Regiochemistry and mechanism of oxidation of N-benzyl-N-alkylhydroxylamines to nitrones

The oxidation of various N-(o-, m-, p-substituted benzyl)-N-alkylhydroxylam ines and their dideuteriobenzyl (PhCD2) counterparts was carried out using mercury(II) oxide and p-benzoquinone (p-BQ) as oxidants. An overwhelming pr eference for the formation of conjugated nitrones is observed in the oxidat ion of N-benzyl-N-isopropylhydroxylamines. Considerable intra- and intermol ecular kinetic isotope effects and negative rho values in the Hammet plots point towards a mechanistic pathway that involves electron transfer from ni trogen to the oxidant followed by hydrogen abstraction. The conformation of unstable (E)-nitrones, which readily isomerize to the more stable (Z)-nitr ones, is deduced from H-1 NMR data. The E = Z isomerization was found to be a bimolecular process. Copyright (C) 2000 John Wiley & Sons, Ltd.