Cyclopropylcarbinyl cyclobutyl cation

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. 

Cyclopropylcarbinyl-cyclobutyl ring expansions (Eq. 5) are facilitated by the presence of the heteroatom substituent in the order O > S > Se. In this case, the heteroatom stabilized cyclobutyl cation (see Eq. 39) can suffer hydrolysis to give the... 

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). ... 

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... 

The planar cyclobutyl cation and the perpendicular cyclopropylcarbinyl cation are of nearly equal energy and much less stable than the bisected cyclopropylcarbinyl or bicyclobutonium ion (ca 36 kcalmol"1) while the latter two structures have very similar energies. Inclusion of correlation at the MP4SDQ/6-31G //MP2-6-31G levels showed that bicyclobutonium ion is more stable than the bisected cyclopropylcarbinyl cation only by 0.7 kcalmol"1. Selected structural parameters for the bisected and perpendicular conformers (6 and 7) of the cyclopropylcarbinyl cation as well as the bicyclobutonium ion 2, as calculated at the MP2/6-31G level, are as shown26. 

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). 

On the other hand, there is now a good deal of evidence that the solvolysis of most cyclobutyl derivatives does lead directly to the cyclopropylcarbinyl cation. For example, orbital symmetry considerations (Section 11.3) indicate that the conversion of cyclobutyl cations into cyclopropylcarbinyl cations should occur by disrotatory ring opening as shown in Figure 6.11 but any steric factors that would hinder such a process decelerate most cyclobutyl solvolyses. Thus 86 See note 84(b). 

The ab /w/nWIGLO/NMR method has been used to determine the relative distribution and stability difference of the cyclopropylcarbinyl cation and cyclobutyl cation in solu-tion. Agreement between IGLO chemical shifts and experimental shifts could only be obtained when assuming a rapid equilibrium between the two cations. Over the range of temperatures considered (-61 to -132° C), a cyclobutyl cation structure with an axial H atom and short 1,3-distances of 1.65 A (bicyclobutonium ion structure) was found to be more stable by 0.5 kcalntoT For the gas phase, however, the cyclopropylcarbinyl cation was calculated to be 0.26 kcalmoT more stable [MP4/6-31G(d)//MP2/6-31G(d) calculations including vibrational corrections]. ... 

Olah and coworkers obtained the parent cyclopropylcarbinyl cation and characterized it by both and NMR spectroscopy. The NMR spectrum of the cation shows two overlapping quartets (/ = 8 and 6.5 Hz) for the methine protons, and two sets of doublets for the methylene protons, <5 H 4.64 and 4.21. Thus the methylene hydrogens are stereochemically non equivalent, which is unexpected for classical cyclopropylcarbinyl or cyclobutyl cations. The NMR spectrum shows only two signals 108.4 (CH), and... 

Phenylethyl cations, phenonium ions and benzyl cations 262 Cyclopropylcarbinyl cation, bicyclobutonium ion, cyclobutyl cation and related systems 265... 

Cyclobutyl- or cyclopropylcarbinyl chloride or the corresponding alcohols react with SbFj to give stable ion solutions with identical H- and C-nmr spectra. These spectra indicate either an ion with three-fold symmetry, the tricyclobutonium ion [32], or a set of rapidly equilibrating, less symmetrical ions with the same effective averaged symmetry. These include cyclopropylcarbinyl cation [33], bicyclobutonium ion [34] or bent cyclobutyl cations [35]. Theoretical calculations predict similar energies for [33]-[35], but indicate that [32] is less likely (Hehre and Hiberty, 1974 Hehre, 1975). Isotopic perturbation studies by Saunders and Siehl (1980) indicate that the... 

CYCLOPROPYLCARBINYL CATION, BICYCLOBUTONIUM ION, CYCLOBUTYL CATION AND RELATED SYSTEMS... 

These extraordinary observations concerning the differences between the two sets of methylene protons, i.e. very different A s and opposite signs of the isotopic perturbations, could most easily be accommodated within a set of equilibrating bicyclobutonium ions [34]. Neither bisected cyclopropylcarbinyl cations [33] nor puckered cyclobutyl cations [35] are expected, among classical models, to have extremely different protons in their methylene groups. However, [34] has one pentaco-ordinated carbon whose attached hydrogens might have unusual chemical shifts and C—H bond force constants. 

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]. 

A recent study of photoelectron spectra of isomeric C4H7 radicals has provided heats of formation of the allylcarbinyl cation (231 3 kcal mol" ) and the cyclobutyl cation (225.1 1.1 kcalmol" ) but the failure of attempts to generate the cyclopropylcarbinyl radical meant that no heat of formation of the corresponding cation could be obtained Similarly, the heats of formation of geometrically reorganized structures such as puckered cyclobutyl cations and bicyclobutonium ions could not be assigned. Further experiments along these lines are clearly indicated, as well as further calculations to resolve the current dichotomy in the theoretical studies. 

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... 

The Spiro [2,2]pentylmethyl system (31) is of considerable interest. The acetolysis products shown below reveal interesting cyclopropylcarbinyl to cyclobutyl cation and... 

In Section 7.4.1 we have been concerned with double bonds predisposed for symmetrical (homoaromatic) participation whereas Section 7.4.2 was devoted to systems where only one end of the double bond is accessible to the incipient carboca-tion. 3-Buten-1-yl substrates (455) cannot approach a symmetrical transition state, but two homoallylic interactions are possible (387), leading toward a cyclopropylcarbinyl cation (388), and (456), leading toward a cyclobutyl cation (458). Structure (387) implies strong interaction between C-l and C-3 with little or no overlap be-... 

Methyl, phenyl, and cyclopropyl substituents at C—4 of (455) promote the formation of cyclopropylcarbinyl products by stabilizing the intermediate (5SS)360. Rate enhancement over the saturated analogs is observed with the activated double bonds but not with the parent (455)361. A 3-methyl group, on the other hand, stabilizes the cyclobutyl cation (458). The fate of the deuterium label in (465) shows that ring closure proceeds directly to a 1-methylcyclobutyl cation362. ... 

Wiberg, K.B. (1968) Application ofthe Pople-Santry-Segal CNDO method to the cyclopropylcarbinyl and cyclobutyl cation and to bicyclobutane. Tetrahedron, 24, 1083-1096. 

Cationic cyclization and its reverse have particular relevance, since ring closure and ring opening have been invoked to account for a large number of biosynthetic pathways.The gaseous cyclopropylcarbinyl-cyclobutyl-homoallyl system represents an extreme instance of a rapid, reversible isomerization of this sort, which has been studied over a period of 20 years both by /1-decay of tritiated cyclobutane and by radiolytic methods. This interconversion is so fast that the three structures... 

Hydrolysis of both the epimeric tosylates (746) in the presence of CaCOj gives similar mixtures of products. The bromo-ketone (748) is accounted for by the intermediacy of a cyclobutyl cation and (747) is formed via a cyclopropylcarbinyl cation (Scheme 56). 

---