5120
J. Am. Chem. Soc. 2001, 123, 5120-5121
Catalytic Dehydrogenative Coupling of Secondary
Silanes Using Wilkinson’s Catalyst
Lisa Rosenberg,*,† Colin W. Davis, and Junzhi Yao
Department of Chemistry, UniVersity of Manitoba
Winnipeg, Manitoba, Canada R3T 2N2
ReceiVed February 20, 2001
ReVised Manuscript ReceiVed April 12, 2001
The pursuit of clean, straightforward alternatives to Wurtz
coupling reactions for the synthesis of polysilanes has focused
attention on dehydrocoupling reactions of silanes catalyzed by
transition metals.1 Among complexes investigated in their capacity
to produce these polymers2 two classes have emerged. One
includes group 4 metallocene derivatives, which are considered
the most active for catalytic dehydrocoupling, since they are
capable of producing oligomers and sometimes polymers from
primary silanes. Catalysts in the second class include many late
metal complexes, such as Wilkinson’s catalyst, (Ph3P)3RhCl, 1,
and other Rh(I) phosphine complexes;3 these are considered
relatively inactive for dehydrocoupling, since they do not produce
long chains, even for primary silane monomers.4 Complex 1 and
other late metal complexes catalyze the coupling of secondary
silanes to di- and trisilanes, but their application to the large-
scale synthesis of these useful, hydrido-substituted oligosilanes
has been discouraged by the known activity of such catalysts for
a competing reaction of silanes: redistribution of substituents at
silicon.5 In this communication we describe results indicating that
1 actually exhibits high activity and chemoselectivity for coupling
of secondary silanes, under appropriate conditions. We have
demonstrated that coupling in this system is extremely fast but is
not thermodynamically favored; chemoselectivity is therefore
exceedingly sensitive to the presence of hydrogen gas, also a
product of the coupling reaction.
Figure 1. Dependence of product distribution on catalyst concentration,
observed for small-scale reactions of Ph2SiH2 where (a) removal of
hydrogen was not rigorously controlled and (b) hydrogen was removed
efficiently (see text for details).
We therefore investigated the effects of varying both substrate
and catalyst concentration on the reactions of diphenylsilane in
the presence of 1. We obtained lower conversions of diphenyl-
silane, with higher relative amounts of redistribution, when we
used solvent in our initial experiments,7 so carried out subsequent
reactions in neat diphenylsilane. Preliminary, small-scale reac-
tions8 of the neat substrate (e.g. Figure 1a) consistently gave more
dehydrocoupling than redistribution9 but with much greater
selectivity for coupling at lower catalyst concentrations. At high
catalyst concentrations, an increase in the amount of redistribution
was apparently at the expense of coupled product.
However, in these small-scale reactions, the product distribu-
tions were surprisingly sensitive to the size of reaction vial relative
to the volume of substrate and to the rate and efficacy of stirring.
Chemoselectivity was enhanced, and its catalyst concentration
dependence became far less pronounced, when these conditions
were manipulated to allow more efficient removal of hydrogen
gas from the reaction mixtures (Figure 1b).10 We obtained
optimum conversion (75-80%, at which point the mixture
solidifies) and minimum redistribution using flat-bottomed vials,
wide enough to allow the substrate to be spread thinly, and high
stir rates, with stir bars of lengths approaching the inner diameter
of the vial. Thus, although redistribution of phenyl groups at
silicon can be a major competing reaction for diphenylsilane in
the presence of 1, it can be kept to a minimum with efficient
removal of H2(g).11 Based on the above results, which suggest
good catalyst activity and selectivity for coupling at low catalyst
concentrations, we prepared 2 on a 5-g scale from neat diphen-
ylsilane using 0.2 mol % 1 as catalyst.12 The reaction proceeds
readily at room temperature and is complete within 2 h, which
Our interest in preparing 1,2-bis(hydrido)-substituted disilanes
on a synthetically useful scale led us to reinvestigate the activity
of 1 in the dehydrocoupling of diphenylsilane (eq 1). Since
previous reports of this reaction invariably included the observa-
tion of considerable amounts of Ph3SiH, arising from redistribution
of the phenyl and hydride groups on silicon,6 we needed to identify
conditions that would favor dehydrocoupling over redistribution.
(7) Product distributions, showing 26% redistribution, were similar to those
reported in ref 3d for this reaction.
† New address as of July 1, 2001: Department of Chemistry, University
of Victoria, P.O. Box 3065, Victoria, B.C., Canada, V8W 3V6.
(1) (a) Tilley, T. D. Comments Inorg. Chem. 1990, 10, 37. (b) Corey, J. Y.
AdV. Silicon Chem. 1991, 1, 327. (c) Gauvin, F.; Harrod, J. F.; Woo, H. G.
AdV. Organomet. Chem. 1998, 42, 363. (d) Reichl, J. A.; Berry, D. H. AdV.
Organomet. Chem. 1999, 43, 197.
(2) Polysilanes are of interest for their unusual electronic and optical
properties, arising from extensive electron delocalization along the all-silicon
backbone: (a) West, R. J. Organomet. Chem. 1986, 300, 327. (b) Miller, R.
D.; Michl, J. Chem. ReV. 1989, 89, 1359. (c) Hamada, T. J. Chem. Soc.,
Faraday Trans. 1998, 94, 509.
(3) Ojima, I.; Inaba, S.; Kogure, T. J. Organomet. Chem. 1973, 55, C7.
(b) Lappert, M. F.; Maskell, R. K. J. Organomet. Chem. 1984, 264, 217. (c)
Brown-Wensley, K. A. Organometallics 1987, 6, 1590. (d) Chang, L. S.;
Corey, J. Y. Organometallics 1989, 8, 1885. (e) Rosenberg, L.; Fryzuk, M.
D.; Rettig, S. J. Organometallics 1999, 18, 958.
(4) For exceptions to this classification of late metal catalysts, see: (a)
Fontaine, F.-G.; Kadkhodazadeh, T.; Zargarian, D. Chem. Commun. 1998,
1253 (b) Chauhan, B. P. S.; Shimizu, T.; Tanaka, M. Chem. Lett. 1997, 785.
(5) Curtis, M. D.; Epstein, P. S. AdV. Organomet. Chem. 1981, 19, 213.
(6) Phenylsilane is also produced by this redistribution reaction, but this
more volatile product is less often detected.
(8) Reactions were carried out at room temperature in small, open vials in
the glovebox. Fixed amounts of substrate (50-200 mg) and reaction times
(1-4 h) were used for each series of experiments, with reactions being
quenched by hexanes elution through Florisil.
(9) Product distributions were determined from the amounts of 2 and Ph3-
1
SiH observed by H NMR spectroscopy.
(10) The apparent dependence on catalyst concentration of chemoselectivity
initially observed (Figure 1a) suggests that very different mechanistic pathways
exist for these rhodium-mediated dimerization and redistribution reactions. A
system exhibiting such concentration dependence presents an attractive
opportunity to identify differences in key intermediates in these competing
reactions. However, almost complete disappearance of the concentration
dependence of this chemoselectivity with efficient removal of hydrogen (Figure
1b) suggests that rhodium intermediates in the two mechanisms may not
actually be so different. Neither mechanism has been firmly established for
the late transition metals: proposed mechanisms rely on either the intermediacy
of transition metal silylene fragments or a series of oxidative addition/reductive
elimination steps (refs 1 and 5), and the involvement of dinuclear intermediates
has not been ruled out (ref 3e, for example).
(11) The maximum amount of Ph2SiH2 undergoing redistribution in these
experiments was 9%.
10.1021/ja015697i CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/05/2001