Thiopyranyl Radicals and Related Species

Radicals Containing a Six-Membered Heterocycle 1. Thiopyranyl Radicals and Related Species 

Thioxanthenes cannot give radicals which are aromatic by the definition adopted here, by simple gain or loss of electrons, as conjugation within the heterocycle is incomplete the cation-radical has been suggested to occur in 

9-Phenylthioxanthenyl (161) has long been known and has been the subject of several investigations. There has been a measure of disagreement over the assignment of hyperfine splittings, but all workers 

9-Phenylthioxanthenyl dimerizes like 160 it has also been implicated in reactions purporting to yield 5-substituted thiaanthracenes the observation of the radical is no doubt valid although the mechanistic inferences drawn have been refuted. ° Incidentally, the work of Mislow and co-workers, ° which clearly demonstrates the intrinsic instability and cyclic ylide character of 5(IV)-thiabenzenes and analogs, possibly implies little chance of successfully preparing heteroaromatic radicals from this class of conjugated heterocycle. 

Consistent with prediction,reduction of thiopyrylium ions with zinc metal in acetonitrile produces transient thiopyranyl radicals which dimerize by coupling at the 4-position. The first thiopyranyl radical to have been characterized by ESR is 2,4,6-triphenylthiopyranyl (159) for which a geometry has been deduced which is closely similar to its oxygen analog (29)  

6-phenyl groups of 31 and 34°, respectively, from the plane of the hetero-cycle.  

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