Singlet Oxygen's Potential Role as a Nonoxidative Facilitator of Disulfide S-S Bond Rotation†

Abstract

The role of singlet oxygen potentially mediating increased conformational flexibility of a disulfide was investigated. Density functional theory (DFT) calculations indicate that the singlet oxygenation of 1,2-dimethyldisulfane produces a peroxy intermediate. This intermediate adopts a structure with a longer S-S bond distance and a more planar torsional angle θ (C-S-S-C) compared with the nonoxygenated 1,2-dimethyldisulfane. The lengthened S-S bond enables a facile rotation about the torsional angle in the semicircle region 0° < θ < 210°, that is ~5 kcal mol^-1 lower in energy than the disulfane. The peroxy intermediate bears n_O → σ_S-S and n_O → σ*_S-S interactions that stabilize the S-O bond but destabilize the S-S bond, which contrasts with stabilizing n_S → σ*_S-S hyperconjugative effects in the disulfane S-S bond. Subsequent departure of O₂ from the disulfane peroxy intermediate is reminiscent of peroxy intermediates which also expel O₂ , yet facilitate cis-trans isomerizations of stilbenes, hexadienes, cyanines, and carotenes. "Non-oxidative" ¹ O₂ interactions with a variety of bond types are currently underappreciated. We hope to raise awareness of how these interactions can help elucidate the origins of molecular twisting.