Asymmetric hydrosilylation of aromatic ketones catalyzed by an economical and effective copper-diphosphine catalytic system in air

Article abstract of DOI:10.1002/cjoc.201500072

In the presence of the inexpensive and non-toxic polymethylhydrosiloxane, the combination of copper(II) acetate and a chiral diphosphine displayed high catalytic efficiency in the asymmetric hydrosilylation of a series of aromatic ketones in air atmospher

Full text of DOI:10.1002/cjoc.201500072

FULL PAPER
DOI: 10.1002/cjoc.201500072
Asymmetric Hydrosilylation of Aromatic Ketones Catalyzed by
an Economical and Effective Copper-Diphosphine
Catalytic System in Air
Minting Liang,a Xiaofeng Xia,a Xiang Liu,*,a and Hexing Lib
a Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material
Engineering, Jiangnan University, Wuxi, Jiangsu 214122, China
b Key Laboratory of the Chinese Ministry of Education in Resource Chemistry, Shanghai Normal University,
Shanghai 200234, China
In the presence of the inexpensive and non-toxic polymethylhydrosiloxane, the combination of copper(II) ace-
tate and a chiral diphosphine displayed high catalytic efficiency in the asymmetric hydrosilylation of a series of
aromatic ketones in air atmosphere and at room temperature. (R)-1-Arylethanols were obtained with up to 99%
yield and 93% enantiomeric excess. Meanwhile, the electron effect and steric hindrance of substituents on the aro-
matic ring had an interesting influence on both the yields and enantioselectivities. Furthermore, a possible mecha-
nism was presented to explain the influence of some key factors on the reaction.
Keywords asymmetric catalysis, hydrosilylation, ketones, copper salt, polymethylhydrosiloxane
Introduction
In fact, in most cases, the catalytic systems for hy-
drosilylation required harsh-low temperature as well as
exclusion of moisture and oxygen. And those strict con-
ditions for hydrosilylation greatly limited the wide-
spread application of AHS. Hence, chemists have never
stopped developing an economical and practical AHS
catalyst system which can display high efficiency and
stability in air atmosphere and at room temperature.
Polymethylhydrosiloxane (PMHS), a by-product of
the organosilicon industry, has been well known for its
low-cost, non-toxicity and air stability.[52] It is a desir-
able hydride donor for economical, environmentally
friendly and practical reduction processes. The combi-
nation of Cu(OAc)2•H2O and chiral ligand 2,2'-bis-
(diphenylphosphino)-1,1'-binaphthalene (BINAP)[53,54]
for the AHS has much potential for the practical appli-
cation because of its high catalytic efficiency and low
cost. Therefore, we became particularly interested in
probing the efficiency of the catalytic system by em-
ploying PMHS as a stoichiometric hydride source and
Cu(OAc)2•H2O/BINAP as catalyst. Herein, the eco-
nomical and effective (R)-BINAP/Cu(OAc)2•H2O/
PMHS system for the asymmetric reduction of aromatic
ketones was investigated in air atmosphere and at room
temperature. The impacts of the electronic effect and
steric hindrance of the substituents on the aromatic ring
on the AHS reaction were also studied.
Asymmetric hydrosilylation (AHS) of prochiral ke-
tones represents one of the most efficient methods for
the preparation of chiral secondary alcohols which are
very important intermediates in organic synthesis and
are used widely in the fields of pharmaceuticals, agro-
chemicals, fragrance and flavor.[1] Distinguished from
other methods of asymmetric reduction (hydrogena-
tion[2,3] and hydrogen transfer[4,5]), AHS does not need
high-pressure condition and it has a good operability.[6-8]
AHS of prochiral ketones has been known since the
early 1970s.[9] Since then, the AHS catalyzed by pre-
cious metals such as ruthenium,[10-13] rhodium[14-17] or
iridium[18-21] has been well established. Recently, AHS
reactions using cheaper and less toxic metals such as
titanium,[22-24] zinc,[25-28] iron[29-32] and copper[33-36] have
emerged. Copper complexes are particularly attractive
owing to their efficiency and environmental benignancy.
The first report on the use of copper hydride along with
a chiral phosphine for AHS was published by Brunner
and Miehling in 1984.[37] Subsequent researches about
the AHS based on CuCl/t-BuONa/chiral ligands[38-42]
led to the development of the copper-catalyzed AHS.
