Chiral N-hydroxybenzamides as potential catalysts for aerobic asymmetric oxidations.

Abstract

Chiral N-hydroxybenzamides (1H-3H) have been synthesized as precursors of chiral short-lived N-oxyl radicals 1(•)-3(•). The latter species have been generated by oxidation of 1H-3H with Pb(OAc)4 or hydrogen abstraction from 1H-3H by the tert-butoxyl radical and characterized by UV-vis spectrophotometry and EPR spectroscopy. Through a kinetic study of the hydrogen atom transfer processes promoted by 1(•)-3(•) from three chiral benzylic substrates (1-phenylethylamine, 1-phenylethanol, and α-vinylbenzyl alcohol), a moderate chiral discrimination has been found, with selectivity factors 0.5 ≤ k(H)(S)/k(H)(R) ≤ 2.