title: 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 lambda(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 CH(2)-groups alpha to the nitrogen atom in X-substituted-(N-acetyl)benzylamines (X-C(6)H(4)CH(2)NHCOCH(3)) have been determined in MeCN solution at 25 degrees C. Correlation of the k(H)(X) data with the Hammett sigma(+) parameters gives a small value for rho (-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 ArCH(2)NHCOMe is accordingly reported.
authors.affiliation: Dipartimento di Chimica, Università La Sapienza, and IMC-CNR Sezione Meccanismi di Reazione, P.le A. Moro 5, 00185, Roma, Italy.
authors.forename: Alessandra
authors.initials: A
authors.lastname: Coniglio
authors.forename: Carlo
authors.initials: C
authors.lastname: Galli
authors.forename: Patrizia
authors.initials: P
authors.lastname: Gentili
authors.forename: Raffaella
authors.initials: R
authors.lastname: Vadalà