Copper-dipyridylphosphine-polymethylhydrosiloxane: A practical and effective system for the asymmetric catalytic hydrosilylation of ketones

Article abstract of DOI:10.1002/adsc.201100105

In the presence of the inexpensive and non-toxic stoichiometric reductant polymethylhydrosiloxane (PMHS), the chiral copper(II)-dipyridylphosphine catalyst displayed high efficiency in the stereoselective hydrosilylation of a wide scope of aryl alkyl and heteroaromatic ketones under an air atmosphere and mild conditions in good to excellent ees (up to 97%). With certain amounts of sodium tert-butoxide and tert-butyl alcohol as additives, the reaction on a 21-g substrate scale can be conveniently completed within a few hours even at a substrate-to-ligand (S/L) ratio of 50,000. Copyright

Full text of DOI:10.1002/adsc.201100105

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DOI: 10.1002/adsc.201100105
Copper-Dipyridylphosphine-Polymethylhydrosiloxane:
A Practical and Effective System for the Asymmetric Catalytic
Hydrosilylation of Ketones
Xi-Chang Zhang,a,b Fei-Fei Wu,b Shijun Li,b Ji-Ning Zhou,b Jing Wu,b,* Ning Li,a
Wenjun Fang,a,* Kim Hung Lam,c and Albert S. C. Chand,*
a
Department of Chemistry, Zhejiang University, Hangzhou 310027, Peopleꢀs Republic of China
Fax : (+86)-571-8795-1895; e-mail: fwjun@zju.edu.cn
College of Material, Chemistry and Chemical Engineering and Key Laboratory of Organosilicon Chemistry and Material
b
Technology of Ministry of Education, Hangzhou Normal University, Hangzhou 310036, Peopleꢀs Republic of China
Fax : (+86)-571-2886-8023; e-mail: jingwubc@hznu.edu.cn
Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hong Kong, Peopleꢀs
c
Republic of China
d
State Key Laboratory of Chiroscience and Institute of Creativity, Hong Kong Baptist University, Hong Kong, Peopleꢀs
Republic of China
Fax : (+852)-3411-2123; e-mail: ascchan@hkbu.edu.hk
Received: February 10, 2011; Revised: March 18, 2011; Published online: June 16, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100105.
Abstract: In the presence of the inexpensive and
non-toxic stoichiometric reductant polymethylhy-
drosiloxane (PMHS), the chiral copper(II)-dipyri-
dylphosphine catalyst displayed high efficiency in
the stereoselective hydrosilylation of a wide scope
of aryl alkyl and heteroaromatic ketones under an
air atmosphere and mild conditions in good to ex-
cellent ees (up to 97%). With certain amounts of
sodium tert-butoxide and tert-butyl alcohol as addi-
tives, the reaction on a 21-g substrate scale can be
conveniently completed within a few hours even at
a substrate-to-ligand (S/L) ratio of 50,000.
systems were promising for this type of reaction.
Since then, a variety of effective copper catalysts
bearing chiral diphosphines or monophosphines[5] has
been developed and applied in the hydrosilylation of
a wide range of ketones[6–10] with moderate to excel-
lent ees.
We have established a catalyst system formed in
situ from CuF2, enantiomeric dipyridylphosphine
(Figure 1, P-Phos 1a or Xyl-P-Phos 1b)[11] and the hy-
dride donor PhSiH3, which allowed for the highly
active and enantioselective hydrosilylation of a broad
assortment of aryl alkyl[8a], diaryl[8a] and heteroaroma-
tic[9b] ketones under a normal atmosphere and mild
conditions. PMHS (polymethylhydrosiloxane), a by-
product of the organosilicon industry, has been well-
known for its low-cost, non-toxicity and air stability.[12]
Thus, it is a desirable hydride donor for economical,
Keywords: asymmetric catalysis; copper; dipyridyl-
phosphine ligands; hydrosilylation; ketones; poly-
methylhydrosiloxane
Enantiomerically pure secondary alcohols are signifi-
cant intermediates for the manufacture of pharma-
ceuticals and advanced materials.[1] The non-precious
metal copper-catalyzed asymmetric hydrosilylation of
prochiral ketones represents a rewarding transforma-
tion toward chiral alcohols due to the economic bene-
fits and mild reaction conditions.[2] The first example
was reported by Brunner and Miehling in 1984[3]
using CuH-DIOP[4] for the hydrosilylation of aceto-
phenone, which revealed that copper/diphosphane Figure 1. Dipyridylphosphine ligands.
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Xi-Chang Zhang et al.
