Dehydrogenative silylation of alkenes

Scheme 14.1 Organometallic intermediate competitive hydrosilylation and dehydrogenative silylation of alkenes.

Dehydrogenative silylation of alkenes is often observed as a side reaction of hydrosilylation (vide supra). However, this reaction becomes a predominant or exclusive process depending on the nature of the catalysts used as well as substrates3,79,80,95-99. 

Vinylsilanes and allylsilanes are prepared by dehydrogenative silylation of alkenes catalysed by Rh and other complexes [221]. A particularly effective catalyst for alkenylsilanes is Ru3(CO)12 [222], Using excess 1-hexene, 1-silyl-1-hexene 575 was... 

As we have already mentioned, ruthenium complexes predominantly catalyze the dehydrogenative silylation of alkenes but competitively with the hydrosilylation so the reaction usually gives a mixture of the dehydrogenative silylation and hydrosilylation products. Ru3(CO)12 appears to be a very active catalyst for the dehydrogenative silylation of styrene, para-substituted styrenes [ 19, 20],trifluoropropene and pentafluorostyrene [21] by trialkyl-, phenyldialkyl-silanes (but also triethoxysilane) (Eq. 10). 

Scheme 19 Dehydrogenative silylation of alkenes catalyzed by Re(I) ctnnplex 16...

Although for the catalytic transformations of organosilicon compounds only hydrosilylation is well known as industrially important process, in the last 20 years other reactions of silicon compounds catalyzed by transition metal complexes have been discovered and developed. They include double (bis)silylation of alkenes and alkynes, silylative coupling of alkenes and alkynes with vinylsi-lanes, dehydrocoupling of hydrosilanes, silylformylation and silylcarbonylation of unsaturated compounds, and dehydrogenative silylation of alkenes and alkynes with hydrosilanes. Only the latter, as related to hydrosilylation (and very often its side reaction), has been discussed here (13). 

The catalytic addition of organic and inorganic silicon hydrides to alkenes, ary-lalkenes, and cycloalkenes as well as their derivatives with functional groups leads to their respective alkyl derivatives of silicon and occurs according to the anti-Markovnikov rule. However, under some conditions (e.g., in the presence of Pd catalysts), this product is accompanied by a-adduct (i.e., the one containing an internal silyl group). Moreover, dehydrogenative silylation of alkenes with hydrosilanes, which proceeds particularly in the presence of iron- and cobalt-triad complexes as related to hydrosilylation (and very often its side reaction), is discussed. 

Over the last two decades, Wilkinson complex and related phosphine complexes of rhodium(I) have been used in numerous reactions for synthetic purposes, such as in the hydrosilylation of styrene and vinylcyclo-propene to yield ring-opening products of vinylamines. The [ (dippe)Rh 2(/u.-H)2] complex [where dippe = l,2-bis(diisopropylphosphino)ethane] is active in the hydrosilylation of olefins by diphenylsilane (4). Rhodium complexes were extremely favorable catalysts for dehydrogenative silylation of alkenes and divinyldiorganosilanes (4,13). 

The use of Ni(0) catalysts for the hydrosilation of phenyl acetylene with PhsSiH and of various internal alkynes and conjugated diynes with Ph2SiH2, PhMc2SiH and EtsSiH has also been reported.It is of interest to note that although the Ni equivalent of Karstedt s catalyst has been prepared, dehydrogenative silylation of alkenes leading to unsaturated products is the dominant reaction and hydrosilation a secondary pathway with this Ni catalyst. 

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