Cyclooctachalcogen ring formation

Elemental sulfur may be oxidized to a variety of polysulfur cations. Thus, oxidation of Sg in liquid SOj with AsFj or SbFj leads to among other species  

The bonding in is probably fairly subtle but one contributing resonance form is expected to be the following  

The process might very well involve single-electron transfers from Sg and various radical intermediates. Here, however, we ask you to write out a polar mechanism for the reaction. 

WeTl now consider the reverse of what we discussed above, that is, how a cyclooctachalco-gen ring is assembled, a very common process in chalcogen chemistry. Consider, as an example, the following reaction  

An examination of the products indicates that a large number of Se-Se bonds must form, strongly suggesting that Se-on-Se nucleophilic attacks must be taking place. With that insight, the first couple of steps are not difficult to surmise  

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In our experience, the mechanisms of cyclooctachalcogen ring formation are perceived as a bit more difficult than nucleophilic breakdown of the same rings. They are certainly quite challenging exercises in arrow pushing. We hope you ll get more comfortable with practice ... 

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