), and Cy3P (0.08 mmol) were used.
Table 4. Asymmetric hydrosilylation of α-keto estersa
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6
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yield (%)b
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1
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8
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7c: Ar = 3-MeC6H4
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99
7g: Ar = 3,5-Me2C6H3 83
7h: Ar = 2-thienyl 87
97
a All reactions were carried out with α-keto esters 6 (0.40
mmol), HB(C6F5)2 (0.04 mmol), chiral diyne 2a (0.02
mmol), Cy3P (0.04 mmol), and PhMe2SiH (1.2 mmol) in
b
toluene (0.8 mL) at 60 °C for 5 h. Isolated yield. c The ee
was determined by chiral HPLC.
In summary, a highly enantioselective hydrosilylation
of 1,2-dikeones was successfully realized for the first
time using the combination of Cy3P and chiral alkenyl-
borane derived in situ from chiral diyne as a FLP catalyst.
A variety of optically active α-hydroxy ketones were af-
forded in 52-98% yields with 86->99% ee’s. Notably, no
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Ar
Ar
O
chiral diyne
HB(C6F5)2, Cy3P
HO
Ar
H
O
R
R
Ar
PhMe2SiH
O
20 examples
52-98% yields
86->99% ee's
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