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environmentally friendly and practical reduction pro-
With the aforementioned preferred conditions in
cesses. In light of this, we became particularly inter- hand, we set out to evaluate the general applicability
ested in probing the efficiency of the catalyst system of the present catalyst system for the asymmetric hy-
by employing PMHS as a stoichiometric hydride drosilylation of a broad range of aryl alkyl ketones in
source. Herein, we report an effective dipyridylphos- air (Table 2). Complete reduction of most substrates
phine/Cu(II)/PMHS system for the asymmetric reduc- by using (S)-1b was realized in 12 h and the stereose-
tion of prochiral ketones.
We firstly examined the influences of a series of
Table 2. Cu(II)-catalyzed asymmetric hydrosilylation of sub-
stituted acetophenones 2b–2o in air.[a]
Cu(I) and Cu(II) precursors on the asymmetric hy-
drosilylation of acetophenone (2a) (Table 1). Previous
studies had indicated that, in a Cu(I) or Cu(II)/di-
phosphine/PhSiH3 system, the counter-ion of the
copper precursor played a key role in the generation
of an active catalyst for the reduction of ketones. In
contrast, by utilizing PMHS as a reductant, the coun-
ter-ion of the copper salts did not show a pronounced
effect on the outcome of the reactions. For instance,
when 2a was submitted to 1 mol% copper salt and
(S)-1b ligand plus 5 equivalent of PMHS in toluene at
À208C in air, the desired product (S)-1-phenylethanol
(3a) was furnished in quantitative yield and moderate
ee after 12 h in most cases (entries 1–3 and 5–6)
except for the use of CuBr2 (entry 4). The catalyst
Entry Ketone
Ligand Cu Salt
Yield
[%][b]
ee
[%]
1
(S)-1b Cu
G
76
2
3
(S)-1b Cu
(S)-1a CuAHCUTNGTRENNUNG
N
67[c]
72[c]
based on CuACHTUNGTRENNUNG(acac)2 (acac=acetylacetonate) gave a
somewhat better ee (entry 6, 82%) than those obtain-
able from other copper sources (78–79% ee). Using
(S)-1a as a chiral ligand, 85% ee was achieved while a
longer reaction time was needed for the full conver-
sion of 2a (entry 7).
4
5
(S)-1b Cu
(S)-1b CuF2
G
85
88
6
7
(S)-1b Cu
(S)-1a CuF2
94
96
8
9
(S)-1b Cu
(S)-1b CuF2
89
91
Table 1. Effects of copper salts on the asymmetric hydrosily-
lation of acetophenone 2a in air.[a]
10
11
12
13
(S)-1b Cu
(S)-1b Cu
(S)-1b Cu
N
92
88
91
93
Entry Ligand Copper Salt
Conv. [%][b] ee [%][c]
1
2
3
4
(S)-1b
(S)-1b
(S)-1b
(S)-1b
(S)-1b
(S)-1b
(S)-1a
CuF2
92
99
99
<5
78
78[f]
79
–
CuCl
CuCl2
CuBr2
5
Cu
Cu(acac)2
Cu(acac)2
A
78
82
85
6
T
ACHTUNGTRENNUNG
92[e]
95
7[f]
(S)-1b Cu
(S)-1b Cu
[a]
Reaction conditions: 1 mmol substrate, substrate concen-
tration=0.8–1M in toluene.
14
15
93
95
[b]
[c]
Determined by NMR and GC analysis.
The ee values were measured by chiral GC analysis. The
absolute configuration was determined by comparing the
retention time with the known data (see Supporting In-
formation).
We got the same results under N2.
The isolated yield was 90%.
Reaction time=24 h
(S)-1a CuF2
16
17
18
(S)-1b Cu
(S)-1b CuF2
(S)-1b Cu
96
97
[d]
[e]
[f]
94[d]
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Copper-Dipyridylphosphine-Polymethylhydrosiloxane: A Practical and Effective System
Table 2. (Continued)
Entry Ketone
Ligand Cu Salt
Yield
[%][b]
ee
[%]
19
20
(S)-1b Cu
(S)-1b Cu
(S)-1b Cu
G
92
81
92
21
Scheme 1. Cu(II)-catalyzed asymmetric hydrosilylation of
simple ketones 4a–4g in air.
[a]
Reaction conditions: 0.5 mmol substrate, substrate con-
centration=0.8–1M in toluene. Full conversions were
observed for all reactions.
Isolated yields.
Reaction time=72 h.
