Phosphonium ionic liquids (PILs) as organocatalysts for green reactions: nucleophilic electrophilic cooperative catalysis

Abstract

With the aim of developing a green synthesis of phosphonium ionic liquids (PILs), we prepared a series of methyltrialkylphosphonium methyl carbonates [Pn,n,n,1]+[O(CO)OCH3]- by the alkylation of trialkylphosphines with the non toxic dimethylcarbonate. These compounds proved to be active organocatalysts for a number of base-promoted organic reactions such as Michael, Henry, and Baylis-Hillman condensations, and transesterification reactions. In addition methyl carbonate salts were a convenient starting material to synthesise a large array of ionic liquids where the phosphonium cation was coupled to weakly basic anions such as bicarbonate, acetate, trifluoroacetate, phenate, chloride, bromide and many more. Mechanistic studies indicated that the anionic and the cationic partners of such ionic liquids acted cooperatively and independently as nucleophilic and electrophilic catalysts. The ambiphilic propensity of these salts was demonstrated by kinetically discriminating the contributions of the anion (nucleophilic catalyst) and of the cation (electrophilic catalyst) on the model solvent-free Baylis–Hillman dimerization of cyclohexenone