Asymmetric hydrosilylation of prochiral ketones catalyzed by nanocrystalline copper(II) oxide

Article abstract of DOI:10.1002/adsc.200600481

Asymmetric hydrosilylation of aryl alkyl ketones to afford chiral secondary alcohols with good yields and excellent enantioselectivity is realized by using nanocrystalline copper(II) oxide and BINAP in the presence of organosilanes as the stoichiometric r

Full text of DOI:10.1002/adsc.200600481

UPDATES
DOI: 10.1002/adsc.200600481
Asymmetric Hydrosilylation of Prochiral Ketones Catalyzed by
Nanocrystalline Copper(II) Oxide
a,
a
a
a
M. Lakshmi Kantam, * Soumi Laha, Jagjit Yadav, Pravin R. Likhar,
a
b
B. Sreedhar, and B. M. Choudary
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad-500007, India
Phone/Fax: (+91)-40-2716-0921; e-mail: mlakshmi@iict.res.in
a
b
Ogene Systems (I) Private Limited, Hyderabad, India
Received: September 19, 2006; Revised: April 26, 2007
Abstract: Asymmetric hydrosilylation of aryl alkyl
ketones to afford chiral secondary alcohols with
good yields and excellent enantioselectivity is real-
ized by using nanocrystalline copper(II) oxide and
BINAP in the presence of organosilanes as the stoi-
chiometric reducing agents.
The AHS reaction using CuH and DIOP, a chiral
phosphine ligand, was first described by Brunner in
[
8]
1984. The utility of copper for hydride delivery was
studied with the Stryker reagent [CuH·PPh ] , a stoi-
3
6
chiometric reducing agent for the reduction of
[
9]
enones. The catalytic version of the reaction was
achieved when a phosphine-copper(I) system such as
[
10]
[
CuH·PPh ] was treated with hydrogen gas
or si-
Keywords: asymmetric hydrosilylation; heterogene-
ous catalysis; nanocrystalline copper(II) oxide; re-
duction; secondary alcohols
3 6
[
11]
lanes
which enabled the generation of the CuÀH
complex. Buchwald described a highly enantioselec-
tive 1,4- reduction of a, b-unsaturated esters and b-
substituted enones using an active catalyst generated
in situ from CuCl/NaO-t-Bu/chiral diphosphine ligand
[
12]
and PMHS.
The effectiveness of [CuH·PPh ] for
3 6
Introduction
the hydrosilylation of carbonyl compounds was re-
[
13]
ported by Lipshutz.
Subsequent studies by the
Asymmetric reduction of ketones represents one of same group led to the development of the highly
the most important methodologies for obtaining enan- enantioselective hydrosilylation of ketones based on
[14]
tiomerically enriched alcohols, which are key inter- CuCl/NaO-t-Bu and chiral diphosphine ligands. Be-
[
1]
mediates for numerous biologically active molecules.
sides Strykerꢀs reagent and copper alkoxides, copper
[15] [16]
Asymmetric hydrogenation and transfer hydrogena- fluoride and copper(II) acetate also catalyze the
tion of carbonyl compounds have been reported by AHS reaction of prochiral ketones in the presence of
Mikami et al. and Tu and co-workers using ruthenium BINAP ligand. Recently, Bellemin-Laponnaz and co-
catalysts with excellent yields and enantiomeric workers reported an efficient AHS reaction using a
excess. Asymmetric hydrosilylation (AHS) reactions chiral
employing organosilanes as the stoichiometric reduc- system.
[2]
BINAP-copper(I)-phenyl(methyl)silane
A
H
R
U
G
[
17]
ing agents lead to very high enantiomeric excesses on
Although homogeneous catalysts are desirable be-
a large range of substrates, employing catalytic cause of their high activity and selectivity, separation
amounts of complexes of transition metal such as rho- of homogeneous catalysts from the products of reac-
[
3]
dium or titanium. Highly enantioselective hydrosily- tion and/or recovery of the catalysts are inherent
lations of prochiral ketones were reported by Fu and problems associated with conventional catalysts. Het-
co-workers using the planar chiral P,N-ferrocene- erogeneous catalysis has potential advantages, such as
based rhodium catalyst at room temperature in the easy separation, efficient recycling and minimization
presence of mesitylphenylsilane as the hydride of metal traces in the product. Very recently Lipshutz
[4]
source. The AHS reaction is also catalyzed by ruthe- reported the heterogeneous copper-catalyzed AHS re-
[
18]
nium complexes in the presence of chiral tridentate li- action.
