Phosphonium salts as organocatalysts for green reactions

With the aim of developing a green synthesis to ammonium and phosphonium ionic liquids, we prepared a series of methyltrialkyl phosphonium methyl carbonates by the alkylation of trialkyl phosphines with the non toxic dimethylcarbonate. These salts were a convenient stepping-stone 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. As well as paving the way to a versatile synthesis of ILs, these compounds proved to be very active organocatalysts for base-promoted C-C bond forming addition reactions such as the Michael, Henry, nitroaldol, and Baylis-Hillman reactions. For these types of reaction, mechanistic studies indicated that the anionic and the cationic partners of ionic liquids acted cooperatively and independently as nucleophilic and electrophilic catalysts. This ambiphilic propensity of these phosphonium 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. Methyl carbonate, hydrogencarbonate, acetate and phenate trioctyl methyl phosphonium salts were also excellent organocatalysts for the synthesis of an array of alkylcarbonates by transesterification of dimethyl- and diethyl-carbonate with primary and secondary alcohols, including benzyl alcohol, cyclopentanol, cyclohexanol, and the hindered menthol.Conditions were optimized to operate with very low catalyst loadings up to 1 mol% and to obtain non symmetrical dialkyl carbonates (ROC(O)OR’; R= Me, Et) with selectivity up to 99% and isolated yields > 90%. Also 1,2 and 1,3 diols could be used to obtain the corresponding cyclic carbonates rapidly (less than half an hour in the case of propylenediol), with very good yields (80-100%) under mild conditions.