Arynes traps

In the context of this section it is important that Ruchardt and Tan (1970 a) found that (solid) benzenediazonium fluoroborate gave benzyne adducts with potassium acetate in the presence of aryne trapping agents such as tetracyclone or anthracene. This is, however, not the case if water is present (Cadogan, 1971). As a consequence of these observations, Cadogan et al. (1971) simplified the formation of arynes from diazonium ions by converting aniline or its substitution products into arynes in a... 

Evidence for arynes as unstable intermediates in dediazoniation comes from product analyses (Scheme 8-23) and from aryne-trapping experiments. A benzenediazonium ion with a substituent in the 4-position gives the two products indicated in Scheme 8-23 when reacted with a Bronsted acid HY. Analogously, two or three isomers respectively are obtained with benzenediazonium ions substituted in the 2-or 3-position. 

The most prominent among these is tetraphenylcyclopentadienone (tetra-cyclone) (151), whose popularity as an aryne trap arises because its thermal stability permits its use at high temperatures. Furthermore, although the initial adduct 152 is readily decarbonylated, the ultimate product 153 is very... 

A second common aryne trap which may lead to an erroneous inference is the o-quinoidal 1,3-diphenylisobenzofuran 159. With benzyne precursors the... 

The claim for this aryne is based on the isolation of the appropriate adducts 631 or 632 when a crown-ether complex of the diazonium carboxylate potassium salt 633 is decomposed in the presence of tetracyclone (151) or the diphenyltetrazine (169). As pointed out in Section II.2.B.C, however, both of these diene traps may be ambiguous as aryne probes because of their tendency to react with the aryne precursors by an addition-elimination process via 634 and 635 rather than by an elimination-addition mechanism involving the aryne 617. The failure of several other dienes with unambiguous records as aryne traps (Section II.2.B.b.d), such as anthracene (147), to give aryne adducts with the precursor 633 tends to support the former mechanism. On the other hand. 

Several examples of such processes have been noted in Section III and include transhalogenation to the very stable a-metallated compounds 678 or 679, ring opening, and stepwise elimination to nonaryne intermediates which can rearrange or attack the aryne traps which are present. The lower resonance energy of five-membered heterocycles compared to benzenoid compounds makes them much more susceptible to addition reactions, and hence both cine-substitution and Diels-Alder products have been shown to sometimes arise by addition-elimination and not elimination-addition mechanisms. A summary of all the demonstrated nonaryne reactions of five-membered hetaryne precursors discussed in Section III is collected in Table 8. 

Scheme 1-236. Diels-Alder-type cycloadducts obtained by aryne trapping with caibocyclic and heterocyclic dienes.

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