Preparation of isocyanates from primary amines and carbon dioxide using Mitsunobu chemistry

Primary alkylamines 1 and hindered arylamines 1 give high yields of isocyan ates 5 when reacted with carbon dioxide and the Mitsunobu zwitterions 4 gen erated from dialkyl azodicarboxylates and Bu3P in dichloromethane at -78 de grees C. Use of Ph3P still gave high yields of isocyanates from reactions o f primary alkylamines, but only low yields were obtained from reactions of aromatic amines. Reactions which failed to give high yields of isocyanates gave either carbamoylhydrazines 6 and/or dicarbamoylhydrazines 10 and/or tr iazolinones 7. The triazolinones were shown to arise from reactions of reac tive aryl isocyanates with the Mitsunobu zwitterion. The carbamoylhydrazine s were shown not to arise from reaction, of isocyanate with reduced dialkyl azodicarboxylates, and a mechanism for their formation is proposed. Single -crystal X-ray analyses confirmed the structures of 6, 7, and 10.