Hydrosilylation of Ketones Catalyzed by Iron Iminobipyridine Complexes and Accelerated by Lewis Bases

Fe-iminobipyridine complexes ((^R BPI^Ar,R' )FeBr₂ , ^R BPI^Ar,R' =iminobipyridine derivatives) were found to exhibit good catalytic activity for hydrosilylation of ketones. The highest TOF (turnover frequency) was obtained for the hydrosilylation of 2-octanone with phenylsilane (4190 min^-1 ). The reactions of various 4-substituted acetophenone derivatives revealed that the introduction of an electron-withdrawing group at the 4-position retarded the reaction. The TOF of the hydrosilylation of 4-chloroacetophenone with diphenylsilane was quite low (30 min^-1 ), however the addition of a catalytic amount of Lewis base, especially pyridine, dramatically accelerated this hydrosilylation (980 min^-1 ). Comparison of this additive effect for several N-donor ligands revealed that the coordination ability of the N-donor ligand was responsible for the acceleration. The rate determining step in the hydrosilylation of ketones appeared to be the reductive elimination of alkoxy and silyl groups from the iron center, which was facilitated by the coordination of N-donor ligand to the iron. This coordination ability of the N-donor ligand, however, inhibited olefin hydrosilylation. Addition of KO^t Bu instead of N-donor also showed the same acceleration and inhibition effects on ketone and olefin hydrosilylations, respectively.