Homoallylic carbocation, -stabilized

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... 

The cation 77 is stabilized by hyperconjugation and -)-I-effect of CHj groups. In nonclassical carbocations the MO is formed, at least partially, by the a-overlapping betwmi atomic orbitals. Such an overlapping may be relatively small, as in the unsymmetrical homoallylic ion 18 or much larger, as in the 7-norbomenyl ion 19 where the C —interaction is apurecr-rc-o lap, while C —and C —are practically P,o-overlaps. In the 2-norbornyl cation 5 (according to Streitwieser) there is a... 

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