Benzenediazonium-2-carboxylates aryne precursors

Although the reaction of arynes with thiophenes as a route to naphthalenes leaves much to be desired, the reaction with thiophene-1,1 -dioxides is synthetically useful. An excess of benzyne precursor (benzenediazonium carboxylate) is used, and the solvent was refluxing 1,2-dichloroethane. Yields of naphthalenes are moderate and conversions are good. Thus 3-methylbenzyne and tetrachlorothiophene dioxide 296 gives naphthalene 297 in 65% yield (96% conversion). The thiophene dioxides may be prepared by m-CPBA oxidation of the corresponding thiophenes. 

The only other useful aryne precursor of the zwitterionic type is diphenyliodonium-2-carboxylate, prepared from 2-iodobenzoic acid. It is much more thermally stable than benzenediazonium-2-carboxylate, decomposing in the range 160-200 °C with elimination of carbon dioxide and iodobenzene (Scheme 7.15). 

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