Pincer Nickel(II) Catalyzed Oxidative Carbonylation of Amines: A Phosgene-Free Synthesis for Isocyanates and Ureas

A novel phosgene-free process for isocyanate and urea formations from amines and cyclic carbonates catalyzed via a nickel(II) pincer complex ( 2 ) is demonstrated. Under relatively mild reaction conditions (2 hours and 25°C for isocyanates; 3 hours and 100°C), various aliphatic and substituted aromatic amines are converted into 32 different isocyanate and urea products. Upon optimization, moderate to good turnovers (TONs) for isocyanate formations were calculated with the highest TON for (3-isocyanatopropyl)benzene ( 4d , 277 ). For ureas, moderate to good TONs were also observed with the highest TON for 1,3-bis(3-phenylpropyl)urea ( 6g , 346 ). Kinetic studies were performed by varying the amount of amine, base, catalyst, and propylene carbonate substrates to determine the order of each individual component and establish the overall order to be 2. With the help of kinetic data and UV-vis spectra, a possible reaction pathway is proposed through a Ni(I)-Ni(III) pathway.