Hydrosilylation of ketone and imine over poly-N-heterocyclic carbene particles

Article abstract of DOI:10.1002/adsc.200800815

N-Heterocyclic carbene (NHC)-catalyzed ketone/imine hydrosilylation, silane alcohol condensation and asymmetric ketone hydrosilylation reactions were demonstrated for the first time over solid, main-chain poly-NHC particles. The stable and robust poly-NHC

Full text of DOI:10.1002/adsc.200800815

FULL PAPERS
DOI: 10.1002/adsc.200800815
Hydrosilylation of Ketone and Imine over Poly-N-Heterocyclic
Carbene Particles
a
a,
a,
MeiXuan Tan, Yugen Zhang, * and Jackie Y. Ying *
a
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669, Singapore
Fax : (+65)-6478-9020; e-mail: ygzhang@ibn.a-star.edu.sg or jyying@ibn.a-star.edu.sg
Received: December 29, 2008; Revised: April 24, 2009; Published online: June 3, 2009
Abstract: N-Heterocyclic carbene (NHC)-catalyzed chiral induction protocol with a cheap and easily ac-
ketone/imine hydrosilylation, silane alcohol conden- cessible secondary alcohol as the chiral source was
sation and asymmetric ketone hydrosilylation reac- also developed in this catalytic system.
tions were demonstrated for the first time over solid,
main-chain poly-NHC particles. The stable and
robust poly-NHC particles were easily recovered, Keywords: heterogeneous catalysis; hydrosilylation;
and exhibited good catalytic recyclability. A novel N-heterocyclic carbenes; organocatalysis
Introduction
cohol condensation and asymmetric ketone hydrosily-
lation reactions using solid poly-NHC particles.
Although organocatalysis has developed rapidly in
recent years, relatively little effort has been devoted
[
1,2]
towards heterogeneous organocatalysis. In general, Results and Discussion
active sites in polymeric catalysts are derived via im-
mobilization or polymerization at a side chain. It is As shown in Scheme 1, poly-imidazolium salt 1 was
quite common in nature for a main-chain polymer to synthesized by condensation of bisimidazole and
function as a catalyst, but this is rarely designed in 2,4,6-tris(bromomethyl)mesitylene in dimethylforma-
[
3]
synthetic polymers. We recently created main-chain mide (DMF) at 1108C for 24–72 h. Spherical particles
poly-N-heterocyclic carbene (poly-NHC) polymer ma- (1.2 mm in diameter) of poly-imidazolium 1 were ob-
[
4]
terials and their poly-organometallic derivatives were tained in 82% yield. Longer reaction times resulted
demonstrated to be excellent heterogeneous cata- in more stable and robust polymer particles with a
[4]
lysts. Herein, the catalytic applications of the stable higher imidazolium component (less terminal groups)
free carbene polymer materials were studied. in the polymer network structure. NHC solid particles
Ketone or imine reduction by hydrogenation, hy- 2 were generated by a basic treatment of 1 (see
droboration and hydrosilylation is one of the most
[
5]
ubiquitous protocols in organic synthesis. Although
hydrogenation is widely practiced industrially, it faces
shortcomings such as, metal leaching, high-pressure
requirements, expensive catalysts, and difficulty in
[
6]
catalyst recycling. Developing effective metal-free
organocatalysts for the hydrosilylation of ketones and
[
7]
imines has attracted great interest. In comparison, a
robust and simple heterogeneous organocatalyst for
ketone hydrosilylation would be even more desirable
since it could be both environmentally friendly and
more economical. NHCs have been known as power-
ful nucleophilic organocatalysts, and are used in cata-
[
8,9]
lyzing the silylcyanation reactions of ketones.
In
this article, we demonstrated for the first time NHC-
catalyzed ketone and imine hydrosilylation, silane al-
Scheme 1. Synthesis of poly-NHC particles.
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Hydrosilylation of Ketone and Imine over Poly-N-Heterocyclic Carbene Particles
FULL PAPERS
[4]
Scheme 1). Typically, NaO-t-Bu (40 mg, 0.5 mmol)
was added to a DMF (10 mL) suspension of 1
(
150 mg) in a reaction flask. The reaction mixture was
stirred for 16 h. The solid product was filtered, and
washed with DMF to remove NaBr salt to obtain a
yellowish brown powder 2 (95 mg). The poly-NHC
13
particles 2 were characterized by C nuclear magnetic
resonance (NMR) spectroscopy, Fourier-transform in-
[
4]
frared (FT-IR) spectroscopy, and elemental analysis.
