Kinetics and mechanism of the titanium tetrachloride-catalysed cyclotrimerisation of aryl cyanates

Abstract

Aryl cyanates are converted cleanly at 25 °C to 1,3,5-triazines by catalytic amounts of titanium tetrachloride in dichloromethane. Based on IR spectroscopic, kinetic and product analysis, a mechanism is proposed involving rate-limiting nucleophilic attack of the cyanate nitrogen on the cyanato carbon of a cyanate–titanium tetrachloride complex. Subsequent steps are fast, with no evidence for dimeric or acyclic trimeric intermediates; it is suggested that these steps involve a series of fast stereoselective reactions of nitrillium ion intermediates.