Molecular sulfide radical cation

An alternative, although less versatile, method to generate molecular sulfide radical cations is based on the reduction of sulfoxides by hydrogen in very acid solution (eq. 41).89,90... 

One-electxon oxidation of organic sulfides (thioethers) leads primarily to a sulfur-centered radical cation, R2S, which is stabilized either by resonance (e.g. if R = Aryl) or interaction with a free electron pair provided by a second sulfur atom (typically in the case of aliphatic sulfides). The latter stabilization occurs inter- or intm-molecularly with the former depending on the sulfide concentration, while the intramolecular process requires a geometry of the two interacting p orbitals which is favorable for overlap. This concept has been verified in numerous experiments and supported by theoretical calculations. ... 

Aromatic sulfides analogous to thiophenols constitute a group of molecules that fulfils the structural conditions necessary for the observation of FET (Sec. 2.4), i.e. they exhibit a low barrier to rotation about the Qp2 S bond. Thus, the torsion motions of the substituents can be accompanied by considerable fluctuation of the electrons in the highest molecular orbitals with two extreme examples of conformers, planar and vertical. The presence of two radical cation conformers was deduced as primary products of the bimolecular free electron transfer (FET) from aromatic sulfides PhSCH2Ph, PhSCHPhj, and PhSCPhg to w-butyl chloride radical based on the nanosecond pulse radiolysis experiments. ... 

The most important feature of the dimer radical cation is, however, its molecular structure in which the two sulfide entities are linked together via a... 

Mann and Cottrell have established that electrochemical oxidation of aliphatic sulfides proceeds through a relatively stable radical-cation 15 localized on the sulfur atom. However, the more stable configuration corresponds to a molecular complex 16 derived from two sulfide molecules according to ESR data. " ° The weak bond that is formed between the interacting sulfur atoms is a two-center three-electron (2c, 3e) bond." Further oxidation results in formation of an ordinary single bond between the two positively charged onium ions. Similar observations were made recently for electrochemical behavior of thianthrene where the ratio of radical-cation to dimer could be easily controlled by concentration and temperature and studied quantitatively using in situ UV/Vis-NIR- and ESR-spectroelectrochemical measurements (Scheme 6). ... 

Proposals for the mechanism of PPS formation include nucleophilic aromatic substitution (Sj Ar) (2radical-cation (27), and radical-anion processes (28,29). Some of the interesting features of the polymerization are that the initial reaction of the sodium sulfide-hydrate with NMP affords a soluble NaSH-sodium 4-(N-methylamino)butanoate mixture, and that polymers of higher molecular weight than pi edicted by the Caruthers equation are produced at low conversions. Mechanistic elucidation has been hampered by the harsh polymerization conditions and poor solubility of PPS in common organic solvents. A detailed mechanistic study of model compounds by Fahey provided strong evidence that the ionic S]s Ar mechanism predominates (30). Some of the evidence supporting the S s(Ar mechanism was the selective formation of phenylthiobenzenes, absence of disulfide production, kinetics behavior, the lack of influence of radical initiators and inhibitors, relative rate Hammet values, and activation parameters consistent with nucleophilic aromatic substitution. The radical-anion process was not completely discounted and may be a minor competing mechanism. 

The difference in reactivity between the a-stannyl sulfide 16A and the a-silyl sulfide 16B can be explained by comparing the two-center energies of their carbon-metal bonds. Semiempirical molecular orbital calculation revealed that the bond energies decrease in the order of 2-silyl, 2-germyl, and 2-stannyl-1,3-dithiane cation radicals. As the silyl dithiane was completely consumed by the oxidation under the... 

---