Cyclopropylcarbinyl cation intermediates

Evidence has been obtained467 for the involvement of a tertiary cyclopropylcarbinyl cationic intermediate in the rearrangement of presqualene diphosphate to squalene. 16-Oximino-17a-benzyl-17//-hydroxy derivatives in the androstane and estrane series have been converted into 16-oxo-17//-benzyl-17a-hydroxy derivatives with inversed configuration at C(17), on treatment with titanium trichloride. It has been suggested468 that the rearrangement occurs through the key intermediate (401) (see Scheme 97). 

Evidence has been obtained467 for the involvement of a tertiary cyclopropylcarbinyl cationic intermediate in the rearrangement of presqualene diphosphate to squalene. 

Sorensen and Ranganayakulu (1970) studied the 1,3,4,4-tetramethyl cyclo-hexenyl cation [274] and using isotopically labelled precursors observed a degenerate rearrangement. The reaction (177) which equilibrated the C(3)-and C(4)-methyl groups was suggested to involve a cyclopropylcarbinyl cationic intermediate or transition state. 

The various carbenium ions /erf-alkyl, bridgehead-, norbornyl-, allyl-, benzyl- or cyclopropylcarbinyl-cations, which are assumed to be intermediates in these decarboxylations are compiled in ref. [293]. 

Solvolytic studies provided the first structural indication for almost every carbocation-ic intermediate and the C4H,+ ion is no exception. Roberts observed that the solvolysis of cyclopropylcarbinyl or cyclobutyl systems and the diazotative deamination reactions of cyclopropylcarbinyl amine or cyclobutyl amine gave similar product mixtures consisting of cyclopropylcarbinyl, cyclobutyl and allylcarbinyl derivatives in essentially the same ratio1,2. A common cationic intermediate of C3v structure, the tricyclobutonium ion 1, was... 

In solvolytic reactions it was observed that cyclopropylcarbinyl cations are formed as transient intermediates from the homoallylic alcohols 4 (equation 5). 

The 2-aryl substituted cyclopropylcarbinyl cations have partial homoallylic character, whose contribution to the resonance hybrid increases when strong electron-withdrawing substituents (e.g. phenyl) are attached at the C2. Thus, 3-arylcyclobutyl tosylates on acetolysis give the homoallylic acetates predominantly, through the intermediate formation of the 2-arylcyclopropylcarbinyl cations (equation 21). 

With the aim of studying a geometrically well-defined cyclopropylcarbinyl cation Baldwin and Foglesong (1968a) prepared the 8,9-dehydro-2-adamantyl 3,5-dinitrobenzoates [125 X = H, D or T]. The solvolysis of [125] in 60% aqueous acetone proceeded with considerable rate enhancement in comparison with 2-adamantyl tosylate. Scrambling of the label to the 8 and 9 positions in the solvolysis of [125 X = D] and [125 X = T] revealed a degenerate rearrangement (90) of the intermediate 8,9-dehydro-2-adamantyl cation [126]. 

The parent 3-homonortricyclyl cation [147] underwent three-fold degenerate rearrangements in superacids, as shown by its temperature dependent nmr spectra, but only at higher temperatures (—85°C to 20 C) than for the corresponding dehydrohomoadamantyl [144 R = H] and dehydroadamantyl cations [126 X = H], The lower rearrangement rate of [147] was explained by a less favourable formation of the puckered cyclobutyl cation intermediates [159] in this geometrically more constrained system. The assignment of a symmetrical cyclopropylcarbinyl cationic structure to [147] was confirmed by comparison of its and C-nmr spectra with the static counterparts [157]. 

Effectively, they undergo ring expansion into cyclobutanones with a variety of electrophilic reagents with hydrobromic, perbenzoic acids and with tert-butyl hypochlorite, 2-alkyl-, 2-hydroxymethyl- and 2-chloroalkylcyclobutanones are obtained, respectively . The rearrangement takes place most probably through an intermediate cyclopropylcarbinyl cation (equation 77). 

Two cationic structures, protonated cyclopropane and the cyclopropylcarbinyl cation, were among the earliest for which non-classical structures were proposed. Structure elucidation for both of these reactive intermediates owes much to the use of labeled cyclopropanes they will be the focus of attention in this section. 

Homoallylic participation was first invoked by Shoppee to account for the high rates and the stereospecificity of 3/3-(415) and i-cholesteryl (416) interconversions339). The double bond in (415) is clearly restricted to participation by one end. We cannot distinguish, however, between a homoallylic and a cyclopropylcarbinyl cation as the intermediate [cf. (387) and (355)] because both would be chiral. The same limitations apply to the 3-cyclohexenyl system. The tosylate (417) acetolyzes slightly more slowly than cyclohexyl tosylate340). The products include the acetate (418) corresponding to the starting material, the bicyclic acetate (419), and (420) as a result of hydro-... 

---