The reaction mechanism in the ethanolysis of urea with transition metal-based catalysts: DFT calculations and experiments

Abstract

Alcoholysis of urea is an attractive methodology for the synthesis of organic acyclic carbonates. It has low environmental impact and represents an indirect way of using CO2. ZnO, known to have interesting performances, is reported in the literature to behave as a homogeneous catalyst in the form of Zn(NCO)2(NH3)2 in the methanolyis of urea. In this paper, theoretical and experimental studies are described aimed at shading light on the reaction mechanism of ethanol with urea under transition-metal (TM) catalysis. The reaction of ethanolysis of urea proceeds in two steps: i) formation of ethylcarbamate (EC) also in absence of catalysts; ii) co-ordination of EC, via the carbonylic group, to a TM centre and alcoholysis to afford diethylcarbonate (DEC). The adduct TM-EC has been isolated using several TMs (Zn, Sc, La) and characterized. It has been shown to react with ethanol to afford DEC. The energetics (free Gibbs energy) of the non-catalyzed and catalyzed ethanolysis of EC have been compared, highlighting the role of the TM as catalyst. The same reaction mechanism has been shown to hold for methanol, also if the energy of the TSs differs from EtOH. The reaction mechanism demonstrated in this paper for the ethanol(methanol)–urea reaction is dissimilar from that reported in the literature for the methanol–urea system, most probably due to both: i) higher level of DFT calculations used in this work that most carefully reveals the energetics of the TSs, and ii) use of Gibbs Free Energy instead of Enthalpy used by other authors.