- Hydrogen atom abstraction reactions from tertiary amines by benzyloxyl and cumyloxyl radicals: Influence of structure on the rate-determining formation of a hydrogen-bonded prereaction complex
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A time-resolved kinetic study on the hydrogen atom abstraction reactions from a series of tertiary amines by the cumyloxyl (CumO?) and benzyloxyl (BnO?) radicals was carried out. With the sterically hindered triisobutylamine, comparable hydrogen atom abstraction rate constants (kH) were measured for the two radicals (kH(BnO ?)/kH(CumO?) = 2.8), and the reactions were described as direct hydrogen atom abstractions. With the other amines, increases in kH(BnO?)/kH(CumO ?) ratios of 13 to 2027 times were observed. kH approaches the diffusion limit in the reactions between BnO? and unhindered cyclic and bicyiclic amines, whereas a decrease in reactivity is observed with acyclic amines and with the hindered cyclic amine 1,2,2,6,6-pentamethylpiperidine. These results provide additional support to our hypothesis that the reaction proceeds through the rate-determining formation of a C-H/N hydrogen-bonded prereaction complex between the benzyloxyl α-C-H and the nitrogen lone pair wherein hydrogen atom abstraction occurs, and demonstrate the important role of amine structure on the overall reaction mechanism. Additional mechanistic information in support of this picture is obtained from the study of the reactions of the amines with a deuterated benzyloxyl radical (PhCD2O?, BnO?- d2) and the 3,5-di-tert-butylbenzyloxyl radical.
- Salamone, Michela,Dilabio, Gino A.,Bietti, Massimo
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supporting information; scheme or table
p. 6264 - 6270
(2011/10/08)
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- Absolute rate constants for the Reactions of tert-Butoxyl, tert-Butylperoxyl, and Benzophenone Triplet with Amines: The Importance of a Stereoelectronic Effect
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Absolute rate constants have been determined for the reactions of tert-butoxyl, tert-butylperoxyl, and benzophenone triplet with a variety of amines.All three reagents abstract hydrogen from the carbon α to nitrogen with rate constants which are much greater than those for the corresponding reactions with hydrocarbons and isostructural ethers.This rate enhancement is attributed to polar effects on the transition state and to the stabilization of α-aminoalkyls.These product radicals are stabilized by conjugation between the unpaired electron and the nitrogen lone pair, and, as a result, the hydrogen abstractions show a pronounced stereoelectronic effect with abstraction being most facile when the C-H bond being broken is eclipsed with the axis of the nitrogen lone-pair orbital.
- Griller, D.,Howard, J. A.,Marriott, P. R.,Scaiano, J. C.
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p. 619 - 623
(2007/10/02)
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