The hydrogen atom transfer reactivity of sulfinic acids.

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 (?H2 ? 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).

SUBMITTER: Griesser M 

PROVIDER: S-EPMC6148200 | biostudies-literature | 2018 Sep

REPOSITORIES: biostudies-literature