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