The Cyclopropylcarbinyl Cation

The cyclopropylcarbinyl cation is characterized, like the previous molecules, by the existence of a high labile 2p orbital which can interact with orbitals of appropriate symmetry in the ring. We can single out the important interactions in the two extreme configura- 

By now the reader will already have been alerted to an important geometrical consequence of the hyperconjugative stabilization of B. Bond 12 should shorten, due to a decreased electron density in the (12 antibonding) Walsh orbital. 

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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-... 

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). 

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. 

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. 

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). 

Figure 6.10 Change in energy of the cyclopropylcarbinyl cation as the cationic center is rotated. From K. B. Wiberg and J. G. Pfeiffer, J. Amer. Chem. Soc., 92, 553 (1970). Reprinted by permission of the American Chemical Society.

If two different C4H7 + ions may exist, which is the more stable The fact that most cyclobutyl derivatives seem to solvolyze directly to the cyclopropylcarbinyl cation strongly suggests that that ion is the more stabilized. Nuclear magnetic resonance studies, however, give conflicting information. The spectrum of the unsubstituted cyclopropylcarbinyl cation in superacid solution is most... 

In discussing the cyclopropylcarbinyl cation before the norbornyl cation we have, chronologically, put the cart before the horse. The first experimentally documented example of anchimeric assistance by a C—C a bond was announced by Winstein and Trifan in 1949.96 These workers studied the solvolysis of exo- and... 

Figure 3.24. 100-MHz 1H NMR spectrum of the cyclopropylcarbinyl cation in SbF5-S02ClF solution at —80°C. (a) 60-MHz spectrum of H2 region (b) 60-MHz spectrum of H2 region from the a,a-dideuteriocyclopropylcarbinyl precursor.171...

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]. ... 

The cyclopropylcarbinyl radical (4), the cyclopropylcarbinyl cation (5) and the cyclopropylcarbinyl anion (6) all dominate the expected reactivity of vinylcyclopropanes since one of these forms will be expected to be a major contributor to either a radical or a polar transition state. It is important to consider the various reactive subunits in some detail in order to understand the big picture reactivity of a vinylcyclopropane system, especially as perturbed by additional substituents. 

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