Copper-Catalyzed enantioselective hydrosilylation of ketones by using monodentate binaphthophosphepine ligands

Article abstract of DOI:10.1002/chem.200902442

"Chemical Equation Presented" No base required: The first copper-catalyzed asymmetric hydrosilylation of carbonyl compounds by using monodentate binaphthophosphepine ligands is presented. After optimization of the reaction parameters, high yields and enantioselectivities (up to 96 % ee) for a broad range of aryl alkyl, cyclic, heterocyclic and aliphatic ketones are achieved without a base.

Full text of DOI:10.1002/chem.200902442

DOI: 10.1002/chem.200902442
Copper-Catalyzed Enantioselective Hydrosilylation of Ketones by Using
Monodentate Binaphthophosphepine Ligands
Kathrin Junge, Bianca Wendt, Daniele Addis, Shaolin Zhou, Shoubhik Das, and
Matthias Beller*[a]
The asymmetric catalytic reduction of carbonyl com-
pounds offers a convenient and efficient preparation of
enantiomerically pure chiral alcohols.[1] In the past, various
catalytic methods such as precious metal-catalyzed hydroge-
nation, transfer hydrogenation, and hydrosilylation have
been intensively studied. Although asymmetric hydrogena-
tions proceed with high enantioselectivities and good to ex-
cellent yields for a wide range of prochiral ketones, often
high pressure, elevated temperatures, and special equipment
are required. In contrast, asymmetric hydrosilylation offers
an attractive alternative due to the smooth reaction condi-
tions and the easy to use reducing agents. Since the first ap-
plications of Wilkinsonꢀs catalyst in catalytic hydrosilyla-
tions[2] three decades ago, much work has been published on
the asymmetric hydrosilylation of ketones. In general, Rh-,
Ru-, Ir-, Ti-, or Zn-based catalysts have been used and more
recently also enantioselective Fe-catalyzed hydrosilylations
have been reported.[3]
In the search for cheap and biomimetic catalysts, copper,
aside of zinc and iron, offers interesting possibilities. Pio-
neering work in the field of copper-mediated enantioselec-
tive hydrosilylations was carried out by Brunner and co-
workers in 1984.[4] Later on, inspired by the potential of
Strykerꢀs reagent [(Ph3P)CuH]6,[5,6] and the work of Buch-
wald and co-workers,[7] Lipshutz and his group[8] developed
a highly effective catalyst system based on CuCl/tBuONa
and chiral diphosphane ligands for asymmetric hydrosilyla-
tions. In the presence of SEGPHOS[8] or BINAP[9] aryl alkyl
and heteroaromatic ketones are reduced with excellent
enantioselectivities up to 98% ee. However, temperatures
below À508C and addition of base were required for opti-
mal enantiomeric excess. Noteworthy, a base-free and air-ac-
celerating copper catalyst for the hydrosilylation of carbonyl
compounds was presented by Riant and co-workers,[10]
[11]
which was composed of copper(II) salts such as CuACTHNUTRGNEUNG(OAc)2
or CuF2
[12]
and BINAP. The resulting catalysts typically
work at mild temperatures (room temperature to À208C),
and a remarkably low substrate-to-ligand ratio (S/L up to
50000 and 100000) is needed for Xyl-P-Phos[12] for the hy-
drosilylation of a wide array of aryl alkyl ketones. Finally, it
should be mentioned that also heterogeneous copper sour-
ces (copper-in-charcoal,[13] copper nanoparticles,[14] and
copper–aluminum hydrotalcide[15]) were applied for enantio-
selective reductions.
Although various copper-based catalysts have been inves-
tigated for the asymmetric hydrosilylation of carbonyl com-
pounds, the number of phosphorus ligands applicable in
these reactions is rather limited, and many ligands, which
are well established in asymmetric hydrogenations, show
low chiral induction.[8c,10b]
In recent years, we and others[16] demonstrated the useful-
ness of monodentate ligands derived from the binaphtho-
phosphepine backbone in asymmetric hydrogenations,[17]
transfer hydrogenations,[18] and hydroformylations.[19] Al-
though the potential of chiral monodentate ligands for hy-
drosilylation reactions has been pointed out in several re-
views,[20] to the best of our knowledge there is no reported
example of such a copper-catalyzed reaction.[21] Monoden-
tate phosphorus ligands have the advantage of easier synthe-
ses and tuning compared with their bidentate counterparts.
