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

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.