Coniglio Alessandra, Galli Carlo, Gentili Patrizia, Vadalà Raffaella
Dipartimento di Chimica, Università La Sapienza, and IMC-CNR Sezione Meccanismi di Reazione, P.le A. Moro 5, 00185, Roma, Italy.
Org Biomol Chem. 2009 Jan 7;7(1):155-60. doi: 10.1039/b815584d. Epub 2008 Nov 7.
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.
氨氧自由基BTNO（苯并三唑 - N - 氧基；>N - O*）由HBT（1 - 羟基苯并三唑；>N - OH）与Ce(IV)盐氧化生成。BTNO呈现出一个λ(max)为474 nm的宽吸收带，这使得通过分光光度法研究其从氢供体底物中夺取氢的动力学成为可能。因此，已在25℃的乙腈溶液中测定了BTNO从X - 取代 -（N - 乙酰基）苄胺（X - C₆H₄CH₂NHCOCH₃）中与氮原子相邻的α - CH₂基团夺取氢的速率常数（k(H)）。k(H)(X)数据与哈米特σ(+)参数的相关性给出了一个较小的ρ值（-0.65），这与自由基夺氢步骤相符。来自适当氘代酰胺底物的动力学同位素效应的相当大的值（k(H)/k(D)=8.8）进一步证实了夺氢是速率决定步骤。获得了立体电子效应相关性的证据，即每当可断裂的C - H键与酰胺部分的氮孤对或相邻芳环共线时，夺氢速度就会加快。据此报道了对ArCH₂NHCOMe中苄基C - H键离解能值的评估。
