Hydrogen Atom Abstraction from C-H Bonds of Benzylamides by the Aminoxyl Radical BTNO: A Kinetic Study

The aminoxyl radical BTNO (benzotriazole-N-oxyl; >N-O?) is generated from HBT (1-hydroxybenzotriazole; >N-OH) by oxidation with a CeIV salt. BTNO presents a broad absorption band with lmax 474 nm that lends itself to investigate the kinetics of H-abstraction from H-donor substrates by spectrophotometry. Thus, rate constants (kH) of H-abstraction by BTNO from CH2-groups a to the nitrogen atom in X-substituted-(N-acetyl)benzylamines (X-C6H4CH2NHCOCH3) have been determined in MeCN solution at 25 oC. Correlation of the kH X data with the Hammett s+ parameters gives a small value for r (-0.65) that is compatible with a radical H-abstraction step. The sizeable value (kH/kD = 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