Singlet Oxygen's Potential Role as aNonoxidativeFacilitator 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 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.

Bluer Phototruncation: Retro-Diels–Alder of Heptamethine Cyanine to Trimethine Cyanine through an Allene Hydroperoxide Intermediate

“…Next, we show that H-abstraction by O2 arises in an intramolecular reaction with a transition state barrier of 6.7 kcal/mol in an exothermic process to reach allene hydroperoxide 6 . Notably, peroxy intermediates arise in other 1 O 2 oxidation reactions, such as R 2 S + OO – persulfoxide, and have been implicated in the lowering of rotational energy barriers. − Similarly, allene hydroperoxides and hydroxides have been previously reported in 1 O 2 oxidation of diene and trienes, and are structurally reminiscent of allene hydroxides. − The protonation and hydration reactions suggest a zwitterionic form of the peroxy intermediate rather than a diradical formation. We note that Li and Smith have reported that long-term storage of indocyanine green under low-intensity light leads to oxidative dimer formation, likely via radical intermediates formed by electron transfer to form superoxide, dimerization, and the subsequent hydrogen-atom abstraction reaction.…”

On the mechanism of visible‐light sensitized photosulfoxidation of toluidine blue O

“…We conducted DFT calculations using Gaussian-16, 27 in which compounds were optimized with M06-2X/6-31G(d,p) in the gas phase in a similar manner as reported recently. 28 Unrestricted M06-2X/6-31G(d,p) calculations were conducted for the peroxyl radical and 3 O 2 , where the latter value incorporated the experimentally known singlet-triplet energy value of 22.5 kcal/mol, which was added to this calculated value of 3 O 2 . Harmonic vibrational frequency calculations were also carried out on the stationary points on the potential energy surface (PES).…”