Homoallylic carbocations

Using methods such as those discussed for the norbornyl cation, nonclassical structures have now been established for a number of carbocations. " Representative examples are shown below. The 7-phenyl-7-norbornenyl cation 19 exists as a bridged strucmre 20, in which the formally empty p orbital at C7 overlaps with the C2—C3 double bond. This example is of a homoallylic cation. The cyclopropyl-carbinyl cation 21, historically one of the first systems where nonclassical ions were proposed, has been shown to exist in superacids mainly as the nonclassical bicyclo-butonium ion 22, although it appears as if there is a small amount of the classical 21 present in a rapid equilibrium. Cations 23 and 24 are examples of p-hydridobridged... 

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... 

Electron demand at the incipient carbocation is also important in determining whether or not homoallylic T aHicipation Takes place. Gassman and Fentiman have plotted the logs of the rates of solvolysis of both 57 and 58 in dioxane-water vs. the Hammett cr+ constants of the X substituents. They... 

Double bonds further removed from the incipient carbocation than the homoallylic position can also assist, in jonizatien-if the..geometry of the system. allows Tt. For example -nitrobenzoate (59) solvolyzes 1500 times faster than the corresponding saturated compound in aqueous acetone. The product is trans,trans-1 -decalol (60). If after 12 half-lives the product is iso-... 

A stereochemical study of the synthesis of unsaturated 1,4-aminoalcohols via the reaction of unsaturated 1,4-alkoxyalcohols with chorosulfonyl isocyanate revealed a competition between an retentive mechanism and an SnI racemization mechanism, with the latter having a greater proportion with systems where the carbocation intermediate is more stable.254 An interrupted Nazarov reaction was observed, in which a nonconjugated alkene held near the dienone nucleus undergoes intramolecular trapping of the Nazarov cyclopentenyl cation intermediate.255 Cholesterol couples to 6-chloropurine under the conditions of the Mitsunobu reaction the stereochemistry and structural diversity of the products indicate that a homoallylic carbocation derived from cholesterol is the key intermediate.256 l-Siloxy-l,5-diynes undergo a Brpnsted acid-promoted 5-endo-dig cyclization with a ketenium ion and a vinyl cation proposed as intermediates.257... 

Further selected examples of the decarboxylative acetoxylation and methoxylations are listed in Table 7 [11-13,17,30,142-147]. The electrogenerated stable carbenium ions, such as benzylic, allylic, homoallylic, and tertiary carbocation intermediates, are likely to... 

Such a homoallylic carbocation may rearrange to either a cyclopropylmethyl or a cyclobutyl carbocation, which leads to the formation of cyclopropane or cyclobutane derivatives. In addition, elimination or direct substitution of the homoallylic system leads to considerable amounts of acyclic alkenes. Therefore, this rearrangement has found little synthetic application as mixtures of products are generally obtained. ... 

The striking influence of y-substitution on product formation is readily explained in terms of the relative charge densities at the cyclopropylmethyl and homoallylmethyl centers in the incipient nonclassical carbocation intermediates. Starting from y-di- and monosubstituted homoallylic iodides such carbocations do have considerable cyclopropyl carbocationic character, which, as a consequence, leads to cyclopropylmethyl derivatives on substitution. In the nonsubstituted case, the intermediate carbocation has predominant homoallylic character which under these conditions results in elimination. 

While several mechanisms for the Prins reaction have been proposed,7,10 there is general agreement that the mechanism shown below is reasonable.7 Attack of olefin 1 on protonated aldehyde 10 gives carbocation 11. This carbocation can then be trapped with another molecule of aldehyde, generating adduct 12. Cyclization of 12 affords dioxane 3. Carbocation 11 can also be captured with water giving diol 4. Finally, elimination of a proton from carbocation 11 affords homoallylic alcohol 5. It is possible that the formation of 5 could also proceed via an ene-type mechanism.8... 

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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