Moreover, other copper sources such as copper
fluoride[43-46] and copper acetate[47-51] were studied as
useful catalyst precursors in the copper-catalyzed AHS.
*
E-mail: liuxiang@jiangnan.edu.cn
Received January 22, 2015; accepted March 20, 2015; published online May 5, 2015.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201500072 or from the author.
578
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Chin. J. Chem. 2015, 33, 578—582

Asymmetric Hydrosilylation of Aromatic Ketones
Experimental
Results and Discussion
In an initial study, acetophenone was chosen as a
reference substrate. (R)-BINAP was used as chiral
ligand and PMHS as reducing agent. The influence of
various Cu(I) and Cu(II) precursors on the asymmetric
hydrosilylation (AHS) was examined and the results
were shown in Table 1. There was no great difference in
the catalytic capabilities between Cu(I) and Cu(II) (Ta-
ble 1, Entries 1 and 2). And similar results on the
asymmetric hydrosilylation of acetophenone catalyzed
by Cu(I) were reported by Issenhuth.[55] In Table 1,
Cu(OAc)2•H2O was the best catalyst precursor for the
AHS (Table 1, Entries 4 and 5). Besides, the additive
t-BuOK, which was expected to improve the activation
of the catalytically active species, did not show the good
effect as reported.[56,57] In some cases, the side products
such as KCl, KNO3 and KOAc (Table 1, Entries 1－4)
which were produced by t-BuOK inhibited the catalytic
activity in the hydrosilylation.[58] In fact, without the
additive t-BuOK, Cu(OAc)2•H2O led to the increase of
the yield (Table 1, Entry 5) because the acetate ion was
similar to t-BuO− and could be a good nucleophile for
the activation of the catalytically active species.
(R)-BINAP and polymethylhydrosiloxane (PMHS)
were purchased from Aladdin Reagent Database Inc.
(Shanghai, China). All other chemicals and reagents
were purchased in analytical grade from Sinopharm
Chemical Reagent Co. Ltd., China. The GC (7890A,
Agilent, USA) was equipped with FID detector and
CP-Chiralsil-DEX chiral capillary column (25 m×0.25
mm; Varian Inc., USA). IR analysis was carried out on
a Nicolet 6700 spectrometer. 1H NMR analysis was car-
ried out on a Varian Unity-300 MHz spectrometer.
General procedure for the copper-catalyzed asym-
metric hydrosilylation of aromatic ketones
Cu(OAc)2•H2O (0.0060 g, 3 mol%), (R)-BINAP
(0.0064 g, 1 mol%) and toluene (3 mL) were placed in a
25 mL round-bottom flask. The mixture was stirred at
25 ℃ for 1 h. PMHS (3 mmol, 0.18 mL) was then
added dropwise. And after 30 min, the aromatic ketone
(1 mmol) was added to the reaction mixture. After com-
pletion of the reaction (monitored by TLC), the reaction
was quenched with MeOH (1 mL) and 2.5 mol/L NaOH
(3 mL), and the mixture was stirred vigorously for 3 h.
The mixture was extracted with CH2Cl2 (5 mL×3) and
the combined organic layer was washed with water,
dried over anhydrous sodium sulfate. The solvent was
removed under vacuum and the residue was purified by
silica gel flash column chromatography (eluent, petro-
leum ether/ethyl acetate) to afford the desired product.
Table 2 Effects of reaction temperature on the copper-catalyzed
asymmetric hydrosilylation of acetophenonea
3 equiv. PMHS
1 mol% (R)-BINAP
O
OH
3 mol% Cu(OAc)2·H2O
MeOH
Toluene, air
NaOH (aq.)
1
All products were characterized by IR and H NMR
spectra. The yields and ees of products were determined
by chiral GC analysis (see Supporting Information).
Entry Temperature/℃
Time/h
Yield/%
99
eeb/%
1
2
3
4
5
6
7
8
55
45
35
25
15
15
0
4
73
75
78
78
79
79
82
82
4
96
Table 1 Effects of copper salt precursors on the asymmetric
hydrosilylationa
4
96
3 equiv. PMHS
[Cu]/t-BuOK
4
94
O
OH
4
65
1 mol% (R)-BINAP
MeOH
17
17
36
88
Toluene, 25 °C, air, 4 h
NaOH (aq.)