[b]
[c]
[d]
Next, the activity of the present system was investi-
gated by performing the hydrosilylation of acetophe-
0.01 mol% (S)-1b, reaction time=24 h.
none 2a in the presence of 1 mol% Cu
ACHTUNGTRENNUNG(acac)2,
0.2 mol% (S)-1b and 5 equivalents of PMHS in air at
lectivities of the reaction were greatly affected by the room temperature. After 1.5 h, 53% conversion and
position of the substituents on the phenyl ring of the 77% ee were observed (Table 3, entry 1). Consistent
acetophenone. The ortho-substituted species (2b and with previous findings,[2f,8b,14] the reaction was acceler-
2c) were transformed to the corresponding alcohol ated by addition of an appropriate amount of either t-
products with moderate enantiopurities (76% and BuOH (66% conversion, entry 2) or t-BuONa (89%
72% ee, entries 1–3) while good to excellent ees (88– conversion, entry 3), respectively, without any erosion
97%, entries 4–18) were attained for both the meta- of the ees. To our delight, a quantitative yield was
and para-substituted substrates (2d–2l). Besides, by achieved when both alcohol (4 equiv.) and base
simply prolonging the reaction time to 24 h, a lower (1 mol%) were added (entry 4).
catalyst loading (0.01 mol% 1b) also drove the reac-
Finally, as illustrated in Scheme 2, the excellent cat-
tion to completion with no essential diminution of ee alytic efficiency of the present system was evidenced
(entry 18 vs. entry 16). Moreover, multi-substituted ar- by conducting the reactions with an S/L ratio of
omatic ketones (2m–2o) were also reduced quantita- 50,000 at room temperature under a normal atmos-
tively in high ees under otherwise identical conditions
(entries 19–21).
Table 3. Effects of additives on the copper-catalyzed asym-
metric hydrosilylation of acetophenone 2a in air.[a]
Furthermore, as the findings in Scheme 1 indicate,
the present catalyst system also worked effectively for
a variety of other aryl alkyl ketonic substrates includ-
ing hetero-aromatic ketones (4a–4g) in air and under
mild conditions leading to optically active alcohols
embodying heterocyclic moieties, which are widely re-
garded as valuable intermediates for the synthesis of
physiologically active products. For example, in the
presence of 1 mol% CuACTHNUTRGNE(UGN acac)2 and (S)-1b, the hydro-
silylation of 2-acetylthiophene 4c proceeded at À208C
to give 89% ee for the desired product (S)-5c which is
the key building block for (S)-duloxetine,[13] an inhibi-
tor of serotonin and norepinephrine uptake carriers.
As for the reduction of acetylpyridine substrates with
different positions between carbonyl and nitrogen
atoms (4e–4g), net conversions were fulfilled under a
given set of conditions albeit with moderate enan-
tioinductions (63–75% ee).
Entry
Alcohol
Base
Conv. [%][b]
ee [%]
1
2
3
4
–
–
–
53
66
89
96
77
76
76
77
t-BuOH
–
t-BuOH
t-BuONa
t-BuONa
[a]
Reaction conditions: 1.0 mmol substrate, substrate con-
centration=0.8–1M in toluene.
Determined by NMR and GC analysis.
[b]
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Xi-Chang Zhang et al.
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Scheme 2. Asymmetric hydrosilylation of 2l on a large scale.
phere. Upon adding 1 mol% t-BuONa and 4 equiva-
lents of t-BuOH, the hydrosilylation of 21 g of 2l pro-
ceeded evenly by using only 2 mg of (S)-1b and
1 mol% CuF2, and furnished (S)-3l neatly within 4 h
with 89% ee. On the other hand, in a side by side
comparison study, the system with CuACTHNUTRGENUG(N acac)2 exhibit-
ed inferior catalytic activity (54% conversion after
4 h) to that of CuF2.
A tentatively proposed mechanism is depicted in
Scheme 3.[2f,14a,c,15] In the initial step, upon combining
(S)-Xyl-P-Phos with CuACHTNUTRGNENG(U acac)2 and t-BuONa, the
chiral Cu(II) complex A most likely forms. Then the
s-bond metathesis between A and PMHS takes place
and affords copper hydride species B, which is conjec-
tured to be the key intermediate responsible for dis-
criminating between the enantiotopic faces of ketonic
substrates. Next, stereoselective insertion of C=O of
À
ketone into the Cu H bond occurs, leading to the for-
mation of a copper alkoxide intermediate C, which
Scheme 3. Plausible reaction mechanism.
may involve
a
four-membered transition state
(TS2).[16] Subsequently C may be protonated by the
alcohol additive to give the desired alcohol product D
and a copper alkoxide A. A may be capable of under-
going s-bond metathesis with PMHS more rapidly ligand along with the inexpensive and innocuous hy-
than copper alkoxide C to regenerate the active spe- dride donor PMHS in air. This system allowed for the
cies B.
asymmetric reduction of a wide spectrum of aryl alkyl
In conclusion, we have developed a highly practical and hetero-aromatic ketones with good to excellent
and efficacious catalyst system generated in situ from ees (up to 97%). With the addition of certain amounts
CuACHTUNGTRENNUNG(acac)2 and an optically pure dipyridyl-phosphine of t-BuOH and t-BuONa, the reaction could be com-
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Copper-Dipyridylphosphine-Polymethylhydrosiloxane: A Practical and Effective System
[2] For recent reviews, see: a) B. H. Lipshutz, in: Modern
pleted smoothly in a few hours at an S/L ratio as high
as 50,000.
