Effect of metal ions on the reactions of the cumyloxyl radical with hydrogen atom donors. Fine control on hydrogen abstraction reactivity determined by Lewis acid-base interactions

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DOIResolve DOI: http://doi.org/10.1021/ja309579t
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TypeArticle
Journal titleJournal of the American Chemical Society
ISSN0002-7863
Volume135
Issue1
Pages415423; # of pages: 9
Subject1 ,4-cyclohexadienes; Alkoxyl radicals; Carbon-centered radicals; Donor ability; Donor substrates; Heteroatoms; Hydrogen abstraction; Hydrogen abstraction reaction; Hydrogen atoms; Lewis acid-base interaction; Lewis acidity; Lone pair; Metal salt; Tertiary amine; Time-resolved kinetic study; Transition state; Abstracting; Acetonitrile; Amines; Metal ions; Rate constants; Substrates; Hydrogen; 1,4 cyclohexadiene; acetonitrile; alkoxy radical; amine; cumyloxyl radical; hydrogen; inorganic salt; Lewis acid; Lewis base; lithium ion; lithium perchlorate; magnesium ion; magnesium perchlorate; metal ion; radical; sodium ion; unclassified drug; acid base balance; acidity; article; chemical reaction; chemical reaction kinetics; concentration response; hydrogen bond; molecular interaction; molecular stability; photolysis; Alcohols; Hydrogen; Ions; Lewis Acids; Lewis Bases; Lithium; Magnesium; Molecular Structure
AbstractA time-resolved kinetic study on the effect of metal ions (Mn+) on hydrogen abstraction reactions from C-H donor substrates by the cumyloxyl radical (CumO•) was carried out in acetonitrile. Metal salt addition was observed to increase the CumO• β-scission rate constant in the order Li+ > Mg2+ > Na+. These effects were explained in terms of the stabilization of the β-scission transition state determined by Lewis acid-base interactions between Mn+ and the radical. When hydrogen abstraction from 1,4-cyclohexadiene was studied in the presence of LiClO4 and Mg(ClO4)2, a slight increase in rate constant (k H) was observed indicating that interaction between Mn+ and CumO• can also influence, although to a limited extent, the hydrogen abstraction reactivity of alkoxyl radicals. With Lewis basic C-H donors such as THF and tertiary amines, a decrease in kH with increasing Lewis acidity of Mn+ was observed (kH(MeCN) > k H(Li+) > kH(Mg2+)). This behavior was explained in terms of the stronger Lewis acid-base interaction of M n+ with the substrate as compared to the radical. This interaction reduces the degree of overlap between the α-C-H σ* orbital and a heteroatom lone-pair, increasing the C-H BDE and destabilizing the carbon centered radical formed after abstraction. With tertiary amines, a >2-order of magnitude decrease in kH was measured after Mg(ClO 4)2 addition up to a 1.5:1 amine/Mg(ClO4) 2 ratio. At higher amine concentrations, very similar kH values were measured with and without Mg(ClO4)2. These results clearly show that with strong Lewis basic substrates variations in the nature and concentration of Mn+ can dramatically influence k H, allowing for a fine control of the substrate hydrogen atom donor ability, thus providing a convenient method for C-H deactivation. The implications and generality of these findings are discussed. © 2012 American Chemical Society.
Publication date
LanguageEnglish
AffiliationNational Research Council Canada (NRC-CNRC); National Institute for Nanotechnology (NINT-INNT)
Peer reviewedYes
NPARC number21269872
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Record identifier05285608-318a-4270-98cd-6be420cb26da
Record created2013-12-13
Record modified2016-05-09
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