Ketenimine Formation Catalyzed by a High-Valent Cobalt Carbene in Bulky Alkoxide Ligand Environment

Abstract

High-valent cobalt carbene Co(OR)2(═CPh2) (OR = OCtBu2Ph) undergoes reaction with various isocyanides CNR′ (CNR′ = 2,6-dimethylphenyl isocyanide, 4-methoxyphenyl isocyanide, 2-chloro-6-methylphenyl isocyanide, adamantyl isocyanide) to yield the corresponding ketenimine. The reaction is accompanied by the formation of cobalt bis(alkoxide) bis(isocyanide) complexes Co(OR)2(CNR)2, which were independently synthesized and characterized. DFT calculations suggest the mechanism proceeds through isocyanide binding to Co, followed by intramolecular insertion into the Co–carbene bond to form the ketenimine. We have also conducted an investigation of the catalytic formation of ketenimines at room temperature using mixtures of diazoalkanes (diphenyldiazomethane, methyl diazo(phenyl)acetate, and ethyl diazoacetate) and isocyanides (2,6-dimethylphenyl isocyanide, 4-methoxyphenyl isocyanide, adamantyl isocyanide, cyclohexyl isocyanide, and benzyl isocyanide). While no catalytic reactivity was observed for diphenyldiazomethane, ester-substituted diazoalkanes (diazoesters) demonstrate catalytic turnover. Relatively high yields are observed for the reactions involving bulkier aliphatic substrates adamantyl and cyclohexyl isocyanides. Mechanistic studies suggest that the lack of catalytic reactivity involving diphenyldiazomethane results from the inability of Co(OR)2(CNR)2 to undergo carbene formation upon reaction with N2CPh2. In contrast, facile reaction is observed between Co(OR)2(CNR)2 and diazoesters, which enables the overall catalytic reactivity.