2011
DOI: 10.1002/chem.201100094
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Kinetics and Mechanism of Hydrogen‐Atom Abstraction from Rhodium Hydrides by Alkyl Radicals in Aqueous Solutions

Abstract: The kinetics of the reaction of benzyl radicals with [L(1)(H(2)O)RhH{D}](2+) (L(1)=1,4,8,11-tetraazacyclotetradecane) were studied directly by laser-flash photolysis. The rate constants for the two isotopologues, k=(9.3±0.6) × 10(7) M(-1) s(-1) (H) and (6.2±0.3) × 10(7) M(-1) s(-1) (D), lead to a kinetic isotope effect k(H)/k(D)=1.5±0.1. The same value was obtained from the relative yields of PhCH(3) and PhCH(2)D in a reaction of benzyl radicals with a mixture of rhodium hydride and deuteride. Similarly, the r… Show more

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Cited by 3 publications
(8 citation statements)
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“…This very small normal KIE is consistent with the reactant-like transition-state geometry, in which the Rh-H bond has stretched only slightly (from 1.566 to 1.571 Å) indicating that the bond is still intact, that the C-H bond is long (2.365 Å compared to 1.09 Å calculated for CH 4 ), and that the CH 3 moiety is still planar. This normal KIE is again consistent with the work of Pestovsky et al [6] While these initial results suggest that the reaction could proceed rapidly through this simple one-step pathway resulting in a small normal KIE, they do not consider what might happen to the Rh complex in solution prior to reaction with · CH 3 nor more complex pathways such as the proposed formation of a seven-coordinate methyl complex. Thus, examination of a number of other possibilities were undertaken: (1) the loss of Cl -or · Cl, (2) formation of various six-and seven-coordinate methyl species, (3) formation of a complex with solvent or one of the other molecules present in the reaction (e.g.…”
Section: Resultssupporting
confidence: 84%
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“…This very small normal KIE is consistent with the reactant-like transition-state geometry, in which the Rh-H bond has stretched only slightly (from 1.566 to 1.571 Å) indicating that the bond is still intact, that the C-H bond is long (2.365 Å compared to 1.09 Å calculated for CH 4 ), and that the CH 3 moiety is still planar. This normal KIE is again consistent with the work of Pestovsky et al [6] While these initial results suggest that the reaction could proceed rapidly through this simple one-step pathway resulting in a small normal KIE, they do not consider what might happen to the Rh complex in solution prior to reaction with · CH 3 nor more complex pathways such as the proposed formation of a seven-coordinate methyl complex. Thus, examination of a number of other possibilities were undertaken: (1) the loss of Cl -or · Cl, (2) formation of various six-and seven-coordinate methyl species, (3) formation of a complex with solvent or one of the other molecules present in the reaction (e.g.…”
Section: Resultssupporting
confidence: 84%
“…The results of this experiment, however, were more complex and unexpected, as the reaction appears to have an inverse KIE of 0.66 Ϯ 0.30 and an overall rate constant of 2.8 ϫ 10 5 m -1 s -1 . While these results are intriguing this rate constant is significantly different from that reported by Bakac in 1998 [5] In an effort to [6] To rationalize the apparent inverse KIE, Kats et al proposed a reaction mechanism that proceeds through an equilibrium between the octahedral complex and a seven coordinate Rh IV methyl intermediate followed by C-H reductive coupling to form a Rh II -methane complex and subsequent loss of methane, Scheme 1. [3] This reaction mechanism could be at work and could potentially result in the apparent inverse KIE because of the equilibrium step (AЈ Ǟ B·CH 4 ) that precedes the rate determining loss of methane.…”
Section: Introductionmentioning
confidence: 81%
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