Direct carboxylation of alcohols to organic carbonates: comparison of the Group 5 element alkoxides catalytic activity. An insight into the reaction mechanism and its key steps

The reaction of carboxylation of alcohols is a sustainable technology for the synthesis of dialkylcarbonates that avoids the use of phosgene. In this paper we describe the behaviour of metal alkoxides of Group 5 elements as homogeneous catalysts in the above reaction, and the isolation and characterization of the active species, i.e. the hemicarbonates formed upon insertion of carbon dioxide into the M-OR bond. Ta and V alkoxo complexes are comparatively discussed with respect to the Nb-analogues, whose synthesis and catalytic properties have been published [M. Aresta, A. Dibenedetto, C. Pastore, Inorg. Chem. 42 (10) (2003) 3256, and references therein]. In this paper, we report for the first time the evidence of insertion of carbon dioxide in tantalum alkoxides [Ta(OR)(5)](2) [R = methyl (Me), ethyl (Et)]. The relevant hemicarbonates Ta(OR)(4)[OC(O)OR] have been characterized using spectroscopic techniques (IR and NMR). Tantalum alkoxides and the vanadium compound VO(OR)(3) are shown to be able to catalyze the carboxylation of alcohols at 413 K and 5.5 MPa of CO2. The trend of reactivity for the Group 5 alkoxides as catalysts for the carboxylation of alcohols is Nb >> Ta > V