Organocopper Chemistry, (Ed.: N. Krause), Wiley-
VCH, Weinheim, 2002, pp 167–187; b) H. Nishiyama,
K. Itoh, in: Transition Metals for Organic Synthesis,
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Experimental Section
General Procedure for the Asymmetric
Hydrosilylation Reaction in Air (Table 1, entry 6)
CuACHTUNGTRENNUNG(acac)2 (2.6 mg, 1 mol%) and (S)-1b (7.6 mg, 1 mol%)
were weighed under air and placed in a 25-mL round-bot-
tomed flask equipped with a magnetic stirring bar. Toluene
(0.4 mL) was added and the resulting solution was stirred at
room temperature for 20 min. To the mixture, PMHS
(330 mL, 5 mmol) in toluene (0.3 mL) was added and the so-
lution was cooled to À208C. A solution of acetophenone 2a
(120 mL, 1.0 mmol) in toluene (0.3 mL) was added under
vigorous stirring, and the flask was stoppered. The reaction
was monitored by TLC. Upon completion, the reaction was
quenched by the addition of an aqueous solution of NaOH
(2.5M, 1 mL) and the mixture was stirred vigorously for 3 h.
The organic product was extracted with ether (3ꢂ2 mL).
The combined extract was washed with water, dried with an-
hydrous sodium sulfate, filtered through a plug of silica gel
and concentrated under vacuu, to give the crude product.
The conversion and the enantiomeric excess of the product
(S)-1-phenylethanol [(S)-3a] were determined by NMR and
chiral GC analysis to be 92% and 82%, respectively
(Column, Chirasil-DEX CB 25 mꢂ0.25 mm, Varian, carrier
gas, N2). The crude product was purified by silica gel chro-
matography (ethyl acetate:petroleum ether=1:10 to 1:4) to
afford the product as a colourless oil; yield: 107 mg (90%
yield).
[3] H. Brunner, W. Miehling, J. Organomet. Chem. 1984,
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[4] DIOP=2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis-(di-
phenylphosphino)butane: T. P. Dang, H. B. Kagan, J.
Chem. Soc. Chem. Commun. 1971, 481.
[5] K. Junge, B. Wendt, D. Addis, S. Zhou, S. Das, M.
Beller, Chem. Eur. J. 2010, 16, 68–73.
[6] Examples of catalytic systems derived from homogene-
ous copper sources and diphosphines for hydrosilyla-
tion of aryl alkyl ketones, see: a) B. H. Lipshutz, K.
Noson, W. Chrisman, J. Am. Chem. Soc. 2001, 123,
12917–12918; b) S. Sirol, J. Courmarcel, N. Mostefai,
O. Riant, Org. Lett. 2001, 3, 4111–4113; c) B. H. Lip-
shutz, K. Noson, W. Chrisman, A. Lower, J. Am. Chem.
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hedron Lett. 2004, 45, 5415–5417; e) B. H. Lipshutz,
B. A. Frieman, Angew. Chem. 2005, 117, 6503–6506;
Angew. Chem. Int. Ed. 2005, 44, 6345–6348; f) B. H.
Lipshutz, A. Lower, R. J. Kucejko, K. Noson, Org. Lett.
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Bellemin-Laponnaz, Adv. Synth. Catal. 2006, 348,
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Riant, Synthesis 2007, 1265–1271.
General experimental information and spectral, analytical
data for all chiral products are available in Supporting Infor-
mation.
[7] Examples of catalytic systems derived from heteroge-
neous copper sources and diphosphines for hydrosilyla-
tion of aryl alkyl ketones: a) B. H. Lipshutz, B. A. Frie-
man, A. E. Tomaso Jr, Angew. Chem. 2006, 118, 1281–
1286; Angew. Chem. Int. Ed. 2006, 45, 1259–1264;
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Sreedhar, B. M. Choudary, Adv. Synth. Catal. 2007, 349,
1797–1802; c) M. L. Kantam, S. Laha, J. Yadav, P. R.
Likhar, B. Sreedhar, S. Jha, S. Bhargava, M. Udayakir-
an, B. Jagadeesh, Org. Lett. 2008, 10, 2979–2982;
d) M. L. Kantam, J. Yadav, S. Laha, P. Srinivas, B.
Sreedhar, F. Figueras, J. Org. Chem. 2009, 74, 4608–
4611.
Acknowledgements
We thank the National Natural Science Foundation of China
(21032003, 20672026), the New Century Excellent Talents in
Chinese University Program awarded to Jing Wu (NCET-07-
0250), and the Natural Science Foundation of Zhejiang Prov-
ince (R406378, Y406193) for financial support.
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