Enhancer carbocation rearrangements

The extent to which rearrangement occurs depends on the structure of the cation and foe nature of the reaction medium. Capture of carbocations by nucleophiles is a process with a very low activation energy, so that only very fast rearrangements can occur in the presence of nucleophiles. Neopentyl systems, for example, often react to give r-pentyl products. This is very likely to occur under solvolytic conditions but can be avoided by adjusting reaction conditions to favor direct substitution, for example, by use of an aptotic dipolar solvent to enhance the reactivity of the nucleophile. In contrast, in nonnucleophilic media, in which fhe carbocations have a longer lifetime, several successive rearrangement steps may occur. This accounts for the fact that the most stable possible ion is usually the one observed in superacid systems. 

For example, acetolysis of r/n /-6-tosyloxytricyclo[5.2.0.02 5]nona-3.8-diene proceeded smoothly at 35 C with stereospeeific rearrangement to c.Y0..mj-9-acetoxylricyclo[4.2.1,02 5]nona-3,7-diene (l).30 Interestingly, the rate of acetolysis of the substrate was considerably enhanced kre[25 C = 6.8 x 104) as compared with that of its bicyclic counterpart 2.30 An important conclusion from these studies is that the anchinteric assistances by cyclobutene in the form of homoallylic participation is effective in the stabilization of the carbocation intermediate. It was found in another study that cyclobutene is better than cyclobutane in terms of anchimeric assistance.31... 

Catalytic reforming92-94 of naphthas occurs by way of carbocationic processes that permit skeletal rearrangement of alkanes and cycloalkanes, a conversion not possible in thermal reforming, which takes place via free radicals. Furthermore, dehydrocyclization of alkanes to aromatic hydrocarbons, the most important transformation in catalytic reforming, also involves carbocations and does not occur thermally. In addition to octane enhancement, catalytic reforming is an important source of aromatics (see BTX processing in Section 2.5.2) and hydrogen. It can also yield isobutane to be used in alkylation. 

A similar stereoselectivity has been described for the Lewis acid catalyzed pinacol rearrangement of vinyl-24 and cyclopropyl-substituLed25 hydroxysulfonates 16 (via mesylation) and 17. Of note is a dramatic rate enhancement observed for the [1,2] rearrangement of silyl-substituted substrates 16, an observation consistent with the well-documented ability of silicon substituents to stabilize / -carbocations. 

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