Hence, we became interested in testing monodentate phos-
phepine ligands 1 (Scheme 1) in the hydrosilylation of pro-
[a] Dr. K. Junge, B. Wendt, D. Addis, S. Zhou, S. Das,
Prof. Dr. M. Beller
Leibniz-Institut fꢁr Katalyse e.V. an der Universitꢂt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
Fax : (+49)381-1281-5000
matthias.beller@catalysis
Scheme 1. Structure of monodentate ligands.
68
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Chem. Eur. J. 2010, 16, 68 – 73

COMMUNICATION
chiral ketones using copper as catalyst metal. As a bench-
mark reaction, the reduction of acetophenone to 1-phenyl-
ACHTUNGTRENNUNG
Next, the influence of the silane was investigated in the
presence of both hydrated and water-free copper acetate.
As illustrated in Table 2, the catalytic activity and stereose-
lectivity depends to a large extent on the nature of the re-
chosen. Initially, the copper source was investigated in detail
(Table 1). In all experiments the corresponding alcohol is
Table 2. Asymmetric hydrosilylation of acetophenone: Effect of silanes.[a]
Table 1. Asymmetric hydrosilylation of acetophenone: Effect of copper
precursors.[a]
Entry Silane
Cu
G
CuACTHUNGETRNNU(G OAc)2·H2O
yield [%] ee [%] (R) yield [%] ee [%] (R)
1
2
3
4
5
6
7
8
Ph2SiH2
PhSiH3
Me2PhSiH
MeEt2SiH
Et2SiH2
AHCTUNGRTEN(NGUN EtO)2MeSiH 46
PMHS
Cl3SiH
86
91
24
12
87
78
83
12
19
17
62
21
80
85
90
25
15
64
36
52
38
83
84
12
2
14
44
15
80
Entry
Copper precursor
Ligand
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
CuCl
CuBr
CuI
CuOTf
Cu2O
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
S-BINAP
83
74
78
55
80
79
81
86
78
81
79
89
82
91
93
rac
rac
7 (R)
5 (R)
rac
61
38
[a] General conditions: copper salt (3 mol%), 1a (6 mol%), acetophe-
none (1 mmol), silane (1 equiv), toluene (1 mL), 3 h, 08C.
CuF2
rac
rac
CuCl2
CuF2·H2O
CuCl2·H2O
CuO
44 (R)
rac
18 (R)
6 (R)
rac
78 (R)
83 (R)
80 (S)
9
10
11
12
13
14[b]
15[c]
ducing agent. The highest yields and enantioselectivities are
obtained by using monoaryl- and diarylsilanes such as
PhSiH3 and Ph2SiH2 (Table 2, entries 1 and 2), while deple-
tion of the reaction rate and lower ee values were observed
with alkyl- and alkyl
entries 3–7).
Interestingly, the economically attractive trichlorosilane
furnished up to 80% ee, but only moderate amounts of
product were obtained. Hence, phenylsilane was chosen for
further studies. Within the process of optimization of the re-
action parameters, we also explored the influence of the
ligand structure (Scheme 2). Here, 13 different monodentate
phosphepines 1a–l and 4, phosphoramidites 5, 7, and 8, and
phosphite 6 were tested in the hydrosilylation of acetophe-
none with copper acetate. As shown in Table 3, the phenyl-
substituted phosphepine ligand 1a showed the best perfor-
mance. After 3 h, yields up to 92% and enantioselectivities
Cu
N
CuSO4·5H2O
Cu
Cu
Cu
N
ACHTUNGTRENUN(NG aryl)silanes or polysiloxanes (Table 2,
[a] General conditions: copper salt (3 mol%), 1a (6 mol%), acetophe-
none (1 mmol), PhSiH3 (1 equiv), toluene (1 mL), 3 h, 08C. [b] 1 h, 08C.
