Contributions of quantum mechanical tunneling to the rate of benzylic hydrogen atom abstraction reactions of triplet diarylcarbenes in fluid solution

Shaffer, Myron W.; Leyva, Elisa; Soundararajan, Ν.; Chang, Edith; Chang, David H. S.; Capuano, Vince; Platz, Matthew S.

doi:10.1021/j100172a033

Cited by 10 publications

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“…On the other hand, Platz and his co-workers have studied KDIEs for the benzylic hydrogen atom abstraction of the triplet states of several diarylcarbenes with toluene−toluene- d 8 in fluid solution in the temperature range −50 to +100 °C, and shown that there is a contribution of quantum mechanical tunneling (QMT), especially to the reactions of the sterically hindered carbenes even at this high temperature. This is interpreted as indicating that the decrease in the reactivities of the hindered triplet carbenes results in the relatively slow rate of the classical atom-transfer reaction which allows a QMT pathway to contribute more prominently to the overall hydrogen atom transfer reaction rate.…”

Section: Summarizing Discussionmentioning

confidence: 99%

Polymethylated and Poly(tert)butylated Diphenylcarbenes. Generation, Reactions, Kinetics, and Deuterium Isotope Effects of Sterically Congested Triplet Carbenes

et al. 1997

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Dimesitylcarbene (2a) was shown to decay by undergoing dimerization and to have a half-life of 160 ms, some 5 orders of magnitude longer-lived than diphenylcarbene. Didurylcarbene (2b) was twice as long-lived as 2a, while decamethyldiphenylcarbene (2c) decayed mainly unimolecularly by abstracting H from the o-methyl group and became shorter-lived than 2b. 2,4,6-Tri(tert-butyl)diphenylcarbene (14) decayed unimolecularly almost exclusively by abstracting H from the o-tert-butyl group and showed the lifetime of 125 μs. The Arrhenius plot for H and D abstraction of 2c and 14 gave data consistent with a classical atom-transfer reaction, although the kinetic parameters are very different between the two systems.

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Section: Summarizing Discussionmentioning

confidence: 99%

Polymethylated and Poly(tert)butylated Diphenylcarbenes. Generation, Reactions, Kinetics, and Deuterium Isotope Effects of Sterically Congested Triplet Carbenes

et al. 1997

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“…The activation energy for the reaction of triplet diphenylcarbene and toluene in solution at temperatures near ambient was found to be 3.2 kcal/mol, in only fair agreement with McCurdy’s QMT calculations . Later work would show that QMT also contributed to this reaction, even in fluid solutions at ambient temperature, depressing the experimental result relative to a QMT free process …”

Section: Carbenes As a Source Of Biradicals?mentioning

confidence: 76%

Section: Carbenes As a Source Of Biradicals?mentioning

confidence: 99%

“…19 Later work would show that QMT also contributed to this reaction, even in fluid solutions at ambient temperature, depressing the experimental result relative to a QMT free process. 20 For over two decades, the Moss laboratory 21 and my own 22 have used ns time-resolved spectroscopy to mainly study the reactivity of carbenes in their ground states. Although fascinating, I had the same frustrations as a graduate student.…”

Section: Carbenes As a Source Of Biradicals?mentioning

confidence: 99%

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A Perspective on Physical Organic Chemistry

Platz

2014

J. Org. Chem.

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Intramolecular reactivity of arylcarbenes: Biphenyl-2-ylcarbenes

Dorra

Gomann

Guth

et al. 1996

J. Phys. Org. Chem.

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Biphenyl-2-ylcarbenes, 2-ArC6H,CR, were generated photolytically and thermally from diazo precursors. Cyclization, leading to fluorenes, competes with capture of the carbenes by methanol but proceeds faster than intramolecular hydrogen shifts (with R = Me) and intermolecular C-H insertion reactions (with R = H in cyclohexane). By comparison of product ratios with kinetic data for related carbenes from the literature, the cyclization rate is estimated as cu 10" s-'. The intramolecular reactivity of biphenyl-2-ylcarbenes is not significantly attenuated by variation of R (R = H, Me, Ph). Very minor effects of triplet sensitization and methanol quenching indicate that fluorenes arise from spin-equilibrated biphenyl-2-ylcarbenes, presumably from the singlet state. When A r = mesityl, the carbene predominantly inserts into C-H bonds of the 2'-methyl groups, giving rise to a dihydrophenanthrene. Formation of a fluorene derivative, by formal insertion into C-C bonds, occurs as a minor process. This unprecedented reaction points to intervention of an o-xylylene in which the methyl group migrates. Laser flash photolysis (LFP) of 2-PhC6H,CN,Ph generates a transient absorption which is due to the To -+ T, transition of 9-phenylfluorene rather than to the presumed o-xylylene. On LFP of 2-ArC,H,CN,Ph in trifluoroethanol-acetonitrile, protonation of the carbenes gives rise to carbocations, 2-ArC,H,CH 'Ph. The transient absorption spectra of these cations are strongly influenced by twisting about the Ar-Ar bond (Ar = P h i o-tolyl< mesityl) whereas the rates of nucleophilic capture vary only slightly. Biphenyl-2-ylcarbenium ions (Ar = R = Ph) cyclize more slowly than the analogous carbenes, by a factor of 310,.

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Contributions of quantum mechanical tunneling to the rate of benzylic hydrogen atom abstraction reactions of triplet diarylcarbenes in fluid solution

Cited by 10 publications

References 0 publications

Polymethylated and Poly(tert)butylated Diphenylcarbenes. Generation, Reactions, Kinetics, and Deuterium Isotope Effects of Sterically Congested Triplet Carbenes

Polymethylated and Poly(tert)butylated Diphenylcarbenes. Generation, Reactions, Kinetics, and Deuterium Isotope Effects of Sterically Congested Triplet Carbenes

A Perspective on Physical Organic Chemistry

Intramolecular reactivity of arylcarbenes: Biphenyl-2-ylcarbenes

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