Cyclopropylcarbinyl cations conformation

A closely related stable cation which also exhibits a strong conformational preference is cyclopropylcarbinyl cation. 

Display electrostatic potential maps for both bisected and perpendicular conformers of cyclopropylcarbinyl cation. For which is the charge more delocalized Is the more delocalized cation also the lower-energy cation ... 

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). ... 

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. 

The rigid geometry of the adamantyl nucleus has also been utilized to quantitatively evaluate the stereoelectronic requirements of cyclopropylcarbinyl cations. The bisected conformation of the cyclopropyl cation, 97, is known to be preferred strongly relative to the perpendicular conformation, 98 280 The adamantyl derivate 99 incorporates a cyclopropyl carbinyl system locked in the... 

Similar results are obtained for the corresponding tosylates 281).In conforma-tionally nonrigid systems, on the other hand, where the preferred bisected (97) conformation may be adopted, cyclopropyl substituents enhance solvolysis rates by a factor of approximately 104 30S). Therefore, a free energy difference between 97 and 98 of about 10 kcal/mole (1.36 log 107) may be estimated for tertiary cyclopropylcarbinyl cations at 25 °C28°). 

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 interaction with a cationic center, as in the cyclopropylcarbinyl cation, has a similar character. The main difference is that whereas a proton has no stereochemical requirements with respect to its bond to carbon, the cationic center may take different orientations with respect to the cyclopropane ring. Experiment and theory both agree that the preferred geometry is that known as bisected and that rotation of the cationic center by 90° will raise the energy by about 14 kcal mol The preferred conformation is that which allows the p orbital at the cationic center to interact with the in-plane carbon orbitals in the highest occupied MO. This type of interaction also may be seen in the energies of radical cation states of cyclopropane derivatives as determined by photoelectron spectroscopy . 

This section is confined to cyclopropenium ions. Cyclopropylcarbinyl cations are not considered although these ions are of interest on two counts (1) the relationship with the bicyclobutonium ion, (2) their bisected geometry in certain cases (which has relevance to the conformation of nitrocyclopropane ) as they have recently been reviewed ... 

The reactivity of a large number of geometrically constrained cyclopropylcarbinyl cations have been examined so that the angular dependence of the ability of the cyclopropyl group to stabilize the developing carbocation center could be assessed. Overlap is optimal when the angle 0 in Figure 1, Section II.C, is 0°, as in 7b, and is at a minimum when 6 is 90°, as in 7d. Relative reactivities for conformationally fixed systems... 

The dominant contributor to the reactivity of vinylcyclopropanes in any radical reaction is the form (4a), the cyclopropylcarbinyl radical system. The opening of a cyclopropylcarbinyl radical to a butenyl radical is among the fastest radical processes known, with a rate constant of 1.3 x 10 sec . The various stereoelectronic effects of this rearrangement have been reviewed. - The structure of (4a), deduced from its ESR spectrum - and in agreement with calculations (STO-36 basis set), is in the bisected conformation shown, predicted to be 1.4 kcal mol more stable than its perpendicularly oriented counterpart. Above -100 C only the butenyl radical (4b) can be detected. Substituent effects do not seem to operate here when the substituents are on the cyclopropane (i.e. product stabilization). The cyclopropylcarbinyl cation and anion have structures similar to (4a), bisected conformations (5) and (6), respectively. A concise summary of solvolytic and mechanistic data for system (5) has recently appeared. Reviews of cyclopropylcarbinyl anions and carbenes are also available. ... 

By the ab initio method Hehre and Hiberty have calculated different conformations of the homoallylic ions C H. and the isomeric bisected cyclopropylcarbinyl cation to show all the homoallylic ions I-IV being energetically less stable than ion V by 20-30 kcal/mole. The solvolysis products whose composition points to the intermediate formation of homoallylic rations can 1% attribute either to the fast trapping of unstable ions by counterions or to the change of the stability ratio of the isomeric ions 4 by solvation effects. 

In conclusion, most of the evidence and molecular orbital calculations (see p. 27) support the view that cyclopropylcarbinyl cations have a bisected conformation that is stabilized because of the increased p-character of the bent C—C cyclopropane bonds which increases greatly their C—C hyperconjugative ability. The most important evidence that has been interpreted as arguing against this formulation is the NMR spectra of the cyclopropylmethyl cation itself, and hence there must be certain reservations about the < bove conclusion concerning this ion. It must also be... 

An early success of NMR studies in superacid media dealt with the determination of the preferred conformation of cyclopropylcarbinyl cations. The NMR spectrum of dimethylcyclopropylcarbinyl cation (entry 14) exhibits signals for two nonequivalent methyl groups, clearly supporting the bisected conformation. An alternative conformation, the perpendicular conformation, would exhibit a single peak for two equivalent methyl groups. Subsequent work established the rotational... 

Molecular orbital concepts provide an explanation for the preference for the bisected conformation. In the bisected conformation of the cyclopropylcarbinyl cation, the bent a bonds of the cyclopropane ring are arranged in such a way as to... 

The 8,9-dehydro-2-adamantyl cation (101) incorporates a cyclopropylcarbinyl system in the preferred bisected (97) conformation. As would be expected, then, 8,9-dehydro-2-adamantyl derivatives (e.g.102) are highly... 

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