[c] Copper salt (3 mol%), S-BINAP (3 mol%), 1 h, 08C.
detected in good yield after basic cleavage of the corre-
sponding silyl ether. However, most of the copper(I) and
copper(II) precursors showed no or poor enantioselectivity.
Nevertheless, promising results were achieved in the pres-
ence of copper acetate (Table 1, entries 13 and 14). By ap-
plying CuACHTUNGTRENNUNG(OAc)2·H2O, 1-phenylethanol is obtained in 83%
ee after 1 h, which is in the same range as achieved with the
diphosphane-based system de-
veloped by Yun and Lee[11]
(Table 1, entry 15). Interesting-
ly, the use of hydrated Cu-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
product yield as well as the
enantioselectivity.
It is known from the work of
Chan et al.[12] and Riant et al.[10]
that fluoride in the copper pre-
cursor is crucial for the genera-
tion of the active catalyst. How-
ever, for our system we did not
observe this effect (Table 1,
entry 6).
Scheme 2. Selected monodentate ligands tested in the asymmetric hydrosilylation.
Chem. Eur. J. 2010, 16, 68 – 73
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69

M. Beller et al.
Table 3. Asymmetric hydrosilylation of acetophenone: Effect of
ligands.[a]
Entry
Ligand
CuN
CuACTHUNGETRNNU(G OAc)2·H2O
yield [%]
yield [%]
ee [%]
1
1a
1a
1b
1c
1d
1e
1 f
1 f
1g
1g
1h
1i
1j
1k
1l
4
91
92
90
48
5
83
92
84
75
89
88
88
25
<2
11
5
83 (R)
87 (R)
37 (R)
29.5 (S)
rac
78 (R)
82.5 (R)
86 (R)
83 (R)
82 (R)
76 (R)
66 (R)
52 (S)
n.d.[c]
67 (S)
44 (R)
57 (S)
5 (S)
rac
11 (S)
90
88
53
80
29
83
90
87
87
–
86
80
78
70
56
<1
–
84 (R)
86 (R)
31 (R)
28 (S)
3 (R)
71 (R)
82.5 (R)
85 (R)
83 (R)
–
65 (R)
51 (R)
44 (S)
77 (R)
72 (S)
n.d.[c]
–
2[b]
3
4
5
6
7
Figure 1. Graphical representation of selectivity in hydrosilylation of ace-
1
tophenone in relation to the coupling constant JPÀSe of binaphthophos-
phenines 1.
8[b]
9
10[b]
11
12
13
14
15
16
17
18
19
20
5
6
7
8
3
18
3
–
–
–
–
–
–
2
[a] General conditions: copper salt (3 mol%), 1, 4–8 (6 mol%), aceto-
phenone (1 mmol), PhSiH3 (1 equiv), toluene (1 mL), 3 h, 08C. [b] 5 h,
À208C. [c] Not determined.
Figure 2. Graphical representation of relation between temperature and
enantioselectivity for the hydrosilylation of acetophenone with [Cu]/1a.
up to 84% ee are obtained (Table 3, entry 1). Two other
aryl-substituted phosphepines (1 f and 1g) provided compa-
rable enantioselectivities to 1a (Table 3, entries 7 and 9). In
general, alkyl-substituted phosphepines gave less favorable
results (Table 3, entries 3–5).
With both catalyst systems at À208C an optimal enantiose-
lectivity up to 86% ee for CuACHTUNTRGNEUNG(OAc)2·H2O and 87% ee for
CuACTHUNTRGENNG(U OAc)2 is obtained, respectively (Table 3, entry 2).
In order to draw conclusions about possible structure–ac-
tivity relationships and to estimate the steric and electronic
properties of ligands 1a–l, the first-order coupling constant
between the phosphorus and selenium atom of the corre-
sponding selenides was analyzed.[19] The magnitude of the
77Se–31P coupling constants reveals the s character of the
phosphorus lone-pair orbital. The greater the coupling con-
stants between 31P and 77Se, the stronger the s bond between
the two, which in turn indicates a less basic phosphane.[22] In
Figure 1, the 77Se–31P coupling constants are correlated with
the enantioselectivity reported for the asymmetric hydrosily-
lation of acetophenone with [Cu]/1 (Table 3). In fact, there
is no clear trend observed: binaphthophosphepine ligands
1g (J=740 Hz) and 1 f (J=721 Hz), which vary electronical-
ly, both show high enantiomeric excess. Hence, we believe
that the enantioselectivity in this copper-catalyzed reduction
is controlled more by steric than by electronic factors.
