Perpendicular cyclopropylcarbinyl conformation

An early attempt to achieve the perpendicular cyclopropylcarbinyl conformation by means of t-butyl-substituted spiro[2,5]octyl derivatives was unsuccessful. Thus, the cis isomer (52) with the t-butyl and leaving groups both equatorial was expected to solvolyze much more slowly than the trans isomer (51) having an equatorial t-butyl group and an axial leaving group. However, the small rate difference of 2 1 revealed that the... 

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. 

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 ... 

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... 

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 -KX) T only the butenyl radical (4b) can be detected. Substituent efiects do not seem to operate here when the substituents are on the cyclopropane (i.e. product stabilization). The cy-clopropylcaibinyl cation and anion have structures similar to (4a), bisect conformations (5) and (6), respectively. A concise summary of solvolytic and mechanistic data for system (5) has recently appeai Reviews of cyclopropylcarbinyl anions and carbenes are also available. - ... 

The stabdities of protonated cyclopropylcarbinyl ketones are a long-standing puzzle. Richie provided evidence that the bisected cyclopropylcarbinyl carbenium ion (66a) was the more stable conformation, rather than the perpendicular geometry (66b). Of the protonated, rigid ketones, (67), (68), and (69), spiro compoimd (67) is most stable, but the bicyclo compound (68) proved more stable than the nortricyclic system (69), although the latter has a bisected geometry, while (68) is imable to achieve this. The anomaly appears to have been resolved by semiempirical calculations of heats of formation of the ketones and ions, and an analysis of the effects of syn- and... 

Cyclopropylcarbinyl radical is indicated by its ESR spectrum to prefer the bisected conformation 142 , and while this species is stable below — 140°C, rearrangement occurs above this temperature and at — 100°C only the allylcarbinyl radical (143) is observed (equation 25). The bisected form (142) is calculated using the STO-3G basis set to be only 1.4 kcal mol more stable than the perpendicular form, whereas ring-opening has been calculated to be exothermic by STO-3G and MINDO-3 methods and by experiment 143 is favored by 5.1 kcalmol" . The ring-opening of equation 25 has a... 

In similar studies by Martin and Ree and Schleyer and Buss " which were published simultaneously, the problem of locking the cyclopropylcarbinyl system into the energetically unfavorable perpendicular conformation was solved by use of a spiroadam-antyl system. The relative rates shown in Scheme 6 were measured in acetic acid at 45 °C. 

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... 

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