Cyclopropylcarbinyl cyclobutyl cation rearrangement

In accordance with the previous observations of Dauben and Schleyer the cyclopropylcarbinyl-cyclobutyl cation rearrangement proceeds with net inversion at the migration origin upon nucleophilic attack. 

The rearrangement of the intermediate alkyl cation by hydrogen or methyl shift and the cyclization to a cyclopropane by a CH-insertion has been studied by deuterium labelling [298]. The electrolysis of cyclopropylacetic acid, allylacetic acid or cyclo-butanecarboxylic acid leads to mixtures of cyclopropylcarbinyl-, cyclobutyl- and butenylacetamides [299]. The results are interpreted in terms of a rapid isomerization of the carbocation as long as it is adsorbed at the electrode, whilst isomerization is inhibited by desorption, which is followed by fast solvolysis. 

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). ... 

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 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]. 

The investigations carried out in this area were done primarily to determine the magnitudes of steric and electronic effects on the solvolytic rates and products of reaction in the cyclopropylcarbinyl cation system. The goal of most of these studies was to learn more about the nature of the charge delocalization in the cyclopropylcarbinyl system and of the stereochemistry of the cyclopropylcarbinyl-cyclobutyl and cyclopropyl-carbinyl-allylcarbinyl cation rearrangements. Key papers in these studies were those in 1966 by Schleyer and Van Dine, in 1971 by Majerski and Schleyer and in 1974 by Poulter and Spillner which demonstrated that in the simple cyclopropylcarbinyl system... 

Paquette and coworkers have carried out a solvolytic study of a tricyclic system in which a 3,4-ethano group is attached to the 2-bicyclo[3.1.0]hexyl system. This was done to assess the importance of cyclopropyl versus cyclobutyl neighboring group involvement. Some relative rate data are given in Scheme 10 for 3,5-dinitrobenzoate hydrolyses in 80 % aqueous acetone at 115 °C. Product results are also shown. Deuterium labeling revealed the lack of any cyclopropylcarbinyl-cyclopropylcarbinyl cation rearrangements. Also, the results showed that cyclopropyl participation predominates over initial involvement of the cyclobutyl group. 

An ion cyclotron resonance study of ion-molecule reactions in benzene has shown that the cyclopropenyl cation arises from a a-type complex between protonated benzene and benzene. The mass spectral fragmentation of cyclopropanol monoethers has been examined and subjected to theoretical study and the fragmentation of a series of underivatized aminocyclitol-aminoglycoside antibiotics is believed to involve the ion (387). The chemical ionization mass spectrum of oct-6-en-3-one has been interpreted as involving the cyclobutyl-cyclopropylcarbinyl cation rearrangement (388)- (389). ... 

Dugan et al. (1965) have proposed that marasmic acid (I) is formed in nature from the cyclobutyl cation II, also a suspected precursor of illudin S and M, which occur in closely related fungi. In the marasmic acid case, II rearranges to give the alternate cyclopropylcarbinyl system, as in III and I. Oxidations at the starred carbons in III give marasmic acid (I). 

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