[5]
gands. The AHS reaction of prochiral ketones using
inexpensive hydride sources such as polymethylhydro-
siloxane (PMHS) and easily accessible and economi-
[7]
[
6]
cal catalysts such as zinc diamine complexes or tin
has also been reported.
Adv. Synth. Catal. 2007, 349, 1797 – 1802
ꢁ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. Lakshmi Kantam et al.
UPDATES
Results and Discussion
However, with triaryl- or trialkylsilanes, only a trace
amount of product was obtained even after prolonged
Nanocrystalline metal oxides find excellent applica- reaction time (Table 1, entries 3 and 6). While PhSiH3
tions as active adsorbents for gases and destruction of is the most active reducing agent with our catalytic
[
19]
hazardous chemicals
and catalysts for various or- system, the cheaper and readily available siloxanes
Very recently, CO and NO such as PMHS (Table 1, entry 7), a by-product of sili-
[
20,21]
ganic transformations.
[
22]
oxidations were reported by CuO nanoparticles
and we reported asymmetric epoxidation, Henry and slower reaction rate.
Michael addition reactions using nanocrystalline In the process of optimization for the AHS reac-
tion, we explored the use of different solvents
cone industry, can also be employed albeit with a
[23]
MgO.
We herein report the nanocrystalline copper oxide (Table 2) and also the reaction was done at different
nano-CuO)-catalyzed asymmetric hydrosilylation of temperatures (Table 3). From Table 2 it is evident
(
prochiral ketones to chiral secondary alcohols in good that toluene is the best solvent both in terms of prod-
yields with excellent enantioselectivity (Scheme 1). uct yield and ee. A significant effect was observed
Initially, the activities of various silanes were tested when the reaction temperature was decreased. As the
in the AHS reaction of acetophenone in presence of temperature decreases, the ee increases with a de-
(
(
(
S)-(À)-2,2’-bis(diphenylphosphino)-1,1’-binaphthyl
BINAP) ligand and nano-CuO at room temperature
Table 1). Although it can be seen from Table 1 that
crease in the rate of reaction (Table 3). The solvent
the level of enantiomeric excess (ee) does not depend
too much on the nature of the reducing agent, the
rate of the reaction is highly dependent on the choice
of the reducing agent. Monoarylsilanes such as phe-
Table 2. Asymmetric hydrosilylation reaction of acetophe-
none catalyzed by nano-CuO with different solvents at
[a]
À208C.
nylsilane (PhSiH ) afforded good enantioselectivity
3
and excellent yield in a short period of time (Table 1,
entry 1), while the use of diaryl- and dialkylsilanes
furnished lower yields, but the stereoselectivity re-
mained almost unchanged (Table 1, entries 2 and 5).
[
a]
Reaction conditions: acetophenone (2 mmol), phenylsi-
lane (2 equivs), nano-CuO (0.010 g), solvent (3 mL)
chiral ligand (BINAP) (0.1 mmol, 0.062 g).
Isolated yields.
Absolute configurations were determined to be (S). ee
were measured by Diacel Chiralcel HPLC using OJ-H
column with 5% 2-propanol in hexane.
[
[
b]
c]
Scheme 1. Asymmetric hydrosilylation reaction of ketones
using nano-CuO in the presence of a chiral ligand.
Table 1. Asymmetric hydrosilylation reaction of acetophe-
none by different silanes at room temperature.
[a]
Table 3. Asymmetric hydrosilylation reaction of acetophe-
[a]
none catalyzed by nano-CuO at different temperatures.
[
a]
[a]
Reaction conditions: acetophenone (2 mmol), silane (2
equivs.), nano-CuO (0.010 g), toluene (3 mL), chiral
ligand (BINAP) (0.1 mmol, 0.062 g).
Reaction conditions: acetophenone (2 mmol), phenylsi-
lane (2 equivs), nano-CuO (0.010 g), toluene (3 mL),
chiral ligand (BINAP) (0.1 mmol, 0.062 g).
Isolated yields.
The ee were measured by Diacel Chiralcel HPLC using
OJ-H column with 5% 2-propanol in hexane.
Using commercial CuO.
[
[
b]
c]
[b]
[c]
Isolated yields.
Absolute configurations were determined to be (S). ee
were measured by Diacel Chiralcel HPLC using OJ-H
column with 5% 2-propanol in hexane.
