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The synthesis of aryl isocyanates by direct carbonylation of nitroarenes is the most straightforward alternative to the current technology, which involves the use of very toxic phosgene at one stage of the process. However, it faces several problems and has been little investigated in recent years. By taking advantage of our previous studies on the related carbonylation reaction yielding carbamates, we herein report that the use of phosphorus acids, diphenylphosphinic acid in particular, as promoters for the palladium/phenanthroline catalytic system, allows an improvement in activity and turnover number of almost one order of magnitude with respect to the best results previously reported. The effect of several experimental variables was investigated. Mechanistic investigations indicates that the enhanced activity is not associated with pathways involving water-assisted processes or aniline intermediates.
Ramadan et al. (Tue,) studied this question.