51
0
93
a Reaction conditions: acetophenone (1 mmol), PMHS (3 mmol),
toluence (3 mL). b Absolute configuration was determined to be
(R).
PPh2
PPh2
(R)-BINAP
The effect of reaction temperature on the asymmetric
hydrosilylation of acetophenone was also investigated in
the presence of (R)-BINAP/Cu(OAc)2•H2O/PMHS in
air atmosphere and the results were demonstrated in
Table 2. It took only 4 h to obtain good yields when the
reaction temperature was between 55 ℃ and 25 ℃
(Table 2, Entries 1－4) while much more time would be
needed when the temperature was reduced (Table 2,
Entries 5－8). In fact, increasing reaction temperature
could conquer the activation energy barrier which was
required for the formation of the transition states. It was
Entry Cu precursor/mol%
t-BuOK/mol% Yield/% eeb/%
1
2
3
4
5
CuCl (3)
3
3
3
3
0
41
47
79
75
－
78
78
CuCl2•2H2O (3)
Cu(NO3)2•3H2O (3)
Cu(OAc)2•H2O (3)
Cu(OAc)2•H2O (3)
＜1
66
94
a Reaction conditions: acetophenone (1 mmol), PMHS (3 mmol),
toluence (3 mL). b Absolute configuration was determined to be
(R).
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Liang et al.
FULL PAPER
favorable to the formation of the silyl ether and the re-
generation of the active species which were the rate de-
termining steps.[59] The change of ees of the product
(R)-1-phenylethanol was smaller when the reaction
temperature was reduced from 55 to 0 ℃. Considering
the reaction time and the efficiency of the hydrosilyla-
tion, the reaction temperature of 25 ℃ was chosen.
Since most hydrosilylation required anaerobic at-
mosphere,[60-62] the influence of air on the hydrosilyla-
tion was examined. Figure 1 demonstrated the effi-
ciency of the AHS catalyzed by the (R)-BINAP/
Cu(OAc)2•H2O/PMHS system in air and nitrogen at-
mosphere. The results showed that compared with ni-
trogen, air just slowed slightly down the reaction rate
and the 94% yield was also achieved after 4 h. The ee of
(R)-1-phenylethanol remained unchanged (78% ee) in
the presence of air or nitrogen. It was similar to Riant’s
results[45,63] that the copper-catalyzed AHS could be
carried out efficiently in the presence of oxygen or air.
In the experiment, it was shown that the (R)-BINAP/
Cu(OAc)2•H2O/PMHS system in the presence of air was
also efficient for the AHS.
stituents on the aromatic ring, for example, chloro-
group in Entries 2－4. The highest ee value was
achieved when the chloro-group was present at the
para-position of carbonyl group of acetophenone (Table
3, Entry 4), and the lowest ee value was obtained when
the chloro-group was present at the ortho-position (Ta-
ble 3, Entry 2). It was shown that the steric hindrance of
the ortho-substituent on the aromatic ring had an obvi-
ous impact on the ees of the product (R)-alcohols. The
results were different from the report published by Shi-
mizu et al. where the highest ee was obtained in the case
of ortho-substituted substrates.[67]
Table 3 Effects of reaction temperature on the copper-catalyzed
asymmetric hydrosilylation of acetophenonea
3 equiv. PMHS
1 mol% (R)-BINAP
O
OH
3 mol% Cu(OAc)2·H2O
MeOH
Toluene, 25 °C, air
NaOH (aq.)
R
R
Entry
R
H
Reaction time/h
Yield/%
94
eeb/%
1
2
3
4
5
6
7
8
9
4
78
62
81
87
93
78
78
82
79
o-Cl
4
99
m-Cl
4
99
p-Cl
4
99
p-NO2
p-CH3
p-CH3
p-OCH3
4
99
4
67
20
4
88
37
p-OCH3 36
73
a Reaction conditions: acetophenone (1 mmol), PMHS (3 mmol),
toluence (3 mL). b Absolute configuration was determined to be
(R).
Figure 1 Effects of air on the copper-catalyzed hydrosilylation
of acetophenone. Reaction conditions: acetophenone (1 mmol),
Cu(OAc)2•H2O (3 mol%), (R)-BINAP (1 mol%), PMHS (3
mmol), toluence (3 mL), 25 ℃.
