Cation geometry bisected

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... 

Extended Hiickel calculations have been carried out on the 1-cyclopropylvinyl cation 156 (122). These results show that the most favorable conformation for this ion is the linear bisected structure 156a. However, Hanack et al. (166b), by means of a modified CNDO technique, calculate the most stable geometry of the intermediate ion resulting from homopropargyl participation to be a bridged cyclobutenyl cation rather than 156a. 

The a-spirocyclopropylcycloalkyl cations have the ideal bisected geometry of the cyclopropyl group with respect to the cation center. Hence solvolysis of such substrates reveals enormous rate accelerations. Thus the solvolytic rate acceleration of 87, as compared with the analogous a,a-dimethyl system 88, is in the range of 10596. Even the tertiary substrate 89 undergoes enhanced solvolytic rates.as compared with its a,a-dimethyl analogue 90. 

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... 

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. 

As pointed out by the pioneer in those studies C4H has some of the characteristics expected for a very rapidly equilibrating mixture of classical cyclopropylmethyl, cyclobutyl, and 3-butenyl cations and yet other characteristics which wholly belie any description that implies conventional charge distributions or geometries derived from structural representations using solid lines representing two-electron bonds . The rapidly equilibrating ions referred to are shown in 59a, 59b and 59c respectively, and the structures with unconventional charge distribution are the bicyclobutonium ion (60) or the bisected structure (61). 

All cyclopropylcarbinyl cations show some charge delocalization into the three-membered ring, but only the primary system (29) is unique in its chemical shifts which suggest a delocalized structure61. The C—1 —H coupling constants, however, do not support this conclusion. The non-equivalence of the methyl groups establishes the bisected geometry of (31), with slow rotation about the exocyclic C—C bond. The... 

The first cyclopropylmethyl cation to be directly observed was the tricyclo-propylmethyl cation (100) and the subsequent study of a variety of cyclopropylmethyl cations " led to the conclusion that the tertiary cations are static and, in the absence of constraining skeletal rigidity, adopt a bisected geometry rather than an eclipsed one (making the ex substituents on the carbenium ion... 

Geometries. The optimized structures for the polyenyl cations are shown in Table IV the bond length found for the allyl cation is in good agreement with values derived from previous theoretical studies (51-59). Our calculations predict that there is a plane of symmetry which bisects the polyenyl chains at the central carbon atom, perpendicular to the path of conjugation. We find no evidence for the distortion shown in Equation 3,... 

Only the bisected conformation aligns the cyclopropyl C—C orbitals for effective overlap. Crystal structure determinations on two cyclopropylmethyl cations with additional stabilizing substituents, C and D, have confirmed the preference for the bisected geometry (Fig. 5.7). 

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