Cyclopropylcarbinyl cation rearrangement

Since there are three possible ways to rearrange cyclopropylcarbinyl cation (86) of the type proposed in presqualene pyrophosphate conversion, and the unwanted cyclopropylcarbinyl (86) to allyl (87) rearrangement has been found to account for 99% of total reaction flux in model studiessqualene synthetase must exert strict regiochemical control in the catalytic steps to produce the enzymatic product squalene via the kinetically and thermodynamically unfavored (ca. 0.04 % of the total non-enzymatic flux) rearrangement process (86 82). A tight enzyme-substrate complex that imposes an energy barrier... 

Vinylcyclopropanes represent particularly useful functionality. They do permit a ring expansion to cyclobutanes via the cyclopropylcarbinyl cation manifold (Eq. 9). Equally important, such systems suffer smooth thermal rearrangement to cyclopen-... 

Artemisyl, Santolinyl, Lavandulyl, and Chrysanthemyl Derivatives.— The presence of (41) in lavender oil has been reported earlier. Poulter has published the full details of his work (Vol. 5, p. 14) on synthetic and stereochemical aspects of chrysanthemyl ester and alkoxypyridinium salt solvolyses (Vol. 3, pp. 20—22) and discussed its biosynthetic implications. Over 98% of the solvolysis products are now reported to be artemisyl derivatives which are formed from the primary cyclopropylcarbinyl ion (93) which results from predominant (86%) ionization of the antiperiplanar conformation of (21)-)V-methyl-4-pyridinium iodide the tail-to-tail product (96 0.01%) may then result from the suprafacial migration of the cyclopropane ring bond as shown stereochemically in Scheme 3. This is consistent with earlier work (Vol. 7, p. 20, ref, 214) reporting the efficient rearrangement of the cyclobutyl cation (94) to (96) and its allylic isomer, via the tertiary cyclopropylcarbinyl cation (95). ... 

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). 

From solvolytic studies of iso topically labeled substrates it was shown that cyclopropyl-carbinyl-cyclobutyl interconversion is stereospecific51 52. The stereospecific interconversion of cyclobutyl cations to the corresponding cyclopropylcarbinyl cation was also cleanly observed in superacid medium, and was used to prepare otherwise unstable cis-(a-methylcyclopropyl)carbinyl cation 1753. Thus ionization of d.s-2-chloro- or cw-3-chloro-l-methylcyclobutane in SbF5-S02ClF at -135 °C yielded the ris-isomer which rapidly rearranged irreversibly into the trans-isomer 18 at about -100 °C. The trows-isomer 18 is the only cation obtained when the preparation was carried out at -80 °C, or when prepared from the cyclopropylmethyl carbinol20b 38 50ac (equation 24). 

The reactive cyclopropylidene cyclopropylcarbinyl cation 37 was generated from the corresponding bromide and silver hexafluoroantimonate, and was reacted in situ with olefins such as cyclohexene to form the addition products (equation 33). On the other hand, under the same conditions, the isopropylidene cyclopropylcarbinyl cation 41 rearranged spontaneously by ring expansion (equation 34)71. 

The 3-homonortricyclyl cation 70 was prepared by the isomerization of bicyclo[3.2.1]oct-3-en-2-yl cation 71 at 20 °C in SbFs/SC ClF solution83. The ion shows a threefold degenerate rearrangement between -85 °C to 20 °C. At 20 °C the C4, C6, C8 and Cl, C3, C7 carbons become equivalent with an average of 36.19 ppm and 135.8 ppm, respectively (equation 43). Below -80 °C the cation is a static secondary cyclopropylcarbinyl cation with the cationic center chemical shift at <5I3C 234.1. 

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. 

Olah and coworkers NMR study of the 8,9-dehydro-2-adamantyl cation, obtained from the corresponding alcohol in superacidic medium, showed the equivalence of C2, C8 and C9 carbons ((5 C 157.0) in agreement with the conclusions of solvolytic studies Even at -120 °C the structure of the cation could not be frozen out to a static cation, showing the extremely fast equilibration of the threefold degenerate cyclopropylcarbinyl cation 74. An identical NMR spectrum was obtained from the ionization of the 2,5-dehydro-4-protoadamantanol, which prompted the suggestion of the intermediacy of the 2,5-dehydro-4-protoadamantyl cation 75. The ion rearranges to an ally lie cation 76 at -78 (equation 45). 

The parent spirocyclopropylbenzenium ion 102 can be prepared by the ionization of -phenylethyl chloride in HF-SbF5-S02ClF at -90 °C followed by warming to -60 °C. At higher temperatures (-27 to -5 C), the ion isomerizes to the a-methylbenzyl cation 103 with an activation energy of 13 kcalmol as shown by the NMR kinetic study (equation 61). The rearrangement probably proceeds through a partially delocalized primary 2-phenylethyl cation 104 ". The NMR spectrum of the ethylenebenzenium ion 102 showed (5 C 68.8 (Cl), 171.8 (C2, C6), 133.4 (C3, C5), 155.4 (C4), 60.7 (CH2). The equivalence of the C3, C5 and C2, C6 carbons as well as the methylene carbons show that the ion has Qv symmetry. The deshielded absorptions for the CH2 carbons are similar to those in other cyclopropylcarbinyl cations. 

Mechanism (82) would, however, produce unusual C-satelIites, and both explanations (83) and (84) were disregarded after comparison with the average chemical shift for the six proton group (6.1 ppm) calculated from 4-methyl-cyclopropylcarbinyl cation [104], a thermodynamically favoured rearrangement product of [113] observed at higher temperatures. Average chemical... 

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]. 

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. 

---