Hydrosilylation and dehydrogenative silylation of alkenes

The concurrent P-H transfer from the two ligands to the metal is a key step for two alternative reactions, namely hydrosilylation and/or dehydrogenative silylation [6, 11]. 

In contrast to the Pt(0) and Pt(II) complexes and the corresponding Rh(I) and Rh(III) complexes, the iridium complexes have rarely been employed as hydrosilylation catalysts [1-4]. Iridium-phosphine complexes with d metal configura-tion-forexample, [Ir(CO)Cl(PPh3)2] (Vaska s complex) and [Ir(CO)H(PPh3)3]-were first tested some 40 years ago in the hydrosilylation of olefins. Although they underwent oxidative addition with hydrosilanes (simultaneously to Rh(I) com- 

Ir(II) andlr(V) complexes stabilized by an O-donor ligand (e.g. [Ir(coe)(triso)] and [Ir(C2H4)2(triso)] (triso = tridentatetris(diphenyloxosphosphoranyl)methanides) are effective catalysts for the dehydrogenative silylation and hydrosilylation of ethylene [16-18]. 

A family of cationic catalysts such as [Ir(cod)(PCy3)Py] [PF6] [19] and zwitter-ionic [20] Ir(I) complexes have been tested in the hydrosilylation of styrene, and represent an effective class of the hydrosilylation catalysts to yield predominantly P-adduct accompanied by a-adduct and traces of unsaturated products (see Table 14.1). 

The hydrosilylation of ethylene by the early-late transition-metal heterodinuclear complexes [CpTa( t-CH2)2lr(CO)2] has been studied mainly in a bid to recognize the mechanism of reaction, which occurs via a predominant alkene/Ir—H insertion pathway over a minor insertion of ethylene into the Ir—Si bonds [21]. 

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

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