Homoallyl carbocation

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... 

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. ... 

A homoallylic carbocation can also be made from methyl vemolate (12,13-epoxyoleate) and again this acts as a precursor of cyclopropane eompounds. As already indicated, methyl epoxystearate and BF3 etherate interaet to give 9- and 10-oxostearates. When this reaetion was applied to methyl vemolate in boiling dioxan, the expeeted 0x0 esters were aeeompanied by an oxo-eyelopropane ester, whieh was shown to have flie sfrueture indieated in Seheme 3. The yield of cyclopropane was inereased when the reaction was conducted in benzene solution (Table 2). The keto esters were mixtures of cis and irons isomers (4). The virtual absence of 12-oxo-oleate from the reaction conducted in benzene was confirmed by Ward and van Dorp (7) who reeommended the reaction as a source of 13-oxo-oleate and its reduetion produet 13 -hydroxy oleate. Mueh of this work was also confirmed by an Italian group (8). 

In order to improve the yield of glycosylation, the same research group proposed the use of an aiyl sulfide for the generation of the homoallylic carbocation 110. After extensive experimentation, it was found that 3a,5a-cyclosteroid-6p-yl atyl sulfides 116 and 117 were significantly more reactive that the C-3 atyl sulfide 114 and 115. 

A chiral homoallyl carbocation/cyclopropyl carbinyl carbocation system has been used to prepare ehiral derivatives of the marine natural product, spiniferin-1. Chiral acetals have been used to prepare optically active diterpenoids in biomimetic cationic cyclizations. ... 

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-... 

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... 

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. 

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