Reactions of silyl-substituted Fischer-type carbene complexes with silanes

Article abstract of DOI:10.1016/S0022-328X(00)00721-X

Silyl-substituted Fischer-type carbene complexes show unique behavior when treated with silanes. In toluene, all three kinds (Cr, Mo, W) of methyldiphenylsilyl-substituted carbene complex react with silanes to give direct insertion products in moderate to

Full text of DOI:10.1016/S0022-328X(00)00721-X

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Journal of Organometallic Chemistry 617–618 (2001) 741–743
Communication
Reactions of silyl-substituted Fischer-type carbene complexes with
silanes
Nobuharu Iwasawa a,*, Masatoshi Saitou b, Hiroyuki Kusama a
a Department of Chemistry, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan
b Department of Chemistry, Graduate School of Science, The Uni6ersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Received 2 August 2000; accepted 28 September 2000
Abstract
Silyl-substituted Fischer-type carbene complexes show unique behavior when treated with silanes. In toluene, all three kinds (Cr,
Mo, W) of methyldiphenylsilyl-substituted carbene complex react with silanes to give direct insertion products in moderate to high
yield. On the other hand, the reactions in THF give different kinds of product depending on the center metal. Thus, in addition
to the direct insertion products (Mo), novel reduction-insertion (W) and reduction-carbonyl insertion-silicon migration (Cr)
products are obtained in good yields. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Silyl-substituted Fischer-type carbene complexes; Insertion; Silane reduction; Carbonyl insertion; Silicon migration
1. Introduction
into three types of product depending on the solvent
and the center metal of the complex employed.
The insertion reaction of the Fischer-type carbene
complexes of Group 6 metals into Si–H bonds is a
well-known process, and sometimes gives synthetically
useful silylated compounds [1–8]. For example, Chan et
al. have reported that reaction of a,b-unsaturated car-
bene complexes with triethylsilane gives allylmethoxysi-
lane derivatives in good yields [4,5]. However, although
silanes are reductants, there are no reports of hydride-
reduction reactions of silanes with such Fischer-type
2. Results and discussion
Three kinds (Cr, Mo, W) of methyldiphenylsilyl-sub-
stituted carbene complex 1 were prepared based on
literature procedures [11,12], and the reaction of these
complexes with dimethylphenylsilane was examined.
When each of the three kinds of silyl carbene com-
plex 1 (M=Cr, Mo, W) was treated with 1.5 molar
amounts of dimethylphenylsilane in toluene, the reac-
tions of the chromium and molybdenum complexes
proceeded at room temperature.3 While the reaction of
the chromium complex was rather sluggish, the corre-
sponding reaction of the molybdenum complex pro-
ceeded cleanly to give (dimethylphenylsilyl)(methyl-
carbene complexes1,2
.
During a study of the chemistry of silyl-substituted
Fischer-type carbene complexes of Group 6 metals [10],
interesting behavior was observed in their reaction with
silanes. In this paper, we report the silane-induced
transformation of silyl-substituted carbene complexes
* Corresponding author. Tel.: +81-3-57342746; fax: +81-3-
57342746.
E-mail address: niwasawa@chem.titech.ac.jp (N. Iwasawa).
To our knowledge, one report refers to this possibility. On
1
3 It is noteworthy that the reaction of phenyl or alkenyl carbene
complexes of Group 6 metals with silanes usually necessitates heating
in a hydrocarbon solvent, sometimes in the presence of a donor
ligand such as pyridine [1–5]. Compared to these, the reactivity of
silyl-substituted carbene complexes towards insertion into the Si–H
bond is considerably higher.
reaction of triphenylsilane with chromium p-methylphenylmethoxy-
carbene complex, formation of 4,4%-dimethylstilbene along with the
normal insertion product was noted. See Ref. [2].
2 For hydride-reduction of Fischer-type carbene complexes, see
Ref. [9].
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00721-X

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N. Iwasawa et al. / Journal of Organometallic Chemistry 617–618 (2001) 741–743
30% yield, without formation of 2. The yield of this
compound improved to 64% on employing 5 molar
amounts of dimethylphenylsilane. Furthermore, when
the chromium complex 1c was treated in the same
manner, a novel silylsiloxyalkene 4 was obtained as the
major product as a single geometrical isomer (Scheme
2). The structure and geometry of this compound were
determined by NMR analysis.5
Scheme 1.
Formation of these three types of product can be
rationalized as follows (Scheme 3). When the reaction is
carried out in a non-polar solvent such as toluene,
insertion of the silyl methoxy carbene into the Si–H
bond is the major reaction pathway regardless of the
center metal. The same direct insertion is observed even
in a polar solvent such as THF when the molybdenum
complex 1a is employed. However, when the tungsten
or chromium complex is employed, the silane first acts
as a reducing reagent to give the hydride-addition
intermediate 6. In the case of the tungsten complex 1b,
elimination of methoxide, presumably as dimethyl-
phenylmethoxysilane, occurs to give a non-heteroatom-
stabilized silyl carbene complex 7, which inserts into the
Si–H bond of another molecule of dimethylphenylsi-
Scheme 2.
diphenylsilyl)methoxymethane (2), a normal insertion
product, in 92% yield. The corresponding reaction of
the tungsten complex did not proceed at room tempera-
ture, but on heating at 60°C the complex was consumed
within 7 h to give the same insertion product 2 in 84%
yield (Scheme 1).
When the same reactions were carried out in THF,
they all proceeded at room temperature within 1 day,
but different products were obtained depending on the
center metal. Thus, when the molybdenum complex 1a
was employed, the same insertion product 2 was ob-
tained as the major product, albeit in moderate yield.4
However, when the corresponding tungsten complex 1b
was employed, a new product, (methyldiphenylsilyl)-
(dimethylphenylsilyl)methane (3), was obtained in about
5 The structure of this compound 4 was determined as follows: In
the 2D-HMBC spectra, correlations between the olefinic proton (H1)
and the methyl carbon of the methoxy group (C1), between the
olefinic carbon (C2) and the methyl proton of the methoxy group
(H2), and between the methyl proton of the methyldiphenylsilyl group
(H3) and the olefinic carbon (C3) were observed. The geometry of the
double bond was determined by observation of an NOE between the
olefinic proton (H1) and the methyl protons of the methyldiphenylsi-
lyl group (H3).
4 Use of 5 molar amounts of the silane improved the yield of 2 to
51%.
Scheme 3.

N. Iwasawa et al. / Journal of Organometallic Chemistry 617–618 (2001) 741–743
743
Acknowledgements
This research was partly supported by the Toray
Science Foundation and a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science,
Sports and Culture of Japan.
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lane to give the reduction-insertion product 3.6 On the
other hand, when the chromium complex 1c is em-
ployed, CO insertion followed by silylation on the
oxygen atom occurs at the stage of the hydride-addition
intermediate 6 to give a new Fischer-type carbene com-
plex 8.7 1,2-Migration of the methyldiphenylsilyl group
occurs at this stage to give the product 4. Such facile
1,2-migration of a silyl group has been observed for
several a-silylated carbene complexes [14,15].
The same type of product formation was observed
when triethylsilane was employed as silane as shown in
Scheme 4.
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3. Conclusion
Silyl-substituted Fischer-type carbene complexes
show unique behavior when treated with silanes. De-
pending on the choice of solvent and center metal, three
different reaction pathways: direct insertion, reduction-
insertion, and reduction-carbonyl insertion-silicon mi-
gration can be made to proceed.
6 We found that when chromium or tungsten phenyl–carbene
complex was treated with silane in THF, reduction-insertion product
was obtained in good yield. The corresponding reaction of molybde-
num phenyl–carbene complex was not examined due to the instabil-
ity of the complex. Full details of these results will be published in
due course. The reason for the difference of reaction pathway be-
tween chromium phenyl–carbene and silyl–carbene is not clear.
7 Facile CO insertion into a carbon–chromium bond is probably
.
due to weaker bond energy of Cr–CO bond rather than Mo–CO or
W–CO bond. See [13].