Hydrogen Atom Abstraction Selectivity in the Reactions of Alkylamines with the Benzyloxyl and Cumyloxyl Radicals. The Importance of Structure and of Substrate Radical Hydrogen Bonding

Abstract

A time-resolved kinetic study on the hydrogen abstraction reactions from a series of primary and secondary amines by the cumyloxyl (CumO(•)) and benzyloxyl (BnO(•)) radicals was carried out. The results were compared with those obtained previously for the corresponding reactions with tertiary amines. Very different hydrogen abstraction rate constants (k(H)) and intermolecular selectivities were observed for the reactions of the two radicals. With CumO(•), k(H) was observed to decrease on going from the tertiary to the secondary and primary amines. The lowest k(H) values were measured for the reactions with 2,2,6,6-tetramethylpiperidine (TMP) and tert-octylamine (TOA), substrates that can only undergo N-H abstraction. The opposite behavior was observed for the reactions of BnO(•), where the k(H) values increased in the order tertiary < secondary < primary. The k(H) values for the reactions of BnO(•) were in all cases significantly higher than those measured for the corresponding reactions of CumO(•), and no significant difference in reactivity was observed between structurally related substrates that could undergo exclusive α-C-H and N-H abstraction. This different behavior is evidenced by the k(H)(BnO(•))/k(H)(CumO(•)) ratios that range from 55-85 and 267-673 for secondary and primary alkylamines up to 1182 and 3388 for TMP and TOA. The reactions of CumO(•) were described in all cases as direct hydrogen atom abstractions. With BnO(•) the results were interpreted in terms of the rate-determining formation of a hydrogen-bonded prereaction complex between the radical α-C-H and the amine lone pair wherein hydrogen abstraction occurs. Steric effects and amine HBA ability play a major role, whereas the strength of the substrate α-C-H and N-H bonds involved appears to be relatively unimportant. The implications of these different mechanistic pictures are discussed.

Why are sec-alkylperoxyl bimolecular self-reactions orders of magnitude faster than the analogous reactions of tert-alkylperoxyls? The unanticipated role of CH hydrogen bond donation

“…45 Rate constant ratios, k(PhCH2O•)/k(PhCMe2O•), were also found to be >>1 for many other HBAs. [46][47][48][49] These studies support the idea that HB formation would occur between the Ca-H of an incipient salkoxyl and an O-atom in the decomposing tetroxide. Furthermore, such a Cα-H•••O HB would be expected to greatly weaken a neighboring O-O bond in the tetroxide since intramolecular HB formation is known to decrease the BDE of the "free" OH (i.e., the HBA OH group) in both catechols and 1,8-dihydroxynaphthalenes 50 (via 5-and 6-membered rings, respectively).…”

Proton‐Coupled Electron Transfer from Hydrogen‐Bonded Phenols to Benzophenone Triplets

“…All experiments were performed in deuterated benzene and acetonitrile under nitrogen atmosphere. tert ‐Octylamine (TOA) was chosen as the reference base because it reacts slowly with ketone triplets due to the lack of cleavable C−H bonds in the α‐position to the N‐atom (see Figure S1 in the Supporting Information) …”

“…tert-Octylamine (TOA) was chosen as the reference base because it reacts slowly with ketone triplets due to the lack of cleavable CÀHb onds in the a-position to the N-atom (see Figure S1 in the Supporting Information). [17,18] Experiments in benzene: 1 HNMR and CIDNP spectrai nC 6 D 6 of 1, 2, 4, 5 and benzophenonet ogether with signal assignments are presented in Figure 1. In C 6 D 6 ,s pectra obtained with 1 and 2 show that most of the CIDNP polarizations stem from the signals attributedt ot he parent phenol and benzophenone.…”

Modulation of Biorelevant Radical Reactions by Non‐covalent Interactions

“…The reactions of cumyloxyl (CumO•) and benzyloxy (BnO•) radicals with branched alkylamines were studied using the LFP technique [65]. These two radicals possess a very similar reactivity toward hydrocarbon substrates, indicating that the steric hindrance plays a minor role in such reactions [65].…”

“…Theoretical calculations have shown that the reaction of BnO• with amines occurs through a rate-determining prereaction complex, which involves an uncommon C─H⋯N interaction between the CH 2 group of the BnO• radical and the amine N-atom (Scheme 20.20) [65]. Theoretical calculations have shown that the reaction of BnO• with amines occurs through a rate-determining prereaction complex, which involves an uncommon C─H⋯N interaction between the CH 2 group of the BnO• radical and the amine N-atom (Scheme 20.20) [65].…”