The hydrogen atom transfer reactivity of sulfinic acids

Sulfinic acids (RSO 2 H) 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 RSO 2 H have relatively similar inherent H-atom transfer reactivity to alkyl radicals (∼10 6 M −1 s −1 ). Counter-intuitively, the trend in rate constants with more reactive alkoxyl radicals follows the reaction energetics: ∼10 8 M −1 s −1 for RSO 2 H, midway between thiols (∼10 7 M −1 s −1 ) and sulfenic acids (∼10 9 M −1 s −1 ). Importantly, since sulfinic and sulfenic acids are very strong H-bond donors ( α H 2 ∼ 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 O 2 (∼10 6 M −1 s −1 ). Sulfinic acids are characterized to be very good H-atom donors to each of alkyl and alkoxyl radicals. In order to participate in useful radical chain reactions, the sulfonyl radicals must undergo fast propagating reactions to avoid autoxidation, which is surprisingly rate-limited by the reaction of sulfonyl radicals with oxygen.