Hetarynium Species

As noted in Section I.2.C, this review will cover five-membered hetarynium species such as 10 and 11 which are isoelectronic with arynes but in which the dehydro bond is exclusively between a carbon and a heteroatom and not between two carbon atoms. Such species may be cationic (10) or neutral (11) depending on the nature of the heteroatom. Hetarynium ions were first defined,proposed,and, in some cases, rejected as possible reaction intermediates in the six-membered pyridine system (9). The observed chemistry of 9 is that expected for either the a-pyridyl cation structure 9b or, preferably, the cyclic nitrilium ion form 9a. No uniquely arynelike properties have been reported, which accounts for the absence of hetarynium species from Hoffmann s monograph.  

One example of a cine-substitution which in principle could involve a hetarynium species recently has been reported in the pyrazole series. Reaction of the dinitropyrazole 636 with secondary amines leads to the cine-substitution product 637. Although an elimination-addition process via the hetarynium species 638 was considered, an AEa mechanism (Section II.2.A.e) involving 639 and 640 was preferred and adequately accounts for all the observations. 

The most likely route to five-membered hetarynium ions related to 10 is probably the dediazonization of the diazonium salts 641, a method which represents the single successful path to phenyl cations. With sulfur, oxygen, 

Two additional intermediates formed by loss of nitrogen from N-unsubstituted diazoazoles have been mentioned but no dual properties noted. Thermolysis of the diazotetrazole 665 has been suggested to proceed via the carbene 666 which disintegrates to atomic carbon and nitrogen, while a radical intermediate (667 ) is preferred, but not actually proposed, to rationalize the products of photolysis of the diazoimidazole 668.  

It seems clear from the above studies that intermediates isoelectronic with five-membered hetarynes can be generated in the AT-unsubstituted azole series, but their reactivities suggest that dipolar (646b) or carbenic (646c) properties predominate rather than arynic (646a) ones which might arise from overlap of the peripheral orbitals as in (11c). 

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