Stable ions rearrangement chemistry

By studying stable ion chemistry of polycyclic aromatic systems, Laali et al.862 observed the ring closure of dicyanometacyclophanediene 254 with the involvement of diprotonated intermediate 255 [Eq. (5.318)]. When product 256 was treated again in superacids under different conditions, rearrangement took place to yield 1-cyanopyr-ene through mono- and diprotonated intermediates [Eq. (5.319)]. 

Thus the interesting chemistry of 2-adamantyl cations [198] has been difficult to study under stable-ion conditions since they undergo facile rearrangement to the more stable tertiary 1-adamantyl cations [199]. These difficulties were... 

It is not possible to examine alkyl cations such as er -butyl cation under similar conditions because of the intervention of a myriad of condensation, cyclization, and rearrangement reactions. In 96% sulfuric acid, fer -butanol is converted within minutes to a mixture containing 50% alkanes and 50% cyclopentenyl cations. " One of the major developments in organic chemistry during the decade of the 1960 s was the application of NMR spectroscopy in so-called superacid media to probe the structure of carbonium ions. The most obvious use of this technique is in examining alkyl cations and other less stable ions, the p s of which are not readily measured. In fact, the method is so versatile and the information gained so much more valuable than simple stability measurements that it is now the method of first choice in probing carbonium ion structure. 

Estimating stability it is possible to apply criteria commonly used in organic chemistry. Tertiary alkyl carbocation is more stable than the secondary one which is in its turn more stable than the primary one. For the carbon ions of this type the row of the stability is reversed. Allyl and benzyl cations are stable due to the resonance stabilization. The latter having four resonance structures may rearrange to be energetically favorable in the gas phase tropilium cation possessing seven resonance forms (Scheme 5.3). 

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