RADICAL-CATION INTERMEDIATES IN N-DEALKYLATION REACTIONS

1. A number of mechanistic possibilities exist for P450-catalysed N-de alkylation and have been considered over the years, including C- and N -hydroxylation and sequential electron transfer (SET). With peroxidase s the evidence strongly favours SET and free radicals can be detected. Any mechanism must account for lack of incorporation of label from (H 2O)-O-18 into product by P450s and the high kinetic deuterium isotope effects that are seen in N-dealkylation reactions catalysed by peroxid ases but not P450s. 2. Several lines of evidence support a role for SE T in P450 amine oxidations, including Hammett analysis, products of di hydropyridine oxidations, and products of mechanism-based inhibition b y strained cycloalkylamines. 3. The hypothesis was considered that the P450s act via base catalysis to deprotonate the aminium radical gener ated by SET, since the pK(a), has been estimated to be similar to 9. D ihydropyridine aminium radicals have low pK(a), (<4) and are generally considered to have considerable kinetic acidity. None of the haemopro teins under consideration (including the peroxidases and haemoglobin) showed high kinetic hydrogen isotope effects for the oxidation of [4-H -2]- or [4-H-3]-labelled 1,4-dihydropyridines. These results are conso nant with the view that P450s catalyse the deprotonation of N,N-dialky laniline aminium radicals. 4. Since low isotope effects were seen with biomimetic metalloporphyrin models as well as P450s, the deprotonatio n is attributed to the (FeO)(2+) entity, expected to be a strong base, and not the apoprotein. Thus, the FeO moiety of peroxidases is shield ed, consistent with evidence by others that SET occurs through the por phyrin edge. Both P450s and peroxidases catalysed the oxidative N-deme thylation of aminopyrine and N,N-dimethylaminothioanisole; however, on ly the peroxidases generated the stable coloured aminium radicals. 5. The rates of N-demethylation of variously para-substituted N,N-dimethy lanilines can be used to undertake Hammett or Marcus analysis. The for mer yields rho = -0.6 and the latter an apparent E(1/2) of similar to 1.8 for the formal (FeO)(3+) entity of P4502B1. 6. Even in the oxidati on of N,N-dialkylanilines, a finite rate of N-oxidation is seen (simil ar to 0.1% of N-dealkylation). The simplest paradigm has N-oxygenation and N-dealkylation both proceeding from a common aminium radical inte rmediate.