Mechanistic studies of palladium(II)-catalyzed hydrosilation and dehydrogenative silation reactions

The cationic Pd(II) complexes, [(phen)Pd(CH3)(L)](+)[BAr'(4)](-) phen = 1,10-phenanthroline; L = Et(2)O, Me(3)SiC=CSiMe(3); Ar' = 3,5-(CF3)(2)C6H3) catalyze the hydrosilation and dehydrogenative silation of olefins. Hydrosilation of ethylene, tert-butylethylene, 1-hexene, and cyclohexene by HSiR(3) (R = CH2CH3, C6H5) occurs in (t)he presence of 1 mol % [(phen)Pd(CH3)(L)](+)[BAr'(4)](-). The reaction of tert-butylethylene with HSi(i-Pr)(3) in the presence of [(phen)Pd(CH3)(L)](+)[BAr'(4)](-) yields neohexane and t-BuCH=CHSi(i-Pr)(3). Low-temperature NMR experiments revealed that the catalyst resting state for the silations of ethylene and alkyl-substituted olefins is [(phen)Pd(SiR(3))(eta(2)-H2C=CHR')](+)[BAr'(4)](-). Evidence for rapid, reversible silyl migration at -70 degrees C was observed by H-1 NMR spectroscopy. Deuterium labeling studies show that the intermediate Pd(II) alkyl complexes can isomerize via a series of P-hydride eliminations followed by reinsertions of olefin prior to reaction with DSiEt(3). Styrene (u)ndergoes both hydrosilation and dehydrogenative silation in the presence of [(phen)Pd(CH3)(L)](+)[BAr'(4)](-) or [(phen)Pd(eta(3)-CH(CH3)C6H5)](+)[BAr'(4)](-) yielding ethylbenzene, R(3)SiCH(2)CH(2)C(6)H(5) and trans-R(3)SiCH=CHPh (R = CH2CH3, CH(CH3)(2)). H-1 NMR spectroscopy revealed that the pi-benzyl complexes [(phen)Pd(eta(3)-CH(CH(2)SiR(3))C6H5)](+)[BAr'(4)](-)and [(phen)Pd(eta(3)-CH(CH3)C6H5)](+)[BAr'(4)](-) are the catalyst resting states for the silation reactions of styrene.