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 ole fins. Hydrosilation of ethylene, tert-butylethylene, 1-hexene, and cyc lohexene by HSiR(3) (R = CH2CH3, C6H5) occurs in (t)he presence of 1 m ol % [(phen)Pd(CH3)(L)](+)[BAr'(4)](-). The reaction of tert-butylethy lene 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 e xperiments revealed that the catalyst resting state for the silations of ethylene and alkyl-substituted olefins is (phen)Pd(SiR(3))(eta(2)-H 2C=CHR')](+)[BAr'(4)](-). Evidence for rapid, reversible silyl migrati on at -70 degrees C was observed by H-1 NMR spectroscopy. Deuterium la beling studies show that the intermediate Pd(II) alkyl complexes can i somerize via a series of P-hydride eliminations followed by reinsertio ns 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)Si CH=CHPh (R = CH2CH3, CH(CH3)(2)). H-1 NMR spectroscopy revealed that t he pi-benzyl complexes )Pd(eta(3)-CH(CH(2)SiR(3))C6H5)](+)[BAr'(4)](-) and [(phen)Pd(eta(3)-CH(CH3)C6H5)](+)[BAr'(4)](-) are the catalyst res ting states for the silation reactions of styrene.