Mechanism of the cobalt-catalyzed carbonylation of ethyl diazoacetate

Abstract

Carbonyl cobalt complexes serve as catalysts or catalyst precursors for the facile and selective transformation of primary diazoalkanes into the corresponding ketene. The mechanism of this carbonylation reaction has been elucidated in the case of ethyl diazoacetate as model diazoalkane using octacarbonyl dicobalt as the catalyst precursor. Dinuclear cobalt complexes having ethoxycarbonylcarbene ligand(s) in bridging position(s) have been identified as active intermediary of the catalytic cycles and their relevant chemical properties have been explored. Key step of the carbonylation is the formation of the highly reactive ethoxycarbonylketene by intramolecular coupling of a carbonyl ligand with the ethoxycarbonylcarbene ligand. DFT calculations reveal that the ketene formation takes place via a rapid coupling of the carbene ligand with one terminal CO followed by coordination of an external carbon monoxide and by a facile intramolecular rearrangement and ketene elimination. The ethoxycarbonylketene can be in situ trapped by OH, NH, or CH acid compounds or by N-substituted imines. In the presence of ethanol diethyl malonate is the only product of the catalytic carbonylation of ethyl diazoacetate. On the bases of the kinetics of the composing steps of the catalytic cycles, localization of the rate-determining step(s) under various reaction conditions has been made.