Cyclotrimerization of aryl isocyanates catalyzed by alkoxyallenes through zwitterionic mechanism

Alkoxyallenes (1) are found to catalyze cyclotrimerization of aryl isocyanates (2) in dimethylformamide (DMF) to afford 1,3,5‐triaryl‐s‐triazine‐2,4,6‐trione (3: cyclic trimer of 2), although the reaction of 1 with 2 in bulk gave the corresponding copolymer. In order to speculate about the mechanism of the cyclotrimerization, macrozwitterion 4, was prepared by copolymerization of 1 and 2 in bulk, and the catalytic activity of 4 was further studied. Cyclotrimerization of 2 was promoted by 4 and quenching of 4 with methanol completely destroyed the catalytic activity. Consequently, the alkoxyallene‐catalyzed cyclotrimerization of 2 is assumed to take place through a zwitterion generated from 1 and 2.