Homoallyl cations

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... 

In suitable cases, allylic alcohols can be converted to the cyclopropylcarbinyl cations by reaction with superacids. The reaction involves the rearrangement of the initially formed ally] cation to the homoallyl cation by a 1,2-hydride transfer followed by its cyclization to the cyclopropylcarbinyl cation19 (equation 9). 

Conversion of the acceptor substituent to a function containing a good leaving group LG allows transformation of 16 to the olefin 18 with the homoallyl cation 17 as an intermediate. 

Later Julia and coworkers developed another important mode of ring breaking addition of Grignard compounds or reduction converts the ester 22 to the corresponding alcohols which are startingpoints for a cyclopropylcarbinyl/homoallyl cation rearrangement. After acid treatment p/y-unsaturated carbonyl compounds (e.g. 23) can be isolated, which sometimes isomerize to the oc,(3-unsaturated systems 10). 

Use of diphenyl phosphite for etherification is limited to A -3-hydroxysteroids (a homoallylic cation is probably involved in this case). Thus cholesterol is converted into 3/f-phcnoxy-A -cholcstcne in 50% yield by treatment with diphenyl phosphite in the presence of an acid catalyst. 

It has been suggested" that the formation of the keto-aldehyde (137) in relatively high yield from the sensitized photo-oxidation of thujopsene (138) can best be explained in terms of a dioxetan intermediate (139), similar examples of which have recently been found in singlet oxygen addition to electron-rich double bonds. An extensive analysis of the products of acid-catalysed rearrangement of thujopsene (138) has been carried out." Under different acid conditions ten products have been isolated and identified these include the known compounds, chami-grene (140), cuparene (141), and widdrol (142 R = H) together with the previously unknown compounds (142 R = Et) and (143)—(148). The authors have put forward a mechanistic scheme to explain the formation of all these compounds based on interconversions of cyclopropylcarbinyl and homoallyl cations. 

A bicyclobutonium ion, which is the resonance hybrid of a cyclopropylcarbinyl, a cyclobutyl and a homoallyl cation is considered to account for the reactivity of this system (equation 72) . 

Support for the intermediacy of a homoallylic cholecalciferyl cation (381) in the formation of the 3,5-cyclo-products (375) and (376) came from the solvolysis of cholecalciferyl tosylate (374) under a variety of conditions. Depending upon the solvent, and any nucleophiles supplied, products showed compositions varying from total retention of configuration to 89% inversion, interpreted as evidence of competing pathways through either the homoallylic cation (381) or an 5n2 substitution, respectively. 

Electrophilic addition of hydrohalic acids (hydrogen fluoride, hydrogen bromide, hydrogen chloride), carboxylic acids (e.g. acetic acid) and halogens (bromine, chlorine, iodine) to bicy-cloheptadiene gave a homoallyl cation which underwent rearrangement to nortricyclyl derivatives, i.e. 5-substituted tricyclo[2.2.1.0 ]heptanes. ... 

It appears that the intermediate primary norbornadien-7-ylmethyl cation did not undergo ring enlargement leading to product arising from a secondary bicyclo[2.2.2]octa-2,5-dienyl cation but, more likely formation of a delocalized homoallyl cation through the double bonds of the norbornadiene system - is responsible for the formation of the tetracyclooctane derivative. Such homoallyl cations have been considered as intermediates in the biosynthesis of terpenes. 

The biosyntheses of other monoterpene hydrocarbons, such as those of sabinene, and (-t-)-car-3-ene, are considered to involve the cyclization of such cyclohexenyl homoallyl cations to three-membered ring derivatives. ... 

Other problems dealt with by the Hehre-Pople method are internal rotation in vinylcyclopropane and vinylcyclobutane 153>, the structure of homoallyl cation 154> and ethylenebenzenium cation 155>, torsional barriers in -substituted phenols 156), inversion barriers in -substituted anilines 157>, the effects of a-substitution in keto-enol tautomerism 158> and the circumambulatory rearrangement in bicyclo [3.1.0] hex-3-en-2-yl cation 159> ... 

Exo brosylate 176 forms the ion 181 which is attacked by a nucleophile on four sides. The less active endo epimer 175 also forms the ion 181, but with a parallel Sfj2-substitution which reduces the amount of nortricyclyl compounds and the fraction of rearranged norbomenyl derivatives. The authors maintain that the concept of equilibrated homoallylic cations is not necessary at all. 

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