Catalytic decomposition of dialkyl pyrocarbonates to dialkyl carbonates and carbon dioxide in dichloromethane by a discrete cobalt(II) alkoxide species generated in situ†

Abstract

Dimethyl pyrocarbonate (dmpc) [dimethyl µ-oxo-bis(dioxocarbonate)] and diethyl pyrocarbonate (depc) were catalytically decomposed to dimethyl and diethyl carbonate respectively and carbon dioxide in the presence of [CoL(OR)]⁺ [L = cis,cis-1,3,5-tris(E,E-cinnamylideneamino)cyclohexane, R = methyl or ethyl] which we propose to be generated in situ during reaction in dichloromethane. The activity of the catalyst is undiminished after 60 000 turnovers. In both cases the catalytic rate enhancement for the decomposition is in excess of 10⁷ dm³ mol^–1 of catalyst. The catalytic process follows Michaelis–Menten type kinetics and k_obs is 2.2(2) s^–1 for dmpc decomposition and 1.3(2) s^–1 for depc decomposition. Activation energies for the catalytic decomposition are E_dmpc = 113(5) and E_depc = 120(11) kJ mol^–1. A mechanism involving cobalt-bound alkoxide attack on dialkyl pyrocarbonate is proposed. The crystal structure of [CoL(Cl)] BPh₄ has been determined by single-crystal X-ray diffraction.