Next, the influence of the temperature on the hydrosilyla-
tion of acetophenone in the presence of 1a was investigated.
As stated before the copper catalysts developed by Lipshutz
et al.[8] gave the best enantioselectivities from À50 to
À788C. Similarly, Riant et al.[10] reported improved enantio-
selectivities at À208C. Our results are depicted in Figure 2.
Finally, the general applicability of our protocol was
tested for a broad range of aryl alkyl, cyclic, heterocyclic,
and aliphatic substrates (Table 4 and Table 5). Complete hy-
drosilylation of most substrates is achieved with PhSiH3 in
the presence of anhydrous Cu
N
5 h. Introduction of electron-withdrawing groups at the para
position of acetophenone resulted in increased enantioselec-
tivity (Table 4, entries 1, 7, and 9), giving 89% ee for 2b,
91% ee for 2g, and 90% ee for 2i, respectively. With respect
to 4-haloacetophenones, the enantiomeric excess decreased
in the sequence F>Cl>Br according to the decreasing in-
duction effect. Electron-donating groups at the ortho and
meta position of the aryl group led to high enantioselectivity
(Table 4, entry 10–12).
Unfortunately, sterically more demanding aryl alkyl ke-
tones were inert, despite running the reaction at ambient
temperature (Scheme 3). However, high yields (83–99%)
are determined for cyclic and heterocyclic ketones (Table 5,
entries 1, 6, and 7), while the level of enantioselectivity
ranges between 14% and 88% ee. In the case of cyclohexe-
nylethanone, regioselective hydrosilylation of the carbonyl
group and no reaction of the C=C double bond takes place
(Table 5, entry 3). Notably, with our catalyst system the
70
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Chem. Eur. J. 2010, 16, 68 – 73

Enantioselective Hydrosilylation of Ketones
COMMUNICATION
Table 4. Asymmetric hydrosilylation of different ketones catalyzed by
Table 5. Asymmetric hydrosilylation of different heterocyclic, cyclic and
Cu
A
aliphatic ketones catalyzed by CuACTHNGUTERNNUG
(OAc)2/1a.[a]
Entry
Substrate
Yield [%]
ee [%]
Entry
1
Substrate
Yield [%]
90
ee [%]
51 (S)
1
94
99
89 (R)
90 (R)
2[b]
4a
4b
4c
4d
4e
4 f
2b
2c
2d
2e
2 f
2g
2h
2i
2
3
4
5
6
90
82
47 (R)
96 (R)
3
79
82
83
78
57
99
75
72
80 (R)
76 (R)
37 (R)
35 (R)
91 (R)
87 (+)
90 (R)
80 (R)
4
À
C C(O) 10
–-
C=C 22
60 (À)
À
C C 24
28 (S)
5
92
83
84 (R)
14 (S)
6
7
7
8
9
4g
4h
4i
92
88 (S)
12 (R)
20 (S)
8
40
9
>99
10
11
12
4j
4k
4l
99
83
99
9 (S)
10
2j
3 (n.d.[b]
22 (R)
)
11
73
99
89 (R)
90 (R)
12[b]
2k
2l
[a] General conditions: copper salt (3 mol%), 1a (6 mol%), ketone 4
(1 mmol), PhSiH3 (1 equiv), toluene (1 mL), 5 h, À208C. [b] Not deter-
mined.