Scheme 2. Poly-NHC-catalyzed cyanation and hydrosilyla-
tion reactions.
The average carbene content in polymer 2 was
2 mmolg based on elemental analysis and metal
À1
~
[
4]
coordination experiments.
Solid particles 2 were first tested as a novel hetero- slower. However, full conversion could be obtained in
geneous organocatalyst for the cyanation reaction be- 45 min in DMF at similar conditions (Table 1, en-
tween trimethylsilyl cyanide (TMSCN) and carbonyl tries 4–6). Prompted by this result, NHC was also in-
compounds. Catalyst 2 generated in situ and used as vestigated for ketone hydrosilylation reactions
isolated particles demonstrated excellent catalytic ac- (Scheme 2).
tivities in this reaction at room temperature. Full con-
Firstly, the hydrosilylation of acetophenone with
versions were achieved in 10 min for aldehyde sub- triethylsilane was tested and, unfortunataly, no reac-
strates with 1 mol% of catalyst loading in tetrahydro- tion was observed in THF at room temperature in
furan (THF), and the colloidal particles were also 24 h. When the more active diphenylsilane was em-
easily recovered and reused (Table 1, entries 1–3). ployed, the hydrosilylation reaction proceeded
The reaction for ketone substrates in THF was much smoothly under ambient conditions in THF. Full con-
versions were achieved in 10–36 h at room tempera-
ture for a wide range of ketone substrates with
[
a]
Table 1. Heterogeneous TMS cyanation reactions over 2.
10 mol% of solid 2 particles (Table 2). The poly-NHC
catalyst was easily recycled by filtration. The washed
and reused catalyst showed similar activity as the
fresh catalyst (Table 2, entries 1–3). Both aryl ketones
and alkyl ketones were found to be active for this re-
action (Table 2). When DMF was used as the solvent
instead of THF, the reaction proceeded very rapidly,
and a product mixture of 1-phenylethanol, diphen-
yl(1-phenylethyloxy)silane and diphenyldi(1-phenyl-
[b]
Entry
1
Substrate
Solvent
THF
Time [min]
10
Yield [%]
99
AHCTUNGTRENNUGNe thyloxy)silane was obtained. This suggested that the
polar solvent DMF increased the activity of NHC,
causing part of the product to be over-reduced by
silane. On the other hand, the reaction did not work
in dichloromethane (DCM) and toluene. Further-
more, the system was successfully extended to imine
hydrosilylation (Table 2, entry 10). In most hydrosily-
lation systems, excess silane (2–5 equiv. to substrate)
was required for the metal complexes or organocata-
lysts employed. In our case, only 1 equiv. of silane
was needed, and a quantitative yield of product was
attained. Since this reaction worked under mild condi-
tions and in a well-controlled manner, its extension to
asymmetric reactions could be promising.
[
[
c]
2
3
4
5
6
THF
THF
DMF
DMF
DMF
10
10
30
45
45
99
99
99
99
99
d]
[
e]
In asymmetric organic reactions, the chirality of
products could be derived from the chiral starting ma-
terials or chiral catalysts. Almost all asymmetric hy-
drosilylation reactions were catalyzed by chiral cata-
[
a]
Typical reaction conditions: 1 mol% of catalyst, 1.2 mmol
of TMSCN, 1 mmol of substrate in THF or DMF (1 mL),
room temperature.
Yields were determined by GC and GC-MS.
Recycled catalyst of entry 1.
[
6]
[
[
[
[
b]
c]
d]
e]
lysts. A chiral silane was very rarely used due to the
[
10]
difficulty in its preparation.
Scheme 3 illustrates a
Recycled catalyst of entry 2.
novel chiral induction protocol for asymmetric hydro-
The products were a mixture of 1,2-addition (70%) and silylation over non-chiral poly-NHC particles. The
1,4-addition (30%).
chirality of the products was transferred from a chiral
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MeiXuan Tan et al.
