Institution: | 1. Dipartimento di Scienze e Tecnologie Chimiche, Università “Tor Vergata”, Via della Ricerca Scientifica, 1, 00133 Rome, Italy;2. Dipartimento di Scienze e Tecnologie Chimiche, Università “Tor Vergata”, Via della Ricerca Scientifica, 1, 00133 Rome, Italy
QBIS Research Group, Institut de Química Computacional i Catàlisi (IQCC) and, Departament de Química, Universitat de Girona, Campus Montilivi, Girona, 17071 Catalonia, Spain;3. Department of Chemistry, The University of British Columbia, 3247 University Way, Kelowna, British Columbia, V1V 1V7 Canada |
Abstract: | Kinetic and product studies on the reactions of tert-alkoxyl radicals with secondary and tertiary alkanamides bearing benzylic α-C?H bonds, isoindoline, tetrahydroisoquinoline and the corresponding N-acetyl derivatives were carried out. Product studies on the reactions with the tert-butoxyl radical (tBuO?) point toward exclusive HAT from the benzylic α-C?H bonds. Comparison of the kH values measured for reaction with the cumyloxyl radical (CumO?) with those obtained previously for the corresponding reactions of N-alkyl- and N,N-dialkylalkanamides, are indicative of a lack of benzylic activation and the operation of steric and stereoelectronic effects. Compared to N-methyl and N-ethyl groups, the presence of N-benzyl groups increases the barrier required to reach the optimal conformation for HAT, where the α-C?H bond to be cleaved is perpendicular to the plane of the amide, precluding concurrent overlap with the phenyl π-system. When the benzylic α-C?H bonds are in a conformation that allows for optimal overlap with both the phenyl π-system and the amide π-system or amine nitrogen lone pair, as in the isoindoline and tetrahydroisoquinoline derivatives, increases in kH that exceed 2-orders of magnitude were observed, highlighting the strong contribution provided by stereoelectronic activation to these HAT processes. |