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 h and 25 °C for isocyanates; 3 h and 100 °C), various aliphatic and substituted aromatic amines are converted into 32 different isocyanate and urea products. Upon optimization, moderate to good turnover numbers (TONs) for isocyanate formations were calculated, with the highest TON for (3-isocyanatopropyl) benzene (4d, TON 277, 55% yield). For ureas, moderate to good TONs were also observed, with the highest TON for 1,3-bis-(3-phenylpropyl) urea (6g, TON 346, 69% yield). Kinetic studies were performed by varying the amounts of amine, base, catalyst, and propylene carbonate to determine the order of each component and establish the overall order as 2. With the help of kinetic data and analytical results, a possible reaction pathway is proposed through a Nickel-(I)-Nickel-(III) pathway.