Bisected cyclopropyl

Schleyer and van Dine. These authors reported rates for three monomethyl derivatives and six dimethyl derivatives, and found that the rate accelerations per methyl group were all in the range of 5-11. It was argued that these results were all consistent with electron donation by bisected cyclopropyl groups. ... 

The lower IP for the conformationally fixed derivative 19 compared to c-PrCMe=CH2 is evidence that bisected geometries have the greatest interactions in the ionized states. Similarly, the lower IP for 21, in which one has bisected cyclopropyl, compared to 20, in which both groups are perpendicular, provides further evidence for this effect ... 

Most interestingly, reduction of 9-cyclopropylanthracene and 1,4-dicyclopropyl-naphthalene led to the planar cyclopropyl conformations in 127 and 128 as opposed to the normally observed bisected cyclopropyl conformations as, for example, in the cyclopropylsemidione 129 . In all other cyclopropane derivatives previously studied except 127 and 128 , the excess charge density present in the relevant p orbital of the electrophore, is zero or positive. Even in semidione radical anion systems is calculated (HMO) to be positive +0.10). Thus it is reasonable to assume that the change in the conformational preference of cyclopropyl may be related to the change in sign of q. ... 

These NMR results are consistent with MO calculations performed independently by Koch et al. (1988) and by the joint groups of Saunders, Schindler, Schleyer and Wiberg (Schleyer et al., 1988) at the MP2/6-31G and higher levels of theory (full fourth-order Moller-Plesset perturbation theory ). The C4Hf-system consists, in the light of these calculations, of a cyclobutyl cation (better described as a nonclassical pentacoordinated bicyclobutanonium ion, 7.135) and a bisected cyclopropyl cation (7.132) with an isomerization barrier of 2.5 kJ mol ... 

Only the bisected conformation aligns the cyclopropyl C—C orbitals for effective overlap. Crystal structure determinations on two cyclopropylmethyl cabons with additional stabilizing substituents, C and D, have confirmed file preference for the bisected geometry. The crystal structures of C and D are shown in Fig. 5.8. 

In ion D, in which the phenyl group would be expected to be coplanar with the cationic center to maximize delocalization, the