Highly stereoselective metathesis reaction between optically active hydrosilane and copper(I) salt in 1,3-dimethyl-2-imidazolidinone

Article abstract of DOI:10.1016/S0022-328X(98)00923-1

Highly stereoselective metathesis reactions between optically active silyl compounds and copper(I) salts in 1,3-dimethyl-2-imidazolidinone (DMI) are reported. The substitution reaction of (+)-α-naphthylphenylmethylsilane with a mixture of lithium tert-butoxide and copper(I) salt smoothly proceeded in DMI to give the corresponding silyl ether with a high degree of retention of the configuration in a quantitative yield. The use of DMI as a solvent and the presence of a chloride ion are necessary for this reaction. An optically active alkynylsilane also reacted with the mixed reagent to afford the corresponding silyl ether. A mechanism of oxidative addition of the hydrosilane and reductive elimination of the silyl ether on copper is proposed.

Full text of DOI:10.1016/S0022-328X(98)00923-1

Journal of Organometallic Chemistry 574 (1999) 102–106
Highly stereoselective metathesis reaction between optically active
hydrosilane and copper(I) salt in 1,3-dimethyl-2-imidazolidinoneꢀ
Hajime Ito, Tomoko Ishizuka, Tomoko Okumura, Hiroshi Yamanaka, Jun-ichi Tateiwa,
Motohiro Sonoda, Akira Hosomi *
Department of Chemistry, Uni6ersity of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
Received 23 June 1998; received in revised form 3 August 1998
Abstract
Highly stereoselective metathesis reactions between optically active silyl compounds and copper(I) salts in 1,3-dimethyl-2-imida-
zolidinone (DMI) are reported. The substitution reaction of (+)-h-naphthylphenylmethylsilane with a mixture of lithium
tert-butoxide and copper(I) salt smoothly proceeded in DMI to give the corresponding silyl ether with a high degree of retention
of the configuration in a quantitative yield. The use of DMI as a solvent and the presence of a chloride ion are necessary for this
reaction. An optically active alkynylsilane also reacted with the mixed reagent to afford the corresponding silyl ether. A
mechanism of oxidative addition of the hydrosilane and reductive elimination of the silyl ether on copper is proposed. © 1999
Elsevier Science S.A. All rights reserved.
Keywords: Stereoselective metathesis; Optically active; Hydrosilane
1. Introduction
metathesis reaction between an optically active hydrosi-
lane and copper(I) salts proceeds smoothly with a high
degree of retention of the configuration on the silicon
atom in the presence of a chloride ion at room temper-
ature (r.t) in DMI.
The metathesis reaction between copper(I) salts and
oraganometallic compounds has a wide applicability
for the preparation of organocopper(I) species [1,2].
Usually a relatively reactive organometallic species has
been used for this purpose. It has been accepted that
four co-ordinate organosilicon compounds do not re-
veal a sufficient reactivity against copper(I) salt to form
an organocopper(I) species [3]. However, we recently
reported metathesis reactions between organosilicon
compounds and copper(I) salts occurring smoothly in
1,3-dimethyl-2-imidazolidinone (DMI) [4–8]. In order
to investigate the reaction mechanism, we employed an
optically active hydrosilane in the reaction with various
copper(I) salts. In this paper we describe how the
2. Experimental details
2.1. General
1H- and 13C-NMR spectra were recorded for solu-
tions in CDCl3 using a JEOL model JNM-GX-270
spectrometer operating at 270 and 67.9 MHz, respec-
tively. IR spectra were recorded for neat using a Shi-
madzu IR-460 spectrometer. GC–MS spectra were
recorded using a Shimadzu QP-5000 mass spectrometer.
Optical rotation was measured with a JASCO DIP-360
ꢀ This paper is dedicated to the brilliant achievements of the late
Professor Rokuro Ogawara.
* Corresponding author. Tel.: +81-298-53-4237; fax: +81-298-53-
6503; e-mail: hosomi@staff.chem.tsukuba.ac.jp.
spectrometer. (+)-h-Naphthylphenylmethylsilane
1
and (+)-h-naphthylphenyl(phenylethynyl)methylsilane
8 were prepared by the reported procedure from (–)-h-
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)00923-1

