Oxidation of (carboxyalkyl)thiopropionic acid derivatives by hydroxyl radicals.: Mechanisms and kinetics of competitive inter- and intramolecular formation of σ- and σ*-type sulfuranyl radicals

The substituent effects on kinetics and yields of specific intermediates and products for the one-electron oxidation by hydroxyl radicals of various (carboxylalkyl)thiopropionic acid derivatives, 3-(methylthio)propionic acid (3-MTPA), 3,3'-thiodipropionic acid (3,3'-TDPA), 3-(carboxymethylthio)propionic acid (3-CMTPA), and 2-(carboxymethylthio)succinic acid (2-CMTPA) have been investigated employing pulse radiolysis on the nanosecond to microsecond time scale, and gamma-radiolysis. For each derivative, the initial step was a formation of a hydroxysulfuranyl radical proceeding with absolute rate constants of k(OH+3-MTPA) = 9.1 x 10(9) M-1 S-1 and k(OH+3,3'-TDPA) = 5.8 x 10(9) M-1 s(-1). The subsequent formation of one-electron-oxidized intermediates such as dimeric sulfur-sulfur (S therefore S)-three-electron-bonded and monomeric sulfur-carboxylate oxygen (S-O)-bonded sulfide radical cations strongly depended on pH, thioether concentration, and the availability of alpha- or beta-positioned carboxylate functions. A spectral resolution procedure permitted the quantification of all transients present in solution at any time after the pulse. Whereas both (S therefore S)- and (S-O)-bonded intermediates were formed for 3-MTPA at neutral solution, electrostatic repulsion nearly prohibited the formation of dimeric (S therefore S)-bonded intermediates between an overall negatively charged sulfide radical cation of 3,3'-TDPA and a second nonoxidized, overall twice negatively charged, molecule of 3,3'-TDPA. Neither sulfide radical cation complex (S therefore S)(+) was observed for 3-CMTPA and 2-CMPTA rationalized by a fast decarboxylation of the ct-positioned carboxylate group, yielding alpha-(alkylthio)alkyl radicals which were the only products optically observed on the pulse radiolysis time scale. For 3,3'-TDPA, the conversion of the initially formed hydroxysulfuranyl radicals into the (S-O)-bonded intermediates occurred unimolecularly with k congruent to 10(8) s(-1) whereas the formation of the (S therefore S)-bonded intermediates proceeded bimolecularly with k = (1.9-2.0) x 108 M-1 s(-1). These processes did not occur competitively, as the intercepts of plots of pseudo-first-order rate constants for the formation of the S therefore S bonded intermediates as a function of thioether concentration were too small (2.7 x 10(7) s(-1)) to contain the unimolecular rate constant for the formation of the (S-O)-bonded intermediate (k = 10(8) s(-1)). Therefore, a mechanism was proposed according to which initially formed hydroxysulfuranyl radicals rapidly converted into the sigma*-type (S-O)-bonded intermediate. Subsequently, these either converted into (S therefore S)-bonded radical cation complexes via a displacement of the carboxylate oxygen by a second nonoxidized sulfide function, or reversibly ring-opened to yield the monomeric sulfur-centered radical cation. The latter either associated with a nonoxidized sulfide or irreversibly cyclized to a sigma-type (S-O)-bonded sulfuranyl radical.