Mechanistic studies of palladium-catalysed carbonylation reactions of nitro compounds to isocyanates, carbamates and ureas

Many different palladium-based catalytic systems have been reported for the carbonylation reactions of organic nitro compounds to isocyanates, carbamates and ureas. Almost all of these can be roughly divided into three groups: (i) those containing a second (or even a third) metal (usually a Lewis acid or a metal oxo compound or both), (ii) those in which phenanthroline or similar chelating nitrogen ligands are used and (iii) those in which monodentate phosphines are employed as ligands. The systems in which chelating phosphines are used as ligands lie in between the last two groups. The reaction mechanisms for the catalytic systems in each group appear to be related. Most of the information available does not derive from strictly mechanistic studies, but rather from synthetic studies and it is here critically analysed and compared with the information obtained from other related fields.