Absolute rate constants for hydrogen atom transfer from tertiary amides to the cumyloxyl radical: evaluating the role of stereoelectronic effects

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DOIResolve DOI: http://doi.org/10.1021/jo5013459
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TypeArticle
Journal titleJournal of Organic Chemistry
ISSN0022-3263
Volume79
Issue15
Pages71797184; # of pages: 6
SubjectAmides; Atoms; Hydrogen; Rate constants; C-H bond; Hydrogen atom transfers (HAT); Hydrogen-atom transfer; Steric bulk; Tertiary amides; Time-resolved kinetic study; Free radical reactions; acetamide derivative; acetonitrile; cumyloxyl radical; nitrogen; chemical bond; chemical procedures; chemical reaction; chemical structure; conformation; reaction analysis; stereochemistry; stereoelectronic effect
AbstractA time-resolved kinetic study of the hydrogen atom transfer (HAT) reactions from a series of alkanamides to the cumyloxyl radical (CumO•) was carried out. With N,N-dialkylformamides HAT preferentially occurs from the formyl C-H bond, while in N-formylpyrrolidine HAT mostly occurs from the ring α-C-H bonds. With the acetamides and the alkanamides almost exclusive HAT from the C-H bonds that are α to nitrogen was observed. The results obtained show that alignment between the C-H bond being broken and the amide π-system can lead to significant increases in the HAT rate constant (k H). This finding points toward the important role of stereoelectronic effects on the HAT reactivity and selectivity. The highest kH values were measured for the reactions of CumO• with N-acylpyrrolidines. These substrates have ring α-C-H bonds that are held in a conformation that is optimally aligned with the amide π-system, thus allowing for the relatively facile HAT reaction. The lowest kH value was measured for the reaction of N,N-diisobutylacetamide, wherein the steric bulk associated with the N-isobutyl groups increases the energy barrier required to reach the most suitable conformation for HAT. The experimental results are well supported by the computed BDEs for the C-H bonds of the most representative substrates.
Publication date
PublisherACS Publications
LanguageEnglish
AffiliationNational Research Council Canada (NRC-CNRC); National Institute for Nanotechnology
Peer reviewedYes
NPARC number21272696
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Record identifier3e63e15e-3fbf-4072-b6aa-9c0387f3b451
Record created2014-12-03
Record modified2016-05-09
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