Cyclopropylcarbinyl cations bisected

Cyclopropylcarbinyl cation (bisected) Cyclopropylcarbinyl cation (perpenticular) Sn2 TS... 

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 ... 

Cyclopropylcarbinyl cation, 48 bisected, 35, 206 bond lengths, 37 perpendicular, 35, 208 rotational barrier, 35... 

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... 

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). 

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. 

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.)... 

Figure 6.9 Orbital representation of the bisected cyclopropylcarbinyl cation.

Some of the evidence for the bisected cyclopropylcarbinyl cation follows. That the charge can be delocalized to both C2 and C3 simultaneously has been shown by the work of Schleyer and Van Dine.81 These workers studied the solvolysis of cyclopropylcarbinyl 3,5-dinitrobenzoates and found that methyl substituents accelerated the rate by an amount dependent only on the number of such substituents and not on their position. Thus 68, 69, and 70 react at almost the same rate. If the transition state for ionization were similar to the bicyclo-butonium ion, two methyl groups at C2 should accelerate the rate more than one at C2 and one at C3. A symmetrical transition state for ionization similar to the bisected cyclopropylcarbinyl cation (67) in which the charge is delocalized over all four carbon atoms best explains the results. 

That maximum acceleration occurs when the vacant p orbital is parallel to the plane of the cyclopropyl ring can be seen from the solvolysis of spiro[cyclo-propane-l,2 -adamantyl] chloride (71). The carbocation formed by departure of Cl is unable to adopt the geometry of the bisected cyclopropylcarbinyl cation, but can orient its empty p orbital properly to form the bicyclobutonium ion. This compound solvolyzes 103 times more slowly than 1-adamantyl chloride.82 On the other hand, 72 solvolyzes 10s times faster than 73. The cation from 72 does have its p orbital parallel to the plane of the ring as in the bisected cyclopropylcarbinyl cation.83... 

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... 

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°). 

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... 

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. 

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 ... 

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. 

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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