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

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DOIResolve DOI: http://doi.org/10.1021/ja206890y
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TypeArticle
Journal titleJournal of the American Chemical Society
ISSN0002-7863
Volume133
Issue41
Pages1662516634; # of pages: 10
SubjectAlkyl amines; Hydrogen abstraction; Hydrogen abstraction reaction; Hydrogen atom abstraction; Lone pair; Prereaction complexes; Primary and secondary amine; Secondary and primary amines; Steric effect; Tertiary amine; Time-resolved kinetic study; Abstracting; Amines; Hydrogen bonds; Rate constants; Substrates; Thermomechanical pulping process; Hydrogen; aliphatic amine; amine; benzyloxyl; carbon; cumyloxyl; hydrogen; n,n,n trimethyltempamine; nitrogen; octylamine; radical; unclassified drug; article; chemical interaction; chemical reaction; chemical structure; hydrogen bond; Alcohols; Amines; Hydrogen; Hydrogen Bonding; Molecular Structure; Quantum Theory
AbstractFigure Presented 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. © 2011 American Chemical Society.
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LanguageEnglish
AffiliationNational Research Council Canada (NRC-CNRC); National Institute for Nanotechnology (NINT-INNT)
Peer reviewedYes
NPARC number21271065
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Record identifierad2870a5-5957-4409-9ccf-a194f080e199
Record created2014-03-24
Record modified2016-05-09
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