Cyclopropylcarbinyl cations equilibrating

The field of long-lived cyclopropylcarbinyl cations has been previously reviewed", emphasizing NMR spectroscopic study of the cations, and their equilibration via degenerate and nondegenerate structures. Hence it was not considered necessary here to report a comprehensive review of the NMR spectral data for all cyclopropylcarbinyl cations, unless it was necessary for the present discussion which centers on recent developments in the field while providing sufficient background for the reader. The reader is referred to previous articles and reviews for further information. 

III. PRIMARY CYCLOPROPYLCARBINYL CATIONS A. Equilibrating (Degenerate) Cations... 

Olah, Roberts and coworkers observed3 1 temperature-dependent chemical shifts for the C4H7+ ion, prepared from cyclopropylcarbinol-l-l3C. They suggested an equilibration involving nonclassical bicyclobutonium ion 2 and the bisected cyclopropylcarbinyl cation 3 (equation 12). 

Distinct evidence for the equilibration of bicyclobutonium with a minor isomer, bisected cyclopropylcarbinyl cation, comes from the ultra-low temperature CPMAS studies of Myhre, Webb and Yannoni25. They have observed a major isomer, the bicyclobutonium ion, with a l3C chemical shift of 15 ppm for the pentacoordinated carbon, and a minor bisected cyclopropylcarbinyl cation, whose cationic center s chemical shift was found to be at 235 ppm. The NMR chemical shifts of the cation are also comparable with those calculated by the IGLO method at that temperature26 27. The energies of these cations were shown to be nearly the same (AAH° = 0.05 kcalmol1). 

There are several examples of secondary and tertiary cyclopropylcarbinyl cations where equilibria are degenerate i.e., the structures that are equilibrating are identical in the absence of deuterium or any other labels. The degenerate equilibria can be broken at lower... 

At the lowest temperatures studied ( —140°C), 13C NMR spectroscopy indicates that 519 is still an equilibrating mixture of bisected o-delocalized cyclopropylcarbinyl cations 521 and bicyclobutonium ion 522.171 172 From the comparison of calculated NMR shifts, the low-lying species is considered to be the bicyclobutonium ion.171,172... 

CH2, Jqh - 189 Hz). Thus all three methylene carbons are identical, and the protons attached to each of them are nonequivalent. Equilibrating classical cyclopropylcarbinyl cations can be ruled out not only based on substantially shielded averaged methylene carbon NMR shift, but also due to the presence of nonequivalent geminal protons on each of the methylene carbons. A pentacoordinated nonclassical bicyclobutonium ion can account for the observed results. The proton NMR absorptions at 3 4.64 and 4.21 are assigned to the endo- and exo-methylene hydrogens, respectively, based on the NMR spectra of the stereospeciflcally deuteriated cyclopropylcarbinyl cations The endo-deuteriated cyclopropylcarbinyl cation was prepared from alcohol 5a, whereas a 1 1 mixture of endo- and exo-deuteriated cations were prepared from alcohol 5b (equations 10 and 11). 

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). 

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... 

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. 

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. 

Considerable support for Roberts suggestion has come from later studies, notably those involving NMR studies in superacid media. Thus, while the spectrum of the a,a-dimethylcyclopropylcarbinyl cation is best rationalized in terms of a classical structure that for the parent system is consistent with neither static nor rapidly equilibrating classical structures The parent system seems best to be described as a mixture of the equilibrating bicyclobutonium ions and, less than 4 kcal mol higher in energy another structure that might be the classical cyclopropylcarbinyl cation ... 

Ion 38 (Fig. 16) could be generated both from cyclobutyl and cyclopropylmethyl precursors. At lowest temperatures studied (= 140 °), ion 38 is still an equilibrating mixture of bisected o-delocalized cyclopropylcarbinyl cations 169 and the bicyclo-butonium ion 170. 

The stereochemistry just described also applies to the transformations of optically active (521) which ionizes to give two distinct bisected cyclopropylcarbinyl cations in a 3 1 ratio407. The tertiary cyclopropylcarbinyl derivative (522), on the other hand, solvolyzes without equilibration of the C(CH3)2 groups4087. A cyclopropylcarbinyl rearrangement with inversion at the migrating carbon is also necessary for... 

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