Carbocations cyclohexenyl

When 4-/-butylcyclohex-1 -enyl(phenyl)iodonium tetrafluoroborate (3) is heated at 60 °C in chloroform, 1-fluorocyclohexene 4, 1-chlorocyclohexene 5 and l-(o-iodophenyl)cyclohexene 6 are formed with accompanying iodobenzene leaving group (eq 2).3 These three substitution products are best accounted for by formation of an ion pair involving cyclohexenyl cation 7. The cyclohexenyl cation 7 formed picks up fluoride from tetrafluoroborate and chloride from chloroform solvent, and recombines with the iodobenzene generated (eq 3). This kind of reactions with a counteranion and solvent are characteristic of unstable carbocations and are known in the case of phenyl cation generated from the diazonium salt in the Schiemann-type reaction.4... 

The presence of cyclohexenyl cation intermediates was firmly established by the observation of carbocation rearrangement during the solvolysis of 25, in which an initially generated bent vinyl cation 26 with sp2 hybridization rearranges to a more stable linear vinyl cation 27 with sp hybridization [Eq. (16)]. The ratio of the rearranged to the unrearranged ketones depends on the nature of solvents used and changed from 14 86 (27a 26a) in 60% aqueous ethanol to 46 54 in the less nucleophilic 2,2,2-trifluoroethanol. 

Later Olah et al. showed that the trans-chloride 605 with SbFj—SO Fl yield a 3-cyclohexenyl cation 606, but not the ion 520 thus, just as under solvolytk conditions, ion 520 is formed only when cr-participation is possible in the transition state of ionisation. In the same paper it was pointed out that ion 520 could be generated from the alcohol if freshly distilled SbFj in SOjFQ (—78 "C) was used the admixtures of proton acids, the authors maintain, favour the opening of the cyclopropane ring before ionization. The ionization of 3-methyl- 607 and 3-phenyl-cis-bicyclo[3,l,0]hexanol-3 608 in SbFj—SOjFCl results in the 3-methyl-cyclo-hexenyl ion 609 and the 3-methyl-l-phenylcyclopentenyl ion 610. That the ionization of tertiary substrates results in classical tertiary carbocations rather than trishomocyclopropenyl ions appears to be due to the hindrance in passing from the conformation of the bath to the chair which is indispensable for the formation of a delocalized ion. 

Explain why the solvolysis of the endo chloride to give the cyclohexenyl product is rapid, while the exo chloride reacts quite slowly. (Hint This reaction involves carbocation intermediates.)... 

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