Sulfur transfer reagents generation

The a,a -dioxothione 109 can be generated from indan-l,2,3-trione using potassium thiotosylate as the sulfur transfer reagent and has been trapped as its [4+2]-cycloadduct 110 (Scheme 66). A 1,3-sulfinate migration from S — O is involved . 

First Storable Source of Diatomic Sulfur S2. Access to diatomic sulfur (a very reactive sulfur species) was limited to methods involving deconposition of reactive intermediates like triph-enylphosphane ozonide or bis(thiobenzoyl)biphenyl (generated in situ from BCI3 and 2,2 -dibenzoylbiphenyl). Their spontaneous decomposition produced the 2-sulfur-transfer reagent. The inconveniencies of this approach lead to a series of ingenious... 

Recently, sulfur transfer to dienes using 6,6-dimethyl-1,4-diseleno-3,7-tetrasulfide and other diselenotetrasulfides was reinvestigated. It is not even necessary to use diselenotetrasulfides as actual reagents. Indeed, they form in the reaction medium from the corresponding diselenides with an excess of elemental sulfur (eq 4). Under the thermal conditions required for efficient sulfur transfers, a homolytic cleavage of the Se-S bond results in generation of active sulfur species delivering sulfur to dienes. 

The homolytic cleavage of the carbon-sulfur bond generates an intermediate radical 17 that adopts an axial orientation. The second electron transfer gives a kinetic a-oriented and configurationally stable lithium reagent 18, which is protonated with retention of configuration. Again, the transient radical defines the stereochemical outcome of the overall process. 

Barton and Crich reported the first examples of the uses of 2-substituted allylic sulfur compounds [53]. Their initial experiments with additions of simple alkyl radicals to allyl sulfides, sulfoxides and sulfones were relatively unsuccessful. This failure was largely due to the fact that the nucleophilic alkyl radicals, which were generated by photolysis of the corresponding Barton ester, underwent addition to a second equivalent of Barton ester faster than they added to the allyl transfer agent. Reactions were much more successful with the electron-deficient acrylate reagent 93 (Fig. 4). Crich was later able to show that this same reagent underwent addition reactions with an acyl radical derived from an acyl phenyl telluride [54]. 

---