Kinetic applications of electron paramagnetic resonance spectroscopy. 32. Further studies of quantum-mechanical tunneling in the isomerization of sterically hindered aryl radicals

Brunton, G.; Gray, J. A.; Griller, D.; Barclay, L. R. C.; Ingold, K. U.

doi:10.1021/ja00481a031

Cited by 80 publications

(62 citation statements)

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“…Finally, let us discuss the well-established example of the isomerization of 2,4,6-tri-tert-butylphenyl radical to 3,5-di-tert-butylneophyl in apolar organic solvents depicted in figure 15, which has been studied by Brunton et al (1978). Various barrier types were used for Bell-type semi-classical tunnelling calculations.…”

Section: Complex H-transfersmentioning

confidence: 99%

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Arrhenius curves of hydrogen transfers: tunnel effects, isotope effects and effects of pre-equilibria

Limbach

López

Kohen

2006

Phil. Trans. R. Soc. B

140

163

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In this paper, the Arrhenius curves of selected hydrogen-transfer reactions for which kinetic data are available in a large temperature range are reviewed. The curves are discussed in terms of the onedimensional Bell-Limbach tunnelling model. The main parameters of this model are the barrier heights of the isotopic reactions, barrier width of the H-reaction, tunnelling masses, pre-exponential factor and minimum energy for tunnelling to occur. The model allows one to compare different reactions in a simple way and prepare the kinetic data for more-dimensional treatments. The first type of reactions is concerned with reactions where the geometries of the reacting molecules are well established and the kinetic data of the isotopic reactions are available in a large temperature range. Here, it is possible to study the relation between kinetic isotope effects (KIEs) and chemical structure. Examples are the tautomerism of porphyrin, the porphyrin anion and related compounds exhibiting intramolecular hydrogen bonds of medium strength. We observe pre-exponential factors of the order of kT/hy10 13 s K1 corresponding to vanishing activation entropies in terms of transition state theory. This result is important for the second type of reactions discussed in this paper, referring mostly to liquid solutions. Here, the reacting molecular configurations may be involved in equilibria with non-or less-reactive forms. Several cases are discussed, where the less-reactive forms dominate at low or at high temperature, leading to unusual Arrhenius curves. These cases include examples from small molecule solution chemistry like the base-catalysed intramolecular H-transfer in diaryltriazene, 2-(2 0 -hydroxyphenyl)-benzoxazole, 2-hydroxy-phenoxyl radicals, as well as in the case of an enzymatic system, thermophilic alcohol dehydrogenase. In the latter case, temperaturedependent KIEs are interpreted in terms of a transition between two regimes with different temperature-independent KIEs.

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Section: Complex H-transfersmentioning

confidence: 99%

“…The solid and dashed lines were calculated as described in the text using the parameters listed in table 2. Data from Brunton et al (1978).…”

Section: K1mentioning

confidence: 99%

Arrhenius curves of hydrogen transfers: tunnel effects, isotope effects and effects of pre-equilibria

Limbach

López

Kohen

2006

Phil. Trans. R. Soc. B

140

163

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“…6 The extent to which deuterium can be used to block intramolecular hydrogen atom transfer is not straightforward to predict, as extremely high kinetic isotope effects have been observed in cases where the predominant mechanism involves quantum mechanical tunneling. 7 Whilst there are many examples of the exploitation of the primary kinetic isotope effect in mechanistic studies, there are relatively few in synthetic applications 8 and hence, we initiated the systematic studies described herein into the effectiveness of deuterium as a "protecting group" for hydrogen atoms bonded to a carbon atom a-to nitrogen. a-Aminoalkyl radicals are valuable reactive intermediates in the preparation of a wide range of amine and amino acid derivatives 9 and their regioselective generation is the key to their effective synthetic use.…”

Section: Scheme 1 15-hydrogen Atom Transfermentioning

confidence: 99%

Investigations into the effectiveness of deuterium as a “protecting group” for C–H bonds in radical reactions involving hydrogen atom transfer