13
14
15
16
17
72
40
90
91
88
82 (R)
89 (R)
84 (R)
39 (R)
19 (S)
2m
2n
2o
2p
Scheme 3. Sterically demanding ketones showing no reaction with [Cu]/
1a.
no general trend regarding a,b-unsaturated aliphatic ketones
(Table 5, entries 3–5). Similarly, ketone 4e is reduced with
both chemo- and enantioselectivity (Table 5, entry 5). Also
various aliphatic ketones could be reduced with good activi-
ty but low to moderate enantiomeric excess (Table 5, en-
tries 9–12).
[a] General conditions: copper salt (3 mol%), 1a (6 mol%), ketone 2
(1 mmol), PhSiH3 (1 equiv), toluene (1 mL), 5 h, À208C. [b] 5 h, À408C.
highest enantiomeric excess (>96% ee) ever reported for
this ketone is obtained. Interestingly, this result represents
Chem. Eur. J. 2010, 16, 68 – 73
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71

M. Beller et al.
2o: ee values were determined by HPLC (Chiralpak AD-H), (R)-3o
9.4 min and (R)-3o 10.4 min (eluent: n-heptane/ethanol 99.5:0.5; flow:
1.0 mLminÀ1).
In summary, the first copper-catalyzed asymmetric hydro-
silylation of carbonyl compounds by using chiral monoden-
tate ligands is presented. Under comparably mild conditions,
high yields and enantioselectivities (up to 96% ee) are
achieved for a broad range of carbonyl compounds such as
aryl alkyl, cyclic, heterocyclic, and aliphatic ketones. Com-
pared to other known asymmetric hydrosilylation catalysts,
advantageously no base or fluoride activation is necessary.
2p: ee values were determined by HPLC (Chiralcel OB-H), (R)-3p
5.1 min and (S)-3p 5.85 min (eluent: n-heptane/ethanol 95:5; flow:
1.0 mLminÀ1).
4a: ee values were determined by HPLC (Chiralcel OD-H), (R)-5a
29.4 min and (S)-5a 31.4 min (eluent: n-heptane/ethanol 98:2; flow:
0.5 mLminÀ1).
4b ee values were determined by GC (50 m Chiraldex ß-PM), (R)-5b
62.4 min and (S)-5b 63.5 min (80/60–6–180/30).
4c ee values were determined by GC (50 m Lipodex E), (S)-5c 66.4 min
and (R)-5c 67.3 min (70/75–6–180).
4d: ee values were determined by HPLC (Chiralcel OB-H), (À)-5d
14.5 min and (+)-5d 16.1 min (eluent: n-heptane/2-propanol 98:2; flow:
0.6 mLminÀ1).
Experimental Section
General procedure: A mixture of the silane (1 mmol), copper precursor
(3ꢄ10À2 mmol), and ligand (6ꢄ10À2 mmol) in toluene (0.5 mL) was
purged with argon in a Schlenk tube and stirred for 20 min at À208C. A
solution of acetophenone (1 mmol) in toluene (0.5 mL) was transferred
by syringe to the in situ generated catalyst. The reaction mixture was
stirred for 3 or 5 h at À208C and quenched with methanol (2 mL) or dis-
tilled water (2 mL) and TBAF (1.2 mL; 1m in THF). After the reaction
mixture had been stirred for 2 h, hexadecane was added as standard. The
yield was determined by GC (30 m HP 5 Agilent Technologies 50–
3008C) and the enantioselectivity was determined by HPLC or GC.
4e: ee values were determined by HPLC (Chiralpak AS-H), (S)-5e
19.8 min and (R)-5e 24.4 min (eluent: n-heptane/2-propanol 99:1; flow:
0.2 mLminÀ1).
4 f: ee values were determined by HPLC (Chiralcel OJ-H), (S)-5 f
13.7 min and (R)-5 f 16.4 min (eluent: n-heptane/ethanol 95:5; flow:
0.7 mLminÀ1).
4g: ee values were determined by HPLC (Chiralcel OJ-H), (S)-5g
13.8 min and (R)-5g 19.3 min (eluent: n-heptane/ethanol 95:5; flow:
0.7 mLminÀ1).
2a: ee values were determined by GC (50 m Lipodex E), (S)-3a 35.7 min
and (R)-3a 36.9 min (85/35–6–180/2–8–200/15).
