Mechanistic Studies of Si−CN and C−NC Bond Activation of Silylnitriles and Alkyl Isonitriles by Rhodium Porphyrin Radical:  Novel Cyanide Transfer

The Si−CN and C−NC bonds of silylnitriles and alkyl isonitriles were activated by (tetramesitylporphyrinato)rhodium(II), Rh(tmp), to give rhodium porphyrin silyls or alkyls and rhodium porphyrin cyanide, respectively. Pyridine and triphenylphosphine promoted the rates and yields of the reactions with silylnitriles, but inhibited the rates and yields of the reactions with isonitriles. The reaction of Rh(tmp) with Me₃SiCN exhibited second-order kinetics (rate = k_obs[Rh(tmp)][Me₃SiCN]) at a sufficiently high concentration of pyridine. The reaction with BuNC showed fourth-order kinetics, second-order in each of the reactants; rate = k_obs[Rh(tmp)]²[BuNC]². A novel cyanide transfer rate-determining step was proposed to account for the reaction mechanism.