Heteroarynes intermediates

The Sn(EA) mechanism involves a preliminary base-promoted elimination to yield a heteroaryne intermediate and the subsequent addition of, e.g., NH3 to yield the substitution product(s).85... 

Dehydroheteroarenes like (10) and (11) have also been proposed as intermediates in nucleophilic substitution.23-25 Some of these reactions were evaluated uncritically and operation of other mechanisms like addition-elimination (AE) and ring opening-ring closure (ANRORC) can now be demonstrated in many such cases. Nevertheless, there is conclusive evidence for heteroaryne intermediacy in some reactions of heterocyclic halides. From the preparative point of view, nucleophilic coupling of such intermediates has found only limited applications.26-28 Reactive intermediates with an additional formal bond between nonadjacent atoms, like (12) and (13), have also been postulated but again hold little synthetic interest. 

Arynes and heteroarynes are reactive intermediates derived formally by the removal of two adjacent hydrogens from, respectively, a carbocyclic or a heterocyclic aromatic ring. Prototypical examples are benzyne (1) and 3,4-didehydropyridine (2). Since these molecules contain a partial triple bond, they are properly regarded as -ynes and, as will be seen, undergo many reactions typical of that functionality, modified by the inherent strain caused by their nonlinearity. 

Most experimental evidence for the existence of five-membered heteroarynes is questionable indeed it was stated in 1967 that no unambiguous evidence is yet available for the formation of dehydro derivatives of five-membered aromatic heterocycles , and the picture has not changed greatly since then, despite a major (with 591 references) and excellent review on the subject. The only possible exception is for 2,3-didehydrothiophene 787 proposed as an intermediate in the pyrolysis of 2,3-thiophenedicarboxylic anhydride and a recent re-investigation using matrix isolation techniques found no direct... 

More interesting results were obtained from photolysis of797 and its various C and isotopomers in an argon matrix at 23 K. The main products were 805 and 806 carbene 808, and 807 were also identified (IR). Two of these substances (806 and 808) are previously unknown isomers of 2,3-didehydrothiophene. It is possible that the heteroaryne is an intermediate in some of these conversions (803 806 and 808 806) but it could not be... 

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