4h: ee values were determined by HPLC (Chiralcel OD-H), (S)-5h
21.4 min and (R)-5h 24.9 min (eluent: n-heptane/ethanol 99:1; flow:
1.0 mLminÀ1).
2b: ee values were determined by HPLC (Chiracel OD-H), (S)-3b
44.5 min and (R)-3b 52.7 min (eluent: n-heptane/ethanol 99:1; flow:
1.0 mLminÀ1).
4i: ee values were determined by HPLC (Chiralcel OB-H), (R)-5i
16.0 min and (S)-5i 17.2 min (eluent: n-heptane/ethanol 98:2; flow:
0.3 mLminÀ1).
2c: ee values were determined by HPLC (Chiralcel OD-H), (S)-3c
26.4 min and (R)-3c 28.7 min (eluent: n-heptane/ethanol 98:2; flow:
0.5 mLminÀ1).
4j: ee values were determined by GC (50 m Lipodex E), (S)-5j 25.9 min
and (R)-5j 29.4 min (40/35–6–180/2–8–200).
2d: ee values were determined by HPLC (Chiralcel OD-H), (S)-3d
30.9 min and (R)-3d 33.7 min (eluent: n-heptane/ethanol 98:2; flow:
0.5 mLminÀ1).
4k: ee values were determined by GC (50 m Chiraldex ß-PM), 5k
13.7 min and 5k 14.2 min (130/30–6–200/30).
4l: ee values were determined by GC (50 m Chiraldex ß-PM), (S)-5l
33.1 min and (R)-5l 33.5 min (40/30–10–180/5).
2e: ee values were determined by HPLC (Reposil 100), (S)-3e 32.0 min
and (R)-3e 33.7 min (eluent: n-heptane/ethanol 99.75:0.25; flow:
0.5 mLminÀ1).
2 f: ee values were determined by GC (50 m Chiraldex ß-PM), (R)-3 f
42.1 min and (S)-3 f 43.0 min (120/30–6–180/15).
Acknowledgements
2g: ee values were determined by HPLC (Chiralcel OD-H), (R)-3g
21.7 min and (S)-3g 24.7 min (eluent: n-heptane/ethanol 98:2; flow:
1.0 mLminÀ1).
The authors thank Dr. C. Fischer, S. Buchholz, S. Schareina, and A.
Kammer (all at the Leibniz-Institut fꢁr Katalyse e.V.) for analytical and
technical support.
2h: ee values were determined by HPLC (Chiralpak AD-H), (+)-3h
29.3 min and (À)-3h 31.7 min (eluent: n-heptane/ethanol 98:2; flow:
0.4 mLminÀ1).
2i: ee values were determined by HPLC (Chiralpak AD-H), (S)-3i
19.4 min and (R)-3i 23.3 min (eluent: n-heptane/ethanol 97:3; flow:
1.0 mLminÀ1).
Keywords: copper
hydrosilylation · ketones · P ligands
·
homogeneous
catalysis
·
2j: ee values were determined by HPLC (Chiralcel OJ-H), (S)-3j
34.4 min and (R)-3j 36.8 min (eluent: n-heptane/ethanol 98:2; flow:
0.5 mLminÀ1).
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2k: ee values were determined by HPLC (Chiralcel OD-H), (R)-3k
42.8 min and (S)-3k 54.3 min (eluent: n-heptane/ethanol 97:3; flow:
0.2 mLminÀ1).
2l: ee values were determined by HPLC (Chiralcel OD-H), (S)-3l
19.0 min and (R)-3l 21.0 min (eluent: n-heptane/ethanol 97:3; flow:
1.0 mLminÀ1).
2m: ee values were determined by HPLC (Chiralpak AD-H), (S)-3m
18.0 min and (R)-3m 20.0 min (eluent: n-heptane/ethanol 99:1; flow:
0.5 mLminÀ1).
2n: ee values were determined by HPLC (Chiralcel OD-H), (R)-3n
8.4 min and (S)-3n 9.7 min (eluent: n-heptane/ethanol 97:3; flow:
1.0 mLminÀ1).
72
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Received: September 3, 2009
Published online: November 27, 2009
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73