Mesomeric betaines valence tautomerism

The preceding sections demonstrate two important general routes to six-membered heterocyclic mesomeric betaines. These are (i) deprotonation of appropriate quaternary salts and (ii) valence tautomerism of bicyclic isomers. Both approaches have been used to prepare isoquinolinium-4-olates (170) (Scheme 7). 

When the diphenyl betaine 310 (R = Ph) is generated in acetic anhydride at 120°C in the absence of dipolarophile, a quantitative yield of 1,2-diphenylacenaphthylene (318 R = Ph) is obtained. This desulfurization occurs by initial valence tautomerism to the episulfide (317 R = Ph) which can be isolated in 40% yield if the reaction is performed at 100°C.2 The episulfide 317 (R = Ph) and the hydrocarbon 318 (R = Ph) are also produced by treating the sulfoxide 312 (R = Ph) with phenyllithium. In the presence of oxygen, 1,8-dibenzoylnaphthalene is also a major product, and these observations suggest that the mesomeric betaine 310 (R = Ph) is a common intermediate in these reactions. ... 

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