Mechanism of Phosphine-Catalyzed Cyclo-Oligomerization of Isocyanates

Abstract

The mechanism of the tri-n-butylphosphine-catalyzed cyclo-oligomerization of alkylisocyanates 1 (R = n-Bu, n-Hex) to give products 3-5 is elucidated using NMR spectroscopy. The authors propose the reaction to be initiated by the nucleophilic attack of the phosphine to the carbonyl group of the isocyanate moiety, followed by reaction with another isocyanate leading to the zwitterionic intermediate A. Intramolecular nucleophilic attack on the carbonyl group of A affords uretdiones 3 and the reaction of A with another isocyanate, followed by intramolecular nucleophilic attack on its carbonyl group results in the formation of isocyanurates 4 and iminooxadiazinediones 5. Intermediate A as the key intermediate of this reaction has been characterized for the first time.