Thiyl displacement process

The reaction of thiyl radicals with disulfides results in even much longer-lived (> 100 fis) intermediates which are embedded in an extended equilibrium system (eq. 37) constituting, in its essence, a thiyl displacement process.7 ... 

Returning to addition reactions, this mechanism is also involved in what from the final outcome looks like a displacement process, namely, in the reaction of thiyl radicals with disulfides [14, 21], reaction (34a) ... 

In organic chemistry, particularly synthesis, all these displacement reactions with disulfides have for the most part served as a means of obtaining the respective molecular products, and the focus has been much less on the study of the thiyl free radicals per se. Radiation chemical investigations on these processes have, however, revealed many interesting details of the respective mechanisms and, as such, supported a number of suggestions and conclusions made in... 

Although the outcome on the product side is eventually the same, whether or not a transitory adduct intermediate is formed, the kinetics of the thiyl radical formation are certainly affected. Experimental verification of the suggested adduct intermediate is, however, still pending. In fact, even positive identification of the adduct would not prove reaction sequence (16) and (17). Only if the disulfide radical anion (or its decay product RS ) and the molecular products are also known can this mechanism be distinguished from possible competing displacement according to reaction (5), since the latter process may proceed via an adduct as well. 

The propagation steps of these chain reactions are given in Scheme 12, which represents a plausible mechanistic rationalization of the results. That is, Bu Sn radical, initially generated by photolysis of ditin, reacts with alkyl halide to form an alkyl radical that attacks the allyl sulfide (Sh2 process) to give the desired product and thiyl radical. The displacement reaction from ditin gives the Bu Sn radical, thus completing the cycle of this chain reaction. An intramolecular version of this process (equation (43)) has been introduced by Ueno et ai [89a] and later applied by Boger and Wysocki as a key step in multistep synthesis of some natural products [89b]. 

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