FULL PAPERS
Table 2. Heterogeneous ketone and imine hydrosilylation silane intermediate was generated through silane/al-
[
a]
with diphenylsilane over 2.
cohol condensation in situ; this was then used to
induce the asymmetric hydrosilylation.
[b]
Entry Substrate
Product
Yield [%]
99
The key challenge for this approach was to gener-
ate the chiral silane intermediate efficiently and
easily. Silane/alcohol dehydrogenative condensation
reactions can be catalyzed by base, Lewis acid and
1
2
3
[
11]
many organometallic complexes. Silanolysis of alco-
hol with R SiH was of particular interest to us since
[
[
c]
99
99
2
2
it could generate chiral silane R ACHTUNGTERNNNU(G R’O)SiH when a
2
chiral secondary alcohol was employed. However, the
selective production of mono-substituted silane
R ACHTUNGTERNNNU(G R’O)SiH was still a great challenge. Herein, poly-
2
d]
NHC was developed as a heterogeneous organocata-
lyst for the silanolysis of secondary alcohols with
Ph SiH under very mild conditions. The reaction was
4
5
99
99
2
2
highly selective in producing the mono-substituted
silane Ph (R’O)SiH in quantitative yield with dihydro-
AHCTUNGTRENNUNG
2
gen as the only by-product. The chiral silane product
of this silane/alcohol condensation reaction could pro-
ceed directly to the ketone hydrosilylation reaction
over poly-NHC upon the addition of a ketone sub-
strate. As (À)-menthol was used as the chiral alcohol,
the hydrosilylation of acetophenone with diphenylsi-
lane over 2 produced (R)-1-phenylethanol in 40% ee.
When (+)-menthol was used in this reaction, the
product was in the (S) form with a similar ee value.
The proposed reaction mechanism is shown in
Scheme 4. In cycle A, silane was activated by nucleo-
philic NHCs, and then reacted with a secondary alco-
hol to generate siloxane 3 and dihydrogen. The high
selectivity for forming mono-substituted silane 3
might be due to the steric requirements of the bulky
alcohol and solid polymer catalyst. In cycle B, the
ketone might be activated by the poly-NHC to form
intermediate 4, and further reacted with chiral diphe-
nylsiloxane 3 to form product 6.
6
7
99
99
8
9
99
99
99
1
0
[
a]
Reaction conditions for ketone: 10 mol% of poly-NHC,
.2 mmol of ketone, 0.2 mmol of diphenylsilane, 1 mL of
0
THF, room temperature, 10–36 h. DMF was used for
imine hydrosilylation.
Products were confirmed by H NMR spectroscopy and
Conclusions
[
b]
1
In conclusion, this study has demonstrated that
ketone and imine hydrosilylation reactions could pro-
ceed very smoothly and cleanly over poly-NHC parti-
cles. This novel heterogeneous catalyst was easily re-
GC-MS. Yield was determined by GC and GC-MS.
Recycled catalyst of entry 1.
Recycled catalyst of entry 2.
[
[
c]
d]
secondary alcohol such as menthol (which is inexpen- cycled for reuse. Only 1 equiv. of silane was needed,
sive and easily obtained from natural products) and a quantitative product yield was attained under
through the silane. In this proposed approach, a chiral mild conditions. Poly-NHC was also an excellent cata-
Scheme 3. Asymmetric hydrosilylation reactions over 2.
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Hydrosilylation of Ketone and Imine over Poly-N-Heterocyclic Carbene Particles
FULL PAPERS
Scheme 4. Proposed mechanism for asymmetric hydrosilylation reactions.
lyst for the dehydrogenative condensation between si- suspension was stirred at room temperature for 2 h. a,a’-Di-
lanes and alcohols. The reaction could be extended to chloro-p-xylene (5 mmol) was added to the residue. The re-
sulting solution was stirred at room temperature for another
an asymmetric version with inexpensive and readily
accessible secondary alcohols as the chiral source.
4
h. The solvent was removed under vacuum. The product
was extracted with DCM, and A was obtained in quantita-
1
tive yield after removing the solvent. H NMR (CDCl ): d=
3
7
4
.55 (s, 2H), 7.13 (s, 4H), 7.10 (s, 2H), 6.89 (s, 2H), 5.12 (s,
H); MS (GC-MS): m/z=238 (M ).
Experimental Section
+
General Information
Preparation of 1
All solvents and chemicals were used as received from com-
mercial suppliers, unless otherwise indicated. Centrifugation
was performed on an Eppendorf Centrifuge 5810R
In
a
pressure flask, 2,4,6-tris(bromomethyl)mesitylene
(
399 mg, 1 mmol) and A (357 mg, 1.5 mmol) were dissolved
1
13
in 100 mL of DMF. The flask was capped and placed in an
oven at 1108C for 24 h. A white solid product was precipi-
tated in the reaction flask, filtered, washed with DMF and
ether, and dried under vacuum. It was collected as a white
powder; yield: 620 mg (82%). Elemental analysis, found: C
48.17, H 4.97, N 9.85. These results corresponded to a poly-
mer with bromo-dominated end group (calcd.: C 50.48, H
(
4000 rpm, 10 min). H and C NMR spectra were recorded
on Bruker AV-400 (400 MHz) instrument. Photoacoustic
FT-IR (PA-FT-IR) spectra were recorded on a Digilab FTS
7000 FTIR spectrometer equipped with a MTEC-300 photo-
acoustic detector. Data for PA-FT-IR are reported in wave-
À1
length (cm ) and intensity (s=strong, m=medium, w=
weak). GC-MS was performed on a Shimadzu GCMS
QP2010. Gas liquid chromatography (GLC) was performed
on an Agilent 6890N gas chromatographs equipped with
split-mode capillary injection system and flame ionization
detector. Elemental analysis (C, H, N) was performed on an
EAI CE-440 Elemental Analyzer. SEM images were ob-
tained on a JEOL JSM-7400F electron microscope (10 kV).
1
3
4
1
1
.64, N 9.81); C NMR (solid): d=18 (CH ), 48–52 (CH ),
3 2
+
15–145 (C=C, C=N); PA-FT-IR n=1150 (s, NR ),
558 cm (s, C=N, C=C).
4
À1
Since the polymer products were insoluble in common
solvents, their molecular weights could not be determined
definitively. When water/methanol (volume ratio=4:1) was
used to extract polymer 1, only 5% of the polymer was dis-
solved. Gel permeation chromatography (GPC) indicated
an average molecular weight (MW) of 31,000 for the soluble
portion.
Preparation of Bisimidazole A
NaH (60% in oil, 440 mg, 11 mmol) was added to a DMF
solution of imidazole (680 mg, 10 mmol), and the resulting
Preparation of 2
NaO-t-Bu (40 mg, 0.5 mmol) was added to a DMF (10 mL)
suspension of 1 (150 mg) in a reaction flask. The reaction
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MeiXuan Tan et al.
FULL PAPERS
mixture was stirred for 16 h. The solid product was filtered, Acknowledgements
and washed with DMF (20 mL) 5 times at room tempera-
ture to remove NaBr salt to obtain a yellow powder 2;
yield: 95 mg. Elemental analysis found: C 58.01, H 6.05, N
This work was supported by the Institute of Bioengineering
and Nanotechnology (Biomedical Research Council, Agency
for Science, Technology and Research, Singapore).
12.25. These results corresponded to a polymer whereby the
imidazolium salt in 1 was transformed to the carbene form
1
3
(
(
calcd.: C 58.71, H 6.62, N 12.84); C NMR (solid): d=16.4
CH ), 48.2 (CH , CÀO), 128–163 (C=C), 218 (C carbene);
3
2
2
À1
PA-FT-IR: n=1558 cm (s, C=C).
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[
2
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(
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2
[
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0
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.57 (m, 1H), 2.18 (d, 1H), 1.4–1.6 (m, 7H), 0.84 (d, 3H),
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6
6
Next, acetophenone (0.18 mmol, 22 mL) was added to the
reaction vial, and the reaction solution was stirred at room
temperature for 72 h. Acetophenone was transformed to the
hydrosilylation product in excellent yield. The enantioselec-
tivity was measured using chiral GC (g-TA) after the prod-
uct had been transformed to alcohol. (R)-1-Phenylethanol
was produced in 40% ee.
1394
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ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 1390 – 1394