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

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 nO → σS–S and nO → σ*S–S interactions that stabilize the S–O bond but destabilize the S–S bond, which contrasts with stabilizing nS → 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" 1O2 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. [ABSTRACT FROM AUTHOR]