Carbonate, Acetate and Phenolate Phosphonium Salts as Catalysts in Transesterification Reactions for the Synthesis of Non Symmetric Dialkyl Carbonates

Methyltrioctyl phosphonium methyl carbonate [P8881]+[MeOCO2]- was prepared by the alkylation of trioctyl phosphine with the non toxic dimethyl carbonate. This salt was a convenient source to synthesize different ionic liquids where the methyltrioctyl phosphonium cation was coupled to weakly basic anions such as bicarbonate, acetate, and phenate. At 90-220 °C, all these compounds [P8881]+X-; X=MeOCO2; HOCO2; AcO; PhO] were excellent organo-catalysts for the transesterification of dimethyl- and diethyl-carbonate with primary and secondary alcohols, including benzyl alcohol, cyclopentanol, cyclohexanol, and the rather sterically menthol. Conditions were optimized to operate with very low catalyst loadings up to 1 mol% and to obtain non symmetric dialkyl carbonates (ROCO2R’; R= Me, Et) with selectivity up to 99% and isolated yields > 90%. The catalytic performance of the investigated ionic liquids was discussed through a cooperative mechanism of simultaneous activation of both electrophilic and nucleophilic reactants.