[d]
[
d]
[e]
5
mmol of PMHS were used.
Using 0.275 g of Cu/C.
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Adv. Synth. Catal. 2007, 349, 1797 – 1802

Asymmetric Hydrosilylation of Prochiral Ketones
UPDATES
Table 4. Asymmetric hydrosilylation reaction of acetophe- tr oe sl uu el tn. e and a temperature of À208C gave the best
none catalyzed by nano-CuO with different ligands at
[a]
À208C.
The AHS reaction of acetophenone with commer-
cial CuO afforded 38% of the desired product with
7
2% ee after 24 h at À208C (Table 3). The nano-CuO
2 [24]
with higher surface area (136 m /g)
and smaller
crystallite size (7–9 nm) than the commercial CuO
2
(
SA: 1.952 m /g) displayed higher activity. The higher
enantioselectivity is correlated to the smaller crystal-
lite size which effectively complexes with the chiral
auxiliary.
Different types of chiral auxiliaries were also tested
and the results are summarized in (Table 4). It was
important to note that diphosphine ligands such as
BINAP were crucial for the catalytic activity of nano-
CuO both in terms of yield as well as ee (Table 4,
entry 1). The use of bidentate nitrogen-based ligands
[
a]
Reaction conditions: acetophenone (2 mmol), phenylsi-
lane (2 equivs), nano-CuO (0.010 g), toluene (3 mL),
chiral ligand (0.1 mmol).
[
[
b]
c]
Isolated yields.
Absolute configurations were determined to be (S). ee
were measured by Diacel Chiralcel HPLC using OJ-H
column with 5% 2-propanol in hexane.
[
d]
Without ligand at room temperature.
(
Table 4, entries 2 and 3), gave poor yield of product
and also the ee decreased significantly. Moreover, the
bidentate oxygen-based ligand BINOL produced only
[a]
Table 5. Asymmetric hydrosilylation reaction of prochiral ketones catalyzed by nano-CuO with phenylsilane at À208C.
[
a]
Reaction conditions: ketone (2 mmol), phenylsilane (2 equivs), nano-CuO (0.010 g), toluene (3 mL), chiral ligand
BINAP) (0.1 mmol, 0.062 g).
Isolated yields.
(
[
[
[
b]
c]
d]
Absolute configurations were determined to be (S).
TOF based on moles of CuO and in parenthesis TOF calculated based on surface area (turnover/nm /sec).
2
Adv. Synth. Catal. 2007, 349, 1797 – 1802
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M. Lakshmi Kantam et al.
UPDATES
Table 6. Recycling of the catalytic system for the asymmetric
Hydrosilylation reaction of acetophenone at À208C.
[
a]
Reaction conditions: ketone (2 mmol), phenylsilane (2
equivs), nano-CuO (0.010 g), toluene (3 mL), chiral
ligand (BINAP) (0.1 mmol, 0.062 g).
Isolated yields.
Absolute configurations were determined to be (S).
Figure 1. TEM images of nano CuO (a) fresh nano-CuO (b)
used nano-CuO.
[
[
b]
c]
trace amounts of product despite a prolonged reaction
time.
To widen the scope, the catalytic activity of nano-
CuO was tested in the AHS reaction of an array of
aryl alkyl ketones using (S)-(À)-BINAP as chiral aux-
iliary and phenylsilane as the stoichiometric reducing
agent, which resulted in excellent enantioselectivity.
Acetophenone is hydrosilylated cleanly with high
enantioselectivity (Table 5, entry 1). Introduction of
electron-withdrawing groups at the para position of
acetophenone (Table 5, entries 2, 6 and 7) produced a
decrease in enantioselectivity although total conver-
sion was achieved in a shorter period of time. Con-
versely, the presence of electron-donating groups at te
para-position of acetophenone (Table 5, entries 3 and
4) has little effect on enantioselectivity but a longer
reaction time is needed for full conversion. Sterically
demanding 2-acetonaphthone (Table 5, entry 9) also
afforded good yields with excellent enantioselectivity.
