Flexible, bowl-shaped n-heterocyclic carbene ligands: Substrate specificity in indium-catalyzed ketone hydrosilylation

A series of benzimidazolium chlorides was synthesized as precursors to N-heterocyclic carbene ligands, with N-substituents varying in size from 3,5-xylyl (la) to first-generation dendritic 3,5-bis(3,5-di-tert-butylphenyl) phenyl (lb), to the second-generation 3,5-bis[3,5-bis(3,5-di-terf-butylphenyl) phenyl]phenyl (1c). The dendritic side groups of lb and 1c form a flexible, bowl-like cavity. Iridium complexes of 1a-c were synthesized and were shown to be catalytically active for the hydrosilylation of aryl methyl ketones. The dendritic ligands lb and 1c effect a moderate level of substrate specificity in the competitive hydrosilylation of ketones of varying size. In the competitive hydrosilylation of acetophenone versus 3-(3,5-di-tert-butylphenyl)acetophenone, acetophenone is consumed approximately 3.7 times more quickly using the second-generation ligand 1c. Using the control ligand la, this ratio is 1.8.