“Synergistic Effects of Two Si−H Groups and a Metal Center” in Transition Metal-Catalyzed Hydrosilylation of Unsaturated Molecules: A Mechanistic Study of the RhCl(PPh₃)₃-Catalyzed Hydrosilylation of Ketones with 1,2-Bis(dimethylsilyl)benzene

Three rhodadisilacyclopentene complexes are synthesized by the reaction of 1,2-bis(dimethylsilyl)benzene with RhCl(PPh3)3 or Rh(H)(PPh3)4, and their contributions to the catalytic hydrosilylation of acetone are discussed. Treatment of the rhodium precursor RhCl(PPh3)3 or Rh(H)(PPh3)4 with 1,2-bis(dimethylsilyl)benzene affords an unstable Rh(V)-trihydride species having a rhodadisilacyclopentene skeleton, Rh(Me2SiC6H4SiMe2)(H)3(PPh3)2 (1), as a primary product, which is formed by double oxidative addition of 1,2-bis(dimethylsilyl)benzene to the rhodium center. The complex 1 eliminates H2 upon concentration to quantitatively form a Rh(III)-monohydride complex, Rh(SiMe2C6H4SiMe2)(H)(PPh3)2 (2). Further oxidative addition of 1,2-bis(dimethylsilyl)benzene to 2 gives a Rh(III)-trisilyl complex, Rh(Me2SiC6H4SiMe2)(η1-HSiMe2C6H4SiMe2)(PPh3) (3), in which there is an agostic interaction between the Si−H bond and the Rh(III) center. Elimination of H2 from 1 is reversible, and the most effective method for preparing 1 in solution is found to be treatment of 2 with H2. The catalytic behavior of these three new rhodadisilacyclic complexes, RhCl(PPh3)3, and Rh(H)(PPh3)4, in the hydrosilylation of acetone with 1,2-bis(dimethylsilyl)benzene was studied. The results suggest the important contribution of the trihydride 1 in the synergistic effect of two proximate Si−H bonds, leading to an unusual rate enhancement in the hydrosilylation of acetone with 1,2-bis(dimethylsilyl)benzene.