CONFORMATIONS AND STEREODYNAMICS OF DISULFIDE RADICAL-ANIONS GENERATED BY PHOTOREACTION OF ALIPHATIC THIOLS

Photolysis of alkyl thiols (RSH) in alkaline solutions yields the corresponding disulfide radical anions (RSSR.-) whose EPR spectra can be detected over a large temperature range. When mixtures of two different thiols are photolyzed (RSH and R'SH) each of the three possible disulfide radical anions are observed, i.e., RSSR.-, R'SSR.-, and R'SSR'.-. In the case of the allylthiol (CH2=CHCH2-SH) both the corresponding disulfide radical anion and the thioaldehyde radical anion (CH2=CHCH=S.-) are simultaneously obtained, the latter exhibiting two rotational conformers due to the restricted rotation about the C-CS bond. Dilute solutions of dithiols yield the cyclic disulfide radical anions: line shape analysis of the EPR spectra of the 5- and 6-membered ring derivatives allowed us to measure the activation parameters for a ring reversal process corresponding to the exchange between the two possible conformational enantiomers (DELTAG* =4.7(5) and 5.8 kcal mol-1, respectively). A detailed analysis of the EPR line width of the linear disulfides of the type RCH2SSCH2R.- (R=Me, Bu(t)) reveals the existence of diastereotopic methylene hydrogens, thus indicating that the CSSC dihedral angle is not planar. Accordingly, the linear radicals RSSR.- must exist as racemic mixtures of pairs of conformational enantiomers.