Promotion of the [PPN][Rh(CO)4]-catalysed carbonylation of nitrobenzene by 2-hydroxypyridine and related molecules: an apparent bifunctional activation

2-Hydroxypyridine and related molecules have a large activating effect on the previously reported [PPN][Rh(CO)4]-based catalytic system for the reductive carbonylation of nitrobenzene to methyl phenylcarbamate (PPN+=(PPh3)2N+). The effect is not due to the known 2-hydroxypyridine–2-pyridone tautomeric equilibrium, since 4-hydroxypyridine, for which the same tautomeric equilibrium exists, completely inhibits the reaction. A promoting effect of 2-hydroxypyridine is also observed in the reactions of a previously isolated metallacyclic complex, [PPN][], believed to be an intermediate in the catalytic reactions. However, the dependence of the rate of the catalytic reactions on the aniline concentration indicates that the effect found for the stoichiometric reaction cannot be the one that is relevant for the acceleration of the catalytic reactions. Thus, two different effects are present, both of which appear to be due to the proximal positions of a basic and an acidic site in the promoter molecules.