When the methyl group of the aryl alkyl ketone is
substituted with an ethyl group, the AHS reaction still
gave excellent enantioselectivity albeit with moderate Figure 2. XRD patterns of nano-CuO (a) 1 cycle, (b) 2
yields (Table 5, entries 5 and 10). It is significant to cycle, (c) 3 cycle, (d) 4 cycle.
note that other substituents like fluoro, bromo, cyano,
st
nd
rd
th
and chloro (Table 5, entries 2, 6, 7 and 10) were not
affected during the hydrosilylation reaction. The cor- Figure 1 a and b shows the TEM images of the fresh
responding chiral alcohols are obtained after hydroly- and the used catalyst after the fourth cycle. Interest-
sis (see Experimental Section).
ingly it is observed that the shape and size of the par-
Catalyst recycle experiments were also carried out ticles remain unchanged. This supports the proposal
using acetophenone as a model substrate. As can be that the morphology of the catalyst remains the same
seen in Table 6, the nano-CuO catalyst can be reused even after recycling. The X-ray powder diffraction
for several cycles without loss of activity and selectivi- (XRD, Figure 2) patterns of the fresh and used nano-
o
o
ty. No reaction occurred when the reaction was con- CuO do not differ in the range 2q=0 –80 which con-
ducted with the filtrate obtained after removal of the firms the fact that the structure and morphology of
solid catalyst. This indicates that the active catalyst the catalyst remains the same during the course of the
species was not leached out of the solid catalyst. The reaction.
absence of copper in the filtrate was also confirmed
by AAS studies.
The plausible mechanism of hydrosilylation may be
through the formation of copper hydride species from
Transmission electron microscopic (TEM) studies the reduction of ligated nano-CuO by the silane. At
of both fresh and used catalysts were carried out to this stage, we are not sure whether the active catalytic
understand the shape and size of the particles. species is a copper(I) hydride or a copper(II) hy-
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Adv. Synth. Catal. 2007, 349, 1797 – 1802

Asymmetric Hydrosilylation of Prochiral Ketones
UPDATES
After the reaction, the XPS of the recovered nano-
CuO (Figure 3d) shows the 2p3/2 peak at 934.3 eV
which corresponds to Cu in the +2 oxidation state
only.
In conclusion, the enantioselective hydrosilylation
of aryl alkyl ketones to afford chiral secondary alco-
hols with good yields and excellent enantioselectivity
is realized by using nanocrystalline copper oxide and
BINAP in the presence of organosilanes as the stoi-
chiometric reducing agents without any inorganic
base.
Experimental Section
General Remarks
TM
Nanocrystalline CuO (NanoActive
CuO abbreviated as
2
nano-CuO S.A: 136 m /g) samples were obtained from
Nano-Scale Materials Inc., Manhattan, Kansas, USA. The
commercial CuO (CM-CuO, S.A: 1.952 m /g) was purchased
2
from Loba Chemie. India. Cu(II)/C was prepared by a pro-
[18]
cess previously reported by Lipshutz . Activated Carbon
2
(
3
100 mesh size, having surface area=1001 m /g and contains
0% H O) was provided by Sud-Chemie India Pvt. Ltd. Cu-
2
A
H
R
U
G
ed, India. From AAS studies the copper loading was found
to be 3.07 wt% of Cu. Rest of the chemicals were purchased
from Aldrich and used as received. All solvents were pur-
chased as analytical grade and used as received from Merck
India Pvt. Ltd. Dry toluene was used for all reactions.
ACME silica gel (100–200 mesh) was used for column chro-
matography and thin layer chromatography was performed
on Merck precoated silica gel 60-F254 plates.
All reactions were conducted at À208C in dry toluene
1
under nitrogen atmosphere. The H NMR spectra were re-
corded on Varian Gemini 200 MHz and Bruker Avance
Figure 3. XPS high resolution narrow scans of Cu 2p3/2 for
3
00 MHz spectrometers. Chemical shifts (d) are reported in
(
a) fresh nano-CuO, (b) nano-CuO treated with BINAP, (c)
ppm, using TMS as an internal standard in CDCl . High per-
3
nano-CuO treated with BINAP and silane, and (d) used
nano-CuO.
