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

Abstract

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 γ-radiolysis. For each derivative, the initial step was a formation of a hydroxysulfuranyl radical proceeding with absolute rate constants of kOH+3-MTPA = 9.1 × 109 M-1 s-1 and kOH+3,3‘-TDPA = 5.8 × 109 M-1 s-1. The subsequent formation of one-electron-oxidized intermediates such as dimeric sulfur−sulfur (S∴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 α- or β-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∴S)- and (S−O)-bonded intermediates were formed for 3-MTPA at neutral solution, electrostatic repulsion nearly prohibited the formation of dimeric (S∴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∴S)+ was observed for 3-CMTPA and 2-CMPTA rationalized by a fast decarboxylation of the α-positioned carboxylate group, yielding α-(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 ≅ 108 s-1 whereas the formation of the (S∴S)-bonded intermediates proceeded bimolecularly with k = (1.9−2.0) × 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∴S bonded intermediates as a function of thioether concentration were too small (2.7 × 107 s-1) to contain the unimolecular rate constant for the formation of the (S−O)-bonded intermediate (k = 108 s-1). Therefore, a mechanism was proposed according to which initially formed hydroxysulfuranyl radicals rapidly converted into the σ*-type (S−O)-bonded intermediate. Subsequently, these either converted into (S∴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 σ-type (S−O)-bonded sulfuranyl radical.