Synthetic and mechanistic aspects of gas–liquid phase-transfer catalysis: carboxylate esters

Abstract

The synthesis of esters from solid alkyl- and aryl-carboxylates and gaseous alkyl halides (RX), that is gas–liquid phase-transfer catalysis (g.l.p.t.c.) is promoted by the classical phase-transfer catalysts, quaternary phosphonium salts. The reaction is run under continuous-flow conditions in a column and the product is collected at the outlet. An investigation of the mechanism of the reaction shows the great importance of the liquid film of melted catalyst in promoting anion exchange between the underlying solid carboxylate salt and the gaseous alkyl halide which diffuses and reacts. This technique does not require the use of organic or aqueous solvents, leads to high conversion into esters for low carboxylate : RX molar ratios, and transforms large quantities of reagents with small volumes of solid bed.