formance liquid chromatography (HPLC) was performed
using the following apparatus; Shimadzu LC-10 AT (liquid
chromatograph), Shimadzu SPD-10 A (UV detector), and
Shimadzu C-R6A Chromatopac. Optical rotations were ob-
[16]
dride. X-ray photoelectron spectroscopic (XPS) in- tained on Jasco P-1020 Polarimeter and reported in absolute
temperature
vestigations of fresh nano-CuO and nano-CuO treat- reactions: [a]D
(concentration c=g/100 mL of sol-
ed with BINAP ligand at the Cu 2p level show 2p3/2 vent). X-ray photoelectron spectra (XPS) were recorded on
a Kratos AXIS 165 with a dual anode (Mg and Al) appara-
lines at 934.4 eV and 934.2 eV, respectively, which cor-
[
25]
tus using the Mg-Ka anode. X-ray powder diffraction
respond to the +2 oxidation state (Figure 3a),
(
XRD) data were collected on a Simens D-5000 diffractom-
whereas XPS spectra of nano-CuO treated with
BINAP and silane under an inert atmosphere show
two Cu 2p3/2 peaks at 934.4 and 933.1 eV, which corre-
spond to Cu in +2 and +1 oxidation states, respec-
tively (Figure 3c). The approximate amounts of
eter using Cu-Ka radiation. The external morphologies of
fresh and used catalyst were observed on a Philips TECNAI
F12 FEItransmission electron microscope (TEM).
copper in the +1 and +2 oxidation states were found General Procedure for the Asymmetric
to be 57% and 43%, respectively. from the area Hydrosilylation Reaction
under the curve of the Cu 2p XPS high resolution
Nano-CuO (0.010 g) and (S)-BINAP (0.1 mmol, 0.062 g)
narrow scan. The copper hydride species reacts with
the ketone resulting in the formation of a copper alk-
were placed in a dried Schlenk flask containing dry toluene
3 mL) at room temperature and stirred for 1 h under nitro-
(
oxide that subsequently undergoes s-bond metathesis gen atmosphere. The resulting mixture was cooled to À208C
[
16]
with the organosilane to afford the silyl ether.
and PhSiH (2 equivs.) was added. To the reaction mixture,
3
Adv. Synth. Catal. 2007, 349, 1797 – 1802
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asc.wiley-vch.de
1801

M. Lakshmi Kantam et al.
UPDATES
ketone (2 mmol) was added at that temperature and stirring
was continued. After completion of the reaction, (as moni-
tored by TLC), the reaction mixture was centrifuged to sep-
arate the catalyst. The catalyst was washed with ether and
was reused for another cycle after vacuum drying. The reac-
[8] H. Brunner, W. Miehling, J. Organomet. Chem. 1984,
275, C17.
[9] a) W. S. Mahoney, D. M. Brestensky, J. M. Stryker, J.
Am. Chem. Soc. 1988, 110, 291; b) T. M. Koenig, J. F.
Daeuble, D. M. Brestensky, J. M. Stryker, Tetrahedron
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tion mixture was quenched with water with the aid of Et O
2
(
(
10 mL), and then tetrabutylammonium fluoride solution
TBAF) (1.0M in THF, 1.2 mL) was added. The reaction
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2000, 56, 2789.
mixture was stirred vigorously for 0.5 h. The organic layer
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Hiyama, Tetrahedron 1999, 55, 4573; b) B. H. Lipshutz,
W. Chrisman, K. Noson, P. Papa, J. A. Sclafani, R. W.
Vivian, J. M. Keith, Tetrahedron 2000, 56, 2779.
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was separated and dried over MgSO and the solvent was
4
removed under reduced pressure. The residue was purified
by silica gel flash column chromatography to afford pure
1
(
S)-1-phenylethanol; yield: 85%. H NMR (300 MHz,
CDCl ): d=1.50 (d, 3H, J=6.6 Hz, CH ), 1.95 (br, 1H,
3
3
OH), 4.89 (q, 1H, J=6.6 Hz, CH), 7.22–7.40 (m, 5H, ArH);
HPLC (Diacel Chiralcel OJ-H, 5% 2-propanol in hexane,
À1
flow rate 0.5 mLmin ): t =22.252 min (major), 24.301 min
R
(
minor).
[
14] a) B. H. Lipshutz, K. Noson, W. Chrisman, J. Am.
Chem. Soc. 2001, 123, 12917; b) B. H. Lipshutz, A.
Lower, K. Noson, Org. Lett. 2002, 4, 4045; c) B. H. Lip-
shutz, K. Noson, W. Chrisman, A. Lower, J. Am. Chem.
Soc. 2003, 125, 8779.
Acknowledgements
[
15] S. Sirol, J. Courmarcel, N. Mostefai, O. Riant, Org.
We wish to thank the CSIR for financial support under the
Task Force Project COR-0003 and Soumi Laha and Jagjit
Yadav thank CSIR, India for their fellowships. Nanocrystal-
line CuO samples were obtained from NanoScale Materials
Inc., Manhattan, KS 66502, USA.
Lett. 2001, 3, 4111.
[
[
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[
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