Design and synthesis of isocyanide ligands for catalysis: application to Rh-catalyzed hydrosilylation of ketones.

New isocyanide ligands with meta-terphenyl backbones were synthesized. 2,6-Bis[3,5-bis(trimethylsilyl)phenyl]-4-methylphenyl isocyanide exhibited the highest rate acceleration in rhodium-catalyzed hydrosilylation among other isocyanide and phosphine ligands tested in this study. 1H NMR spectroscopic studies on the coordination behavior of the new ligands to [Rh(cod)2]BF4 indicated that 2,6-bis[3,5-bis(trimethylsilyl)phenyl]-4-methylphenyl isocyanide exclusively forms the biscoordinated rhodium-isocyanide complex, whereas less sterically demanding isocyanide ligands predominantly form tetracoordinated rhodium-isocyanide complexes. FTIR and 13C NMR spectroscopic studies on the hydrosilylation reaction mixture with the rhodium-isocyanide catalyst showed that the major catalytic species responsible for the hydrosilylation activity is the Rh complex coordinated with the isocyanide ligand. DFT calculations of model compounds revealed the higher affinity of isocyanides for rhodium relative to phosphines. The combined effect of high ligand affinity for the rhodium atom and the bulkiness of the ligand, which facilitates the formation of a catalytically active, monoisocyanide-rhodium species, is proposed to account for the catalytic efficiency of the rhodium-bulky isocyanide system in hydrosilylation.