et al. 2008

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Competition experiments have been carried out to determine the extent to which deuterium can be used as a protecting group for carbon-hydrogen bonds in radical-based intramolecular hydrogen atom transfer processes.Translocation of carbon-centred radicals via intramolecular hydrogen atom transfer, in particular from highly reactive aryl and vinyl radicals, represents a popular and powerful method for the remote functionalisation of positions in molecules which would traditionally be regarded as unreactive. 1 High regioselectivity in the hydrogen atom removal/radical generation is of prime importance for such a process to be of synthetic value, and the stereoelectronic desirability for a linear transition state often results in an overwhelming preference for 1,5-hydrogen atom transfer over other possible modes of radical translocation (Scheme 1).Scheme 1 1,5-Hydrogen atom transfer.In certain instances when using radical-based methodology however, hydrogen atom transfer may occur as an unwanted sidereaction. For example, in the total synthesis of the antitumour agent fredericamycin A, 2 Clive et al. found that 1,7-hydrogen atom transfer from an aryl methoxy group (used as a protecting group) to an intermediate vinyl radical occurred after a radical spirocyclisation onto an alkyne. This led to the formation of a substantial quantity of an undesired by-product with the intramolecular hydrogen atom transfer process being found to be competitive with the required quenching of the vinyl radical with triphenyltin hydride. 3 A solution to this problem was found in deuteration of the methyl group, with the primary kinetic isotope effect for deuterium vs. hydrogen atom transfer efficiently retarding the competing side-reaction. 3,4During our initial studies into the radical-based functionalisation of b-amino alcohols, via 1,5-hydrogen atom transfer, 5 we encountered a similar problem. 6 The extent to which deuterium can be used to block intramolecular hydrogen atom transfer is not straightforward to predict, as extremely high kinetic isotope effects have been observed in cases where the predominant mechanism involves quantum mechanical tunneling. 7 Whilst there are many examples of the exploitation of the primary kinetic isotope effect in mechanistic studies, there are relatively few in synthetic applications 8 and hence, we initiated the systematic studies described herein into the effectiveness of deuterium as a "protecting group" for hydrogen atoms bonded to a carbon atom a-to nitrogen. a-Aminoalkyl radicals are valuable reactive intermediates in the preparation of a wide range of amine and amino acid derivatives 9 and their regioselective generation is the key to their effective synthetic use.Competition experiments using two heterocyclic systems were chosen for these studies, in which protecting-radical translocating (PRT) groups 10 were positioned such that 1,5-transfer of hydrogen or deuterium could occur after initial radical generation using standard tin hydride-based methodology. Initial studies were carried out w...

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“…This also decayed with first-order kinetics and yielded k H 3 /k D 3 » 50 at -30 C. It was thought probable that reaction (28.3) would provide one of the first clear and unequivocal examples of QMT in an H-atom transfer. This reaction and related reactions were therefore examined in considerable detail [4,5].…”

Section: Introductionmentioning

confidence: 99%

“…If this were the case, the reaction could be studied at temperatures below 113-K where the effects of QMT should become even more pronounced. Benzene, perdeuteriobenzene and neopentane were used as the solid matrices and kinetic measurements on the isomerization of 1 H were made over a range of temperatures down to the boiling points of liquid nitrogen (77 K) and liquid neon (28 K) [5]. In the overlapping temperature range, the rate constants for isomerization of 1 H were essentially the same for reactions in the solid state and in solution, see Fig.…”

Section: Introductionmentioning

confidence: 99%

Quantum Mechanical Tunneling of Hydrogen Atoms in Some Simple Chemical Systems

Ingold

2006

Hydrogen‐Transfer Reactions

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Not only is it difficult to prove that a given reaction proceeds via tunneling, it is even difficult to define the term tunneling unambiguously. The hydrogen atoms themselves, one presumes, are unaware that they are tunneling: from their vantage point the barrier is, like beauty, only present in the eye of the observer. Moreover, not all observers will see the barrier but only those that have not yet overcome their classical prejudices.Willem Siebrand 1984 IntroductionIn 1933 Bell [1] predicted that, due to quantum mechanical effects, the rate of transfer of a hydrogen atom (H-atom) or proton would become temperature independent at low temperatures. Since that time, kineticists have embraced the concept of quantum mechanical tunneling (QMT) so enthusiastically that it is frequently invoked on the flimsiest of experimental evidence, often using data obtained at, or above, room temperature. At such elevated temperatures, conclusive evidence that the rate of an H-atom or proton transfer is enhanced above that due to "over the top of the barrier" thermal activation, and can only be explained by there being a significant contribution from QMT, is rare. Significant has been italicized in the foregoing sentence because QMT will always make some contribution to the rate of such transfers. The QMT contribution to the transfer rate becomes more obvious at low temperatures. For this reason, the unequivocal identification of QMT in simple chemical systems requires that their rates of reaction be measured at low temperatures.In this chapter, a few simple unimolecular and bimolecular reactions will be described in which the rates of H-atom motion were measured down to very low temperatures. These kinetic measurements provide unequivocal evidence that QMT dominates the reaction rates over a wide range of temperatures. There are two common themes. First, all the experimental data were generated in my own Hydrogen-Transfer Reactions. Edited by

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Kinetic applications of electron paramagnetic resonance spectroscopy. 32. Further studies of quantum-mechanical tunneling in the isomerization of sterically hindered aryl radicals

Cited by 80 publications

References 3 publications

Arrhenius curves of hydrogen transfers: tunnel effects, isotope effects and effects of pre-equilibria

Arrhenius curves of hydrogen transfers: tunnel effects, isotope effects and effects of pre-equilibria

Investigations into the effectiveness of deuterium as a “protecting group” for C–H bonds in radical reactions involving hydrogen atom transfer

Quantum Mechanical Tunneling of Hydrogen Atoms in Some Simple Chemical Systems

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