Hydrogen atom abstraction from C–H bonds of benzylamides by the aminoxyl radical BTNO: A kinetic study

Abstract

The aminoxyl radical BTNO (benzotriazole-N-oxyl; >N–O˙) is generated from HBT (1-hydroxybenzotriazole; >N–OH) by oxidation with a Ce^IV salt. BTNO presents a broad absorption band with λ_max 474 nm that lends itself to investigate the kinetics of H-abstraction from H-donor substrates by spectrophotometry. Thus, rate constants (k_H) of H-abstraction by BTNO from CH₂-groups α to the nitrogen atom in X-substituted-(N-acetyl)benzylamines (X-C₆H₄CH₂NHCOCH₃) have been determined in MeCN solution at 25 °C. Correlation of the k_H^X data with the Hammett σ⁺ parameters gives a small value for ρ (−0.65) that is compatible with a radical H-abstraction step. The sizeable value (k_H/k_D = 8.8) of the kinetic isotope effect from a suitably deuteriated amide substrate further confirms H-abstraction as rate-determining. Evidence is acquired for the relevance of stereoelectronic effects that speed up the H-abstraction whenever the scissile C–H bond is co-linear with either the nitrogen lone-pair of the amide moiety or an adjacent aromatic group. An assessment of the dissociation energy value of the benzylic C–H bond in ArCH₂NHCOMe is accordingly reported.