H. Ito et al. / Journal of Organometallic Chemistry 574 (1999) 102–106
103
naphthylphenylmethylmethoxysilane [11,17]. Copper(I)
2.2.4. Preparation of (–)-h-naphthylphenylmethyl-tert-
tert-butoxide was prepared by the reported procedure
[13]. All operations were conducted under nitrogen
atmosphere. Solvents were dried by the standard
procedure.
butoxysilane 2 using copper(I) tert-butoxide and
tetrabutylammonium chloride
To a DMI (2.0 ml) suspension of copper(I) tert-bu-
toxide (280 mg, 2.06 mmol) and dry tetrabutylammo-
nium chloride (556 mg, 2.0 mmol), (+)-h-naphthyl-
phenylmethylsilane 1 (248 mg, 1.0 mmol) was added.
After stirring for 2 h at r.t. the reaction mixture was
diluted with chloroform. Filtration through a short-
2.2. Preparation of (–)-h-naphthylphenylmethyl-tert-
butoxysilane 2 using copper(I) salts
column
chromatography
(Frolisil®,
ethyl
ac-
2.2.1. Preparation of (–)-h-naphthylphenylmethyl-tert-
butoxysilane 2 using copper(I) chloride
etate:hexane=1:20) followed by purification using
column chromatography (SiO2, ethyl acetate:hexane=
1:100) gave (–)-h-naphthylphenylmethyl-tert-butoxysi-
lane 2 (285 mg, 89%, [h]D= −31.0° (c 4.6, pentane)).
tert-Butyl alcohol (111 mg, 1.5 mmol) was added to
a hexane solution of butyllithium (0.63 ml, 1.0 mmol).
After stirring for 10 min, the solvent and excess tert-
butyl alcohol were removed in vacuo. Copper(I) chlo-
ride (99 mg, 1.0 mmol) and DMI (1.0 ml) were added
to the resultant residue. The mixture was stirred for 10
min to give a pink suspension. Addition of (+)-h-
naphthylphenylmethylsilane 1 (124 mg, 0.5 mmol) gave
a black–red solution. After stirring for 2 h at r.t. the
reaction mixture was diluted with chloroform. Filtra-
tion through a short-column chromatography (Frol-
isil®, ethyl acetate:hexane=1:20) followed by
purification using column chromatography (SiO2, ethyl
acetate:hexane=1:100) gave (–)-h-naphthylphenyl-
methyl-tert-butoxysilane 2 (161 mg, 100%, [h]D= −
2.3. Reduction of (–)-h-naphthylphenylmethyl-tert-
butoxysilane 2
Reduction of (–)-h-naphthylphenylmethyl-tert-bu-
toxysilane 2 (985 mg, 3.06 mmol, [h]D= −31.7°) was
carried out under reported conditions [12]. After the
usual workup, purification using preparative thin-layer
chromatography (SiO2, hexane) gave (+)-h-naph-
thylphenylmethylsilane 1 (126 mg, 17%, [h]D= +30.9°
(c 5.7, pentane)).
2.4. Reaction of (+)-h-naphthylphenyl(phenylethynyl)
methylsilane 8 using the mixed reagent of copper(I)
chloride and lithium tert-butoxide
1
29.8° (c 2.3, pentane)); H-NMR(CDCl3) l=1.90 (3H,
s), 1.27 (9H, s), 7.25–7.51 (6H, m), 7.53–7.61 (2H, m),
7.79–7.91(3H, m), 8.19–8.22 (1H, m); 13C-NMR
(CDCl3) l=1.8, 32.4, 74.2, 125.4, 125.8, 125.9, 128.1,
129.1, 129.7, 129.8, 130.8, 133.9, 134.5, 134.9, 136.9,
137.4, 139.7; MS m/z (relative intensity) 320 (M+, 33),
305 (11), 249 (100). Anal. Calc. for C21H24OSi: C,
78.70; H, 7.55. Found: C, 78.49; H, 7.59.
tert-Butyl alcohol (111 mg, 1.5 mmol) was added to
a hexane solution of n-butyllithium (0.63 ml, 1.0
mmol). After stirring for 10 min the solvent and excess
t-butyl alcohol were removed in vacuo. Copper(I) chlo-
ride (99 mg, 1.0 mmol) and DMI (1.0 ml) were added
to the resultant residue. The mixture was stirred for 10
min to give a pink suspension. Addition of (+)-h-
naphthylphenyl(phenylethynyl)methylsilane 8 (130 mg,
0.373 mmol, [h]D= +5.3° (c 4.3, pentane), 58% ee)
gave a black–red solution. After stirring for 45 h at r.t.
the reaction mixture was diluted with chloroform. Fil-
tration through a short-column chromatography (Frol-
isil®, ethyl acetate:hexane=1:20) followed by
purification using column chromatography (SiO2, ethyl
acetate:hexane=1:100) gave (+)-h-naphthylphenyl-
methyl-tert-butoxysilane 2 (83 mg, 69%, [h]D= +18.5°
(c 4.2, pentane), 58% ee).
2.2.2. Preparation of (–)-h-naphthylphenylmethyl-tert-
butoxysilane 2 using copper(I) cyanide
An almost similar procedure to that described above
using copper cyanide (89 mg, 1.0 mmol) gave product 2
(160 mg, 100%, [h]D= −31.7° (c 7.6, pentane)).
2.2.3. Preparation of (–)-h-naphthylphenylmethyl-tert-
butoxysilane 2 using copper(I) tert-butoxide and lithium
chloride
To a DMI (1.7 ml) suspension of copper(I) tert-bu-
toxide (230 mg, 1.68 mmol) and lithium chloride (80
mg, 1.89 mmol), (+)-h-naphthylphenylmethylsilane 1
(211 mg, 0.85 mmol) was added. After stirring for 2 h
at r.t. the reaction mixture was diluted with chloroform.
Filtration through a short-column chromatography
(Frolisil®, ethyl acetate:hexane=1:20) followed by
purification using column chromatography (SiO2, ethyl
acetate:hexane=1:100) gave (–)-h-naphthylphenyl-
methyl-tert-butoxysilane 2 (270 mg, 99%, [h]D= −
31.0° (c 4.6, pentane)).
3. Results and discussion
There have been extensive studies on the stereochem-
istry of a nucleophilic attack on optically active
organosilicon compounds [9]. Retention of the configu-
ration was normally observed in the case of a poor
leaving group such as hydride [10]. We already reported

