Carboxylation of Styrene Oxide Catalyzed by Quaternary Onium Salts under Solvent-free Conditions

Abstract

Green chemistry has attracted much attention in organic chemistry. Major goal in this area is to maximize the efficient use of raw materials and simultaneously to minimize waste. In this context, conversion of conventional organic processes into the highly atom-efficient protocol with good selectivity is the most desirable. The carboxylation of epoxides shows an interesting application in the utilization of carbon dioxide to produce cyclic carbonates. These compounds are useful as monomers, polar aprotic solvents, and intermediates for the synthesis of pharmaceuticals and fine chemicals. Recently, we have studied the coupling of CO2 with styrene oxide as a model reaction as the carboxylation of epoxides and have noticed the processes using quaternary onium salts such as tetraalkylammonium and phosphonium halides are of full-atom-economy-type to convert styrene oxide into styrene carbonate: efficient, clean and by-productfree under solvent-free reaction conditions. We found the detailed information on the use of onium salts for this reaction under solvent-free conditions is surprisingly limited except a few examples. The patent by McMullen et al. described the coupling reaction of ethylene oxide with CO2 to produce ethylene carbonate in up to 99% yield when catalyzed by several ammonium salts such as tetraethylammonium chloride and a few other ammonium halides. However, the detailed studies of the reaction profile and the catalytic efficiency of quaternary salts in the coupling of CO2 with epoxides are still lacking so far. Historically, quaternary ammonium halides have been often used as complexed with transition metal catalysts or as the reaction ionic medium to promote the reactions of the coupling of CO2 with epoxides rather than used alone. Recent mechanistic studies suggest on the role of these catalytic systems that an epoxide is coordinated to the halogenated metal anion, generated from the interaction of the metal halide with the halide ion, and the resulting coordinated epoxide ring is opened by the halide ion to form an haloalkoxy species. Nucleophilic attack of the haloalkoxy species on CO2 leads to a linear halocarbonate that is transformed into a cyclic carbonate by the intramolecular substitution of the halide. Herein, we report on the detailed reaction profile of carboxylation of styrene oxide catalyzed by various onium salts, particularly such as quartenary ammonium and phosphonium halides under various temperatures and CO2 pressures as shown in Scheme 1. Quatenary ammonium salts-catalyzed reaction. Table 1 shows the results of the experiments performed to optimize the conditions for the carboxylation of styrene oxide in the Scheme 1