Carbocations, benzylic detection

When an aromatic gronp is placed o / to the leaving gronp, a new set of products is formed 332 (Scheme 51). Benzofnranones are formed in poor to good yields with no detection of the prodnct. The authors argue that carbocation intermediate LXXXII is formed due to stabilization at the benzylic position followed by formation and snbsequent nncleophilic attack of the acyl anion eqnivalent. 

The fragmentation in the mass spectrometer of the molecular ion of aromatic compounds to produce benzylic carbocations was discussed in Section 15.6. Actually, the benzylic carbocation is thought to rearrange to an even more stable carbocation, also with m/z 91. that is the actual species that is detected. Suggest a structure for this carbocation and explain why it is so stable. 

Acid-catalyzed hydrolysis of 65a yields tram diol as the only detectable product,69 and this diol product is rationalized by axial attack of water on carbocation 66a. Since the lifetime of a simple benzyl carbocation is very short, an argument might be made that the trans diol is instead formed by an A-2 mechanism. However, we have also synthesized the methoxy derivative 65b, and its acid-catalyzed hydrolysis also yields >98% trans diol.70 The lifetime of carbocation 65b is sufficiently long that it can be captured, subsequent to its rate-limiting formation, by azide ion. This result clearly shows that the trans diol product from acid-catalyzed hydrolysis 65b is formed by axial attack of water on a discrete carbocation (66b). 

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