To better illustrate the effects of some factors dis-
cussed about the hydrosilylation, a possible mechanism
of the copper-catalyzed hydrosilylation was depicted in
Scheme 1. In the initial step, upon combining
(R)-BINAP with Cu(OAc)2, a chiral complex (A) was
likely formed. Then (A) could react with PMHS to form
the copper hydride species (B) via a transition state (TS1)
where the acetate ion attacked the Si－H bond as nu-
cleophile. It suggested that the acetate ion favored the
formation of (B). When t-BuOK was added, t-BuO−
would compete with AcO−. This competition was not
conductive to the formation of TS1 and (B), and ulti-
mately affected the yields (see the results of Table 1).
The active species (B) would have a tendency to aggre-
gate for a dimeric species (C).[68] At this stage we were
not sure whether the active catalytic species (B) was
copper(I) or copper(II) hydride.[69] In fact, CuCl and
CuCl2 had the similar catalytic effects which could be
observed in Table 1. Next, the stereoselective insertion
of C＝O of ketone into the Cu－H bond occured, which
involved a four-membered transition state (TS2), leading
to the formation of the copper alkoxide intermediate (D).
The electronic effect and steric hindrance of the sub-
stituents on the aromatic ring played an important role
in AHS.[64-66] Hence, the AHS of a series of aromatic
ketones was investigated and the corresponding alcohols
were obtained in good ees and yields. The results were
listed in Table 3. Introduction of an electron-with-
drawing group on the aromatic ring was better for the
yields (Table 3, Entries 2－5). A higher ee was obtained
when the para-position of carbonyl group was substi-
tuted by an electron-withdrawing group such as chloro
and nitro groups. However, the presence of an electron-
donating group at the para-position (Table 3, Entries
6－9) had little effect on enantioselectivity but was bad
for the yields and much more reaction time was re-
quired.
Another important result was that the ees of product
(R)-alcohols were dependent on the position of sub-
580
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© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2015, 33, 578—582

Asymmetric Hydrosilylation of Aromatic Ketones
Scheme 1 Possible mechanism of the asymmetric hydrosilylation catalyzed by copper(II) acetate
This was the enantioselectivity determining step (Step
1). As calculated by Issenhuth,[70] in the reaction of (B)
with the aromatic ketone, the most stable diastereoi-
someric form of the transition state TS2 was the (R) one
[(R)-TS2]. The (R)-TS2 generated the diastereoisomer
(R)-(D) and ultimately led to the formation of the
(R)-1-arylethanols (F). As for the AHS of 2'-chloro-
acetophenone (Table 3, Entry 2), the large steric hin-
drance between the phenyl group of the ligand and the
ortho-substituent of the aromatic ring in 2'-chloro-
acetophenone weakened the stability of the (R)-TS2 and
resulted in the low ee of the product (R)-1-2'-chlorophe-
nylethanol. Subsequently, (D) reacted with PMHS to
form the silyl ether (E) via a transition state (TS3) and
the active species (B) was regenerated in cycle. This
was the rate determining step of the reaction (Step 2).
And further hydrolysis of the silyl ether (E) was needed
to afford the product alcohol (F). Scheme 1 also showed
that the presence of air did not substantially affect the
reaction.
cient catalyst system generated in situ from Cu(OAc)2•
H2O and a commercial diphosphine ligand along with
the inexpensive non-toxic reducing agent PMHS in air
atmosphere and at room temperature. This system al-
lowed the AHS of a series of aromatic ketones with
good ees (up to 93%) and excellent yields (up to 99%).
The electronic effect and the steric hindrance of sub-
stituents on the aromatic ring had a clear influence on
the yield and enantioselectivity of the asymmetric hy-
drosilylation. A plausible mechanism of the copper-
catalyzed hydrosilylation was depicted and confirmed
the effects of some factors on the AHS.
Acknowledgement
This research was financially supported by the Na-
tional Natural Science Foundation of China (No.
21402066) and the Open Project of the Key Laboratory
of the Chinese Ministry of Education in Resource
Chemistry.
Supporting Information
Conclusions
Available experimental details, IR, 1H NMR spectra,
In conclusion, we developed an economical and effi-
Chin. J. Chem. 2015, 33, 578—582
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