Oxidation of N,N-Benzylalkylamines to Nitrones by Mo(VI) and W(VI) Polyperoxo Complexes

Oxidation of N,N-benzylalkylamines in chloroform by Mo(VI) and W(VI) polyperoxo complexes (PPC) of general formula Q+3{PO4[MO(O2)2]4}3− (Q+ = onium ion) yield the corresponding nitrones as oxidized products quantitatively. Only in the case of N,N-benzylmethylamine the formation of nitrone is accompanied by 25% of benzaldoxime. Oxidation of N,N-benzyltertbutylamine and N,N-benzylisopropylamine follows second order kinetics. This finding does not disqualify the hypothesis that the reaction might occur by a rate determining nucleophilic attack of the amine onto the peroxide oxygens leading, through a Bartlett-type transition state, to the probable formation of the corresponding hydroxylamine, which then is converted to nitrone in a faster step. Under this respect PPC behavior seems related to that of the corresponding anionic monuclear oxidants Q+[MO(O2)2L]−) (L = anionic ligand). On the other hand PPC toward N,N-benzylmethylamine behave similar to neutral mononuclear oxidants, MO(O2)2L, since in both cases the formation of an oxidant-substrate association complex appears a probable event along the reaction coordinate. However, whereas for the neutral mononuclear oxidants this oxidant-substrate adduct seems to react further toward external amine molecules through a Bartlett-type transition state, for PPC such an adduct seems to evolve to products through unimolecular events akin enzymatic processes.