Azaphosphatrane/Ag2O catalyzed the carboxylative cyclization of propargylic alcohols and CO2

Abstract

The utilization of carbon dioxide (CO2) to synthesize various chemicals is of great importance. Herein, various azaphosphatranes and silver salts were applied to catalyze the carboxylative cyclization of CO2 and propargylic alcohols to prepare α-methylene cyclic carbonates. The influences of catalyst structures, solvents and reaction parameters on catalytic performance were systematically studied. The combination of tris(4-methoxybenzyl) azaphosphatranes (PMB-AZAP) and Ag2O with molar ratio of 0.5:1 gave α-methylene cyclic carbonate in the yield of >99 % at room temperature. Meanwhile, other azaphosphatranes (i-Bu-AZAP, Me-AZAP) gave the product in slightly lower yields. The mechanism exploration revealed that PMB-AZAP and Ag2O formed a silver complex PMB-AZAP-Ag during the reaction, which could synergistically catalyze carboxylative cyclization reaction with PMB-AZAP, resulting in a high product yield. In contrast, AgCl or AgNO3 couldn't react with PMB-AZAP to form silver complexes, and the corresponding catalytic system had no activity. The difference of the catalytic performance indicated that PMB-AZAP-Ag was a key active species for the carboxylative cyclization reaction. The in situ NMR studies concretely demonstrated that PMB-AZAP could activate propargylic alcohols through hydrogen bonding. Furthermore, the catalytic system had good substrate generality. The PMB-AZAP/Ag2O was also applied for the one-pot reaction of propargylic alcohol, CO2 and CH3OH, affording the β-carbonyl carbonate with yield of 80 % at CH3OH/propargylic alcohols ratio of 20:1.