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Issue 36, 2018
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The hydrogen atom transfer reactivity of sulfinic acids

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Abstract

Sulfinic acids (RSO2H) have a reputation for being difficult reagents due to their facile autoxidation. Nevertheless, they have recently been employed as key reagents in a variety of useful radical chain reactions. To account for this paradox and enable further development of radical reactions employing sulfinic acids, we have characterized the thermodynamics and kinetics of their H-atom transfer reactions for the first time. The O–H bond dissociation enthalpy (BDE) of sulfinic acids was determined by radical equilibration to be ∼78 kcal mol−1; roughly halfway between the RS-H BDE in thiols (∼87 kcal mol−1) and RSO-H BDE in sulfenic acids (∼70 kcal mol−1). Regardless, RSH, RSOH and RSO2H have relatively similar inherent H-atom transfer reactivity to alkyl radicals (∼106 M−1 s−1). Counter-intuitively, the trend in rate constants with more reactive alkoxyl radicals follows the reaction energetics: ∼108 M−1 s−1 for RSO2H, midway between thiols (∼107 M−1 s−1) and sulfenic acids (∼109 M−1 s−1). Importantly, since sulfinic and sulfenic acids are very strong H-bond donors (αH2 ∼ 0.63 and 0.55, respectively), their reactivity is greatly attenuated in H-bond accepting solvents, whereas the reactivity of thiols is largely solvent-independent. Efforts to measure rate constants for the reactions of sulfinic acids with alkylperoxyl radicals were unsuccessful. Computations predict these reactions to be surprisingly slow; ∼1000-times slower than for thiols and ∼10 000 000-times slower than for sulfenic acids. On the other hand, the reaction of sulfinic acids with sulfonylperoxyl radicals – which propagate sulfinic acid autoxidation – is predicted to be almost diffusion-controlled. In fact, the rate-determining step in sulfinic acid autoxidation, and the reason they can be used for productive chemistry, is the relatively slow reaction of propagating sulfonyl radicals with O2 (∼106 M−1 s−1).

Graphical abstract: The hydrogen atom transfer reactivity of sulfinic acids

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Publication details

The article was received on 31 May 2018, accepted on 30 Jul 2018 and first published on 31 Jul 2018


Article type: Edge Article
DOI: 10.1039/C8SC02400F
Chem. Sci., 2018,9, 7218-7229
  • Open access: Creative Commons BY-NC license
    All publication charges for this article have been paid for by the Royal Society of Chemistry

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    The hydrogen atom transfer reactivity of sulfinic acids

    M. Griesser, J. R. Chauvin and D. A. Pratt, Chem. Sci., 2018, 9, 7218
    DOI: 10.1039/C8SC02400F

    This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Material from this article can be used in other publications provided that the correct acknowledgement is given with the reproduced material and it is not used for commercial purposes.

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