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 t he previously reported [PPN][Rh(CO)(4)]-based catalytic system for the redu ctive carbonylation of nitrobenzene to methyl phenylcarbamate (PPN+ = (PPh3 )(2)N+). The effect is not due to the known 2-hydroxypyridine-2-pyridone ta utomeric equilibrium, since 4-hydroxypyridine, for which the same tautomeri c equilibrium exists, completely inhibits the reaction. A promoting effect of 2-hydroxypyridine is also observed in the reactions of a previously isol ated metallacyclic complex, [PPN][Rh(CO)(2)ON(Ar)C(O)O], believed to be an intermediate in the catalytic reactions. However, the dependence of the rat e of the catalytic reactions on the aniline concentration indicates that th e effect found for the stoichiometric reaction cannot be the one that is re levant for the acceleration of the catalytic reactions. Thus, two different effects are present, both of which appear to be due to the proximal positi ons of a basic and an acidic site in the promoter molecules. (C) 2000 Elsev ier Science S.A. All rights reserved.