Osmium complexes with silyl ligands

Detailed mechanistic studies with respect to the application of Speier s catalyst on the hydrosilylation of ethylene showed that the process proceeds according to the Chalk-Harrod mechanism and the rate-determining step is the isomerization of Pt(silyl)(alkyl) complex formed by the ethylene insertion into the Pt—H bond.613 In contrast to the platinum-catalyzed hydrosilylation, the complexes of the iron and cobalt triads (iron, ruthenium, osmium and cobalt, rhodium, iridium, respectively) catalyze dehydrogenative silylation competitively with hydrosilylation. Dehydrogenative silylation occurs via the formation of a complex with cr-alkyl and a-silylalkyl ligands ... 

Intermolecular addition of carbon nucleophiles to the ri2-pyrrolium complexes has shown limited success because of the decreased reactivity of the iminium moiety coupled with the acidity (pKa 18-20) of the ammine ligands on the osmium, the latter of which prohibits the use of robust nucleophiles. Addition of cyanide ion to the l-methyl-2//-pyr-rolium complex 32 occurs to give the 2-cyano-substituted 3-pyrroline complex 75 as one diastereomer (Figure 15). In contrast, the 1-methyl-3//-pyrrolium species 28, which possesses an acidic C-3-proton in an anti orientation, results in a significant (-30%) amount of deprotonation in addition to the 2-pyrroline complex 78 under the same reaction conditions. Uncharacteristically, 78 is isolated as a 3 2 ratio of isomers, presumably via epimerization at C-2.17 Other potential nucleophiles such as the conjugate base of malononitrile, potassium acetoacetate, and the silyl ketene acetal 2-methoxy-l-methyl-2-(trimethylsiloxy)-l-propene either do not react or result in deprotonation under ambient conditions. 

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