Conformationally equilibrated carbocation

Fig. 6. Reaction profiles of carbocation conformers, Ci, C2. a) Rapid equilibration b) conformational control...

AG <5 kcal/mole, AG 2 >10 kcal/mole). If the rate of reaction is fast with respect to conformational equilibration the relative populations of the conformers will be of major importance (Fig. 6b). This situation is often encountered with conformationally rigid systems (e.g., polycyclic compounds). Obviously, conformational control can also be brought about by increasing the rate of reaction. We have seen (Section 3) that some 1,2 shifts in carbocations are extremely fast (AG <5 kcal/mole). We expect, therefore, that rearrangements of carbocations may be competitive, at least, with conformational equilibration even in acyclic systems. 

Most of these results have been obtained in methanol but some of them can be extrapolated to other solvents, if the following solvent effects are considered. Bromine bridging has been shown to be hardly solvent-dependent.2 Therefore, the selectivities related to this feature of bromination intermediates do not significantly depend on the solvent. When the intermediates are carbocations, the stereoselectivity can vary (ref. 23) widely with the solvent (ref. 24), insofar as the conformational equilibrium of these cations is solvent-dependent. Nevertheless, this equilibration can be locked in a nucleophilic solvent when it nucleophilically assists the formation of the intermediate. Therefore, as exemplified in methylstyrene bromination, a carbocation can react 100 % stereoselectivity. 

Hyperconjugation has a profound effect on structure and stability of cyclohexyl cations. An elegant study combined theoretical results with experimental data to confirm that different hyperconjugative stabilization patterns lead to the formation of two equilibrating conformers of the 1-methyl-1-cyclohexyl cation where the carbocation p-orbital is oriented either pseudoaxiaUy or pseudoequatoriaUy. 

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