Five-Membered Carbocyclic Arynes

Since the cyclopentadiene anion is isoelectronic with the parent five-membered heterocycles it seems appropriate to consider the corresponding arynes in this review in spite of the absence of a heteroatom. Two such arynes have been suggested as intermediates, ferrocyne (616) and the didehydrocyclo-pentadiene anion (617) itself. The first of these was proposed as one of several possible explanations for the formation of the substitution products 618a and 618b from the reaction of chloroferrocene 619 with a large excess of butyl-lithium. In the presence of added lithium piperidide some piperidylferrocene 

Such associative interactions are well known from both kinetic and structural studies. 

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. 

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As will be described in Section III investigations of the existence of five-membered hetarynes have relied heavily on analogies to the chemistry of the well-authenticated six-membered carbocyclic arynes. It will therefore be the purpose of this section to briefly review and update those aspects of aryne chemistry which have played an important role in this regard. For a more complete coverage of aryne chemistry the reader is referred to the several reviews previously cited and especially to the excellent monograph by Hoffmann. ... 

The remaining categories of arynes to be considered are all on the periphery of the scope of this review. The first of these involves benzo derivatives of five-membered heterocycles where the arynic bond appears in the carbocyclic ring. As mentioned at the outset (Section I.2.B) such species are included on the assumption that the heteroatom in a neighboring ring may effect the reactivity of the aryne. 

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