Cyclopropylcarbinyl cations structure

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]. 

L. R. Schmitz and T. S. Sorensen, J. Am. Chem. Soc., 104,2605 (1982). This paper is concerned mostly with substituent effects on the classical cyclopropylcarbinyl cation structure, but footnote 2 in this reference provides a useful listing of all earlier calculations on the system. 

The l3C NMR spectrum of the C4H7+ cation in superacid solution shows a single peak for the three methylene carbon atoms (72) This equivalence can be explained by a nonclassical single symmetric (three-fold) structure. However, studies on the solvolysis of labeled cyclopropylcarbinyl derivatives suggest a degenerate equilibrium among carbocations with lower symmetry, instead of the three-fold symmetrical species (13). A small temperature dependence of the l3C chemical shifts indicated the presence of two carbocations, one of them in small amounts but still in equilibrium with the major species (13). This conclusion was supported by isotope perturbation experiments performed by Saunders and Siehl (14). The classical cyclopropylcarbinyl cation and the nonclassical bicyclobutonium cation were considered as the most likely species participating in this equilibrium. 

The crystal structures of several different cyclopropylcarbinyl cations, each with a hydroxy function on the carbinyl carbon, have been reported59"61. Of these structures some five represent relatively simple systems and in each of these cases a bisected or close to... 

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. 

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. 

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... 

X-ray crystallographic studies of the cyclopropylcarbinyl cations have so far been mainly confined to the hydroxycyclopropylcarbinyl cations, generated from the corresponding carbonyl compounds. Typically, the corresponding hexafluoroantimonate salts were prepared by treating the carbonyl compounds with HF-SbF5, and single-crystal X-ray diffractions were carried out. Childs and coworkers have obtained several X-ray structures on such cations as 135-14017. 

Since Roberts . work, A,great deal of evidence, both experimental and theoretical, has accumulated that indicates that the hicyclobutoninm ion is not the first-formed ion upon solvolysis of unstrained cyclopropylcarbinyl systems. Instead, the structure of the ion apparently is the bisected cyclopropylcarbinyl cation, which is shownin ajlatted-line formulation in 67 aniirr.airorbital dia-, gram fn Figure 9. (See also Section 10.6.)... 

J. D. Roberts,seems to have first used the term nonclassical ion when he proposed the tricyclobutonium ion structure for the cyclopropylcarbinyl cation. See J. D. Roberts and R. H. Mazur, J. Am. Chem. Soc. 73, 3542 (1951). Winstein referred to the nonclassical structure of norbornyl, cholesteryl, and 3-phenyl-2-butyl cations. See S. Winstein and D. 

Two limiting conformations can be drawn for the cyclopropylcarbinyl cation, C3H5-CH2+. Structure 14 has the vacant p orbital lying in the cyclopropane plane whereas 15 has them orthogonal. 

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 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 4-methylcyclopropylcarbinyl [104] and the 4,4-dimethylcyclopropyl-carbinyl [105] cations have been shown by Olah et al. (1970a, 1973a,b Olah and Liang, 1975) to be stable bisected cyclopropylcarbinyl cations. Attached to the structures [104] and [105] are and (ref. CSj) chemical... 

Molecular structures of the cyclopropylcarbinyl cation salts lOa-c have been determined by X-ray. Both 10a and 10b show a bisected geometry that optimizes cyclopropyl conjugation with the cationic center, whereas in 10c such conjugation could lead to antiaromatic destabilization, and it was suggested that the structure reflected geometrical distortion to avoid interaction of the cyclopropyl group with the allyl cation portion of 10c. 

The structure of the cyclopropylcarbinyl cation has remained one of the most subtle and elusive questions in organic chemistry, and despite the increasingly more sophisticated experimental and theoretical techniques applied to this question there are still unresolved questions regarding this species. 

Molecular orbital calculations using 4-3IG and 6-3IG basis sets led to the conclusion that the minimum energy structure for C4Ht was the bisected cyclopropylcarbinyl cation 61 (Table 12) with a plane of symmetry defined by the formally cationic carbonA second structure (64) was also found as an energy minimum that was subtly different from 61, but possessed an unsymmetrical cyclopropyl ring with one corner bent toward the... 

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