MECHANISM OF HOMOGENEOUS HYDROSILATION OF ALKENES BY (ETA-5-CYCLOPENTADIENYL)RHODIUM

The mechanism of hydrosilation of ethene with R3SiH catalyzed by (eta(5)-C5H5)Rh(C2H4)(SiR3)H (principally with R = Et) has been studied by NMR and GC/MS methods. Comparison with related compounds indicates that activity is confined to this group of compounds at room temperature. The mercury test demonstrates that the reaction is homogeneous, in contrast to recent evidence on platinum catalysts. The reaction mechanism was investigated by deuterium labeling of either substrate and variation of alkene and trialkylsilane. These experiments demonstrated conclusively that (eta(5)-C5H5)Rh(C2H4)(SiR3)H acts as a precursor to the catalytically active species but is not within the catalytic cycle. In consequence, the Chalk-Harrod mechanism cannot apply in this system. An alternative cycle is proposed which is initiated by a hydrogen shift to form (eta(5)-C5H5)Rh(C2H5)(SiR3). The latter generates the hydrosilation products via oxidative addition of R3SiH to form an intermediate containing two silicon atoms. The initial hydrogen shift is consistent with studies of intramolecular exchange and stoichiometric reactions of the starting material. The mechanism is equivalent to the one proposed by Wrighton and Seitz for hydrosilation catalyzed by cobalt carbonyls.