104
H. Ito et al. / Journal of Organometallic Chemistry 574 (1999) 102–106
such as THF and xylene, the desired product was
scarcely obtained in the same reaction period (entries 3,
4). A mixed reagent of copper(I) cyanide and lithium
t-butoxide gave a similar result (entry 5). An optically
pure product is assumed to have [h]D= −31.7° on the
basis of the product. This value is slightly greater than
the previously proposed one (−28°) [12]. Optical pu-
rity of the hydrosilane derived from 2 obtained in entry
5 indicates the present reaction proceeded quite stereo
selectively. Surprisingly, the consumption of h-naph-
thylphenylmethylsilane 1 did not occur with use of
sublimed copper(I) tert-butoxide (entry 6) [13]. A mix-
ture of copper(I) tert-butoxide and lithium chloride
gave the corresponding product again in high yield and
with retention of the configuration (entry 7). Addition
of dry tetrabutylammonium chloride to copper(I) tert-
butoxide in DMI is also effective for the reaction (entry
8). These results revealed that the chloride ion is essen-
tial for both acceleration of the reaction rate and the
stereoselectivity.
In an aprotic polar solvent, nucleophilicity of the
chloride ion was increased and formation of a cuprate-
like species would be favored (Scheme 1). This electron-
rich cuprate-like complex is probably the actual
reagent. We can assume three types of reaction paths.
The first one is a SN2-Si and pseudo-rotation mecha-
nism (path A). In this path, the nucleophilic attack of
an alkoxide from the opposite side of the hydride
leaving group of the asymmetric silicon center first
occurred and a single pseudo-rotation followed to give
the product with retention of the configuration. The
other two reaction mechanisms involve firstly a cleav-
that displacement of a hydride group in a hydrosilane
with a copper(I) salt proceeds smoothly in DMI [6]. We
applied this metathesis reaction to optically active silyl
compounds under various conditions in order to eluci-
date the mechanism of the metathesis reaction between
a hydrosilane and a copper(I) salt. The results are
shown in Table 1.
Firstly, we tested the nucleophilic reaction of (+)-h-
naphthylphenylmethylsilane 1 with tert-butoxide of al-
kali metal which displays strong nucleophilicity (entry
1) [11]. Although it was reported that the stereochem-
istry of the nucleophilic displacement of the hydride on
the optically active hydrosilane with potassium tert-bu-
toxide gave a product with retention of the configura-
tion in tert-butyl alcohol, in our case, a racemic silyl
ether was afforded in a moderate yield in DMI [10].
However, we found that the reactivity and the degree of
stereoselectivity dramatically changed with the addition
of copper(I) chloride in this reaction. The reaction of
(+)-h-naphthylphenylmethylsilane 1 and the mixed
reagent prepared from copper(I) chloride and lithium
tert-butoxide proceeded to form the corresponding silyl
ether 2 in a quantitative yield (entry 1). Moreover, this
reaction
gave
(-)-h-naphthylphenylmethyl-tert-bu-
toxysilane 2 with a high degree of optical purity and the
optical rotation of this product indicates that this
metathesis reaction proceeds with retention of the
configuration.
It was discovered that an excess of the mixed reagent
(CuCl+t-BuOLi) was necessary to ensure complete
conversion of the hydrosilane. This reaction was
strongly dependent on the solvent. In less polar solvents
Table 1
Metathesis reaction between hydrosilane 1 and copper(I) reagents
Entrya
Nucleophile
Solvent
Yield (%)b
Predominant stereochemistry (%)c
1
2
3
4
5
6
7
8
tBuOLi
DMI
DMI
THF
Xylene
DMI
DMI
DMI
DMI
37
100
Trace
0
99
0
Racemic
Retention, 98
–
–
Retention, 100
–
Retention, 100
Retention, 100
CuCl+tBuOLi
CuCl+tBuOLi
CuCl+tBuOLi
CuCN+tBuOLi
tBuOCu
tBuOCu+LiCl
tBuOCu+Bu4NCl
99
89
a A mixture of a copper(I) salt (1.0 mmol) and a hydrosilane (0.5 mmol) was stirred at r.t. for 2 h in a solvent (1.0 ml).
b Isolated yield.
c Based on the specific rotation of 2 [h]D +31.7°.

H. Ito et al. / Journal of Organometallic Chemistry 574 (1999) 102–106
105
Scheme 1. Proposed mechanism of the metathesis reaction.
age of the silicon–hydrogen bond on copper (pathes B,
C). A concerted mechanism with four center transition-
state 7 can explain the stereospecificity (path C). The
most plausible pathway is a redox mechanism on cop-
per metal. Oxidative addition of hydrosilane to a
cuprate-like species forms copper(III) intermediate 6
and subsequent reductive elimination of silyl ether 2
takes place (path B) [14–16]. The oxidative addition
and subsequent reductive elimination (path B) most
likely proceed with retention of the configuration. A
high degree of retention of the configuration and the
reactivity support the idea that the early step of the
reaction is a cleavage of the silicon–hydrogen bond on
the copper center rather than nucleophilic attack of
alkoxide. We also reported that a metathesis reaction
between copper(I) chloride and trimethylsilylalkynylsi-
lane proceeded smoothly in DMI [4]. Our next interest
is the stereochemistry of a substitution reaction on the
silicon center with an alkynyl leaving group. Optically
active alkynylsilane 8 was prepared by the reported
procedure [17]. Treatment of this substrate with the
mixed reagent of copper(I) chloride and lithium t-bu-
toxide also provided the corresponding ether with re-
tention of the configuration (Scheme 2).
In conclusion, we have shown a metathesis reaction
of copper(I) salt and optically active silyl compounds
proceeds stereospecifically on the chiral silicon center.
The oxidative addition and reductive elimination path-
way on the copper center was proposed. These results
provide not only valuable insight into the stereochem-
istry of optically active silyl compounds but also useful
information of the reaction mechanism of the silicon–
copper exchange reaction.
Acknowledgements
Financial support for this work is partly provided by
Grants-in-Aid for Scientific Research on Priority Areas
from the Ministry of Education, Japan and Pfizer Phar-
maceuticals Inc. We thank Dow Corning Toray Sili-
cone Co. Ltd., Chisso Co. Ltd., and Shin-Etsu
Chemical Co. Ltd. for
compounds.
a gift of organosilicon
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