The hydrogen atom transfer reactivity of sulfinic acids

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 similar to 78 kcal mol(-1); roughly halfway between the RS-H BDE in thiols (similar to 87 kcal mol(-1)) and RSO-H BDE in sulfenic acids (similar to 70 kcal mol(-1)). Regardless, RSH, RSOH and RSO2H have relatively similar inherent H-atom transfer reactivity to alkyl radicals (similar to 10(6) M-1 s(-1)). Counter-intuitively, the trend in rate constants with more reactive alkoxyl radicals follows the reaction energetics: similar to 10(8) M-1 s(-1) for RSO2H, midway between thiols (similar to 10(7) M-1 s(-1)) and sulfenic acids (similar to 10(9) M-1 s(-1)). Importantly, since sulfinic and sulfenic acids are very strong H-bond donors (alpha(H)(2) similar to 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; similar to 1000-times slower than for thiols and similar to 10000000-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 (similar to 10(6) M-1 s(-1)).