Silicon-transition-metal complexes alkenes

Transition metals have been used to trap and stabilize many different types of reactive intermediates, such as carbenes. Reactive silicon intermediates have only recently yielded to this approach. In the case of alkenes, for instance, transition metal complexes are generally made by exposing the alkene to a transition metal bearing suitable leaving groups (e.g., carbonyl). Unlike carbon-based intermediates, however, silicon-based analogs have been very difficult to prepare until recently. Unless... 

Silicon hydrides can also oxidatively add to low-valent transition metal complexes forming a metal hydride silyl complex which can undergo subsequent insertion reactions. This elementary step forms the basis for the hydrosilylation process for alkenes and ketones. 

Carbon-shielding tensors, alkene and alkyne transition metal complexes, 1, 472 Carbon—silicon bond formation... 

The reaction is catalysed by many transition-metal complexes, and a mechanism for the hydrosilylation of an alkene under transition-metal catalysis is depicted in Figure Si5.7. Initial coordination of the alkene to the metal is followed by cis addition of the silicon-hydrogen bond. A hydride migratory insertion and elimination of the product silane complete the cycle. 

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 stereochemistry of addition to acetylenes has been studied for various types of homogeneous catalysts. In practice, cis addition seems to be a common feature of hydrosilylation catalyzed by transition metal complexes, although this mechanistic scheme does not require a cis addition. Benkeser (39) found that the addition of trichlorosilane to 1-alkenes in the presence of homogeneous (chloro-platinic acid) catalysts induces a cis approach and results in the trans product. Several series of experiments with optically active trisubstituted silanes have indicated that when catalyzed by platinum complexes, the hydrosilylation process occurs overall with retention of configuration at the asymmetric silicon center. 

The hydrosilylation of alkenes and alkynes catalyzed by transition metal complexes is often accompanied by side reactions such as isomerization, polymerization, and hydrogenation of unsaturated compounds and/or redistribution and dehydrogenation of silicon hydrides as well as reactions in which both substrates take part, such as dehydrogenative silylation (4,13). The latter reaction, which in certain conditions permits direct production of unsaturated silyl compound, has been a subject of intense study over the last two decades. 

Alkenes have been found to react reversibly with phosphine-silylene complexes (199) containing a Si(II)-Sn bond to produce alkyl silylene complexes (200). Calculations suggest that the insertion reaction proceeds through oxidative addition and migratory insertion in a two-step process, suggesting that silicon(II)-phosphine complexes may behave like transition-metal complexes. ... 

The silicon hydrides do not spontaneously add to alkenes either. However, the hydrosilation, or hydrosilylation reaction, of olefins is of significant utility in the preparation of alkyl-subtituted silanes with the use of either radical or transition metal catalysis. The preferred metal catalysts for hydrosilation are platinum complexes. Chloro-platinic acid will catalyze hydrosilations with halosilanes, alkylarylhalosilanes, alkoxy-silanes, and siloxanes that in many cases are quantitative under ambient conditions. Yields and conversions are generally poorer for alkyl,- and arylsilanes. Many other coordination complexes have been found to catalyze the hydrosilation reaction, and these can provide certain advantages, particularly in regiochemistry. Some typical hydrosilation reactions are shown in Table... 

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