Ylide compounds thiophene derivatives

The parent nonclassical annelated thiophenes represented by structure (Id X = S, Y = O, NR, S, Se) are unknown however, there is considerable information on their characteristic reactions, mainly as ylides in the case of those derivatives which have been either generated in solution or isolated as stable solid compounds. 

The known nonclassical A,B-diheteropentalenes consist of compounds containing an annelated thiophene or selenophene ring, the only uncharged nonradical representations of which contain a tetravalent sulfur or selenium, while the charged structures represent carbonyl, azomethine, thiocarbonyl or selenocarbonyl ylides. The parent systems have not yet been synthesized, only substituted compounds being known. The properties of these substituted derivatives provide a good measure of understanding of the reactivities of the parent systems. 

Likewise, several stable S,C-ylides derived from benzo[ >]thiophene and some 2,3-disubstituted benzo[i>]thiophenes have been reported recently <9iT86l l> by rhodium(II) acetate-catalyzed reaction with -icarbonyl diazo compounds. Ylides (e.g. (140)) from benzo[ >]thiophene and dibenzo-thiophene have also been reported by franj-ylidation using phenyliodonium bis(phenylsulfonyl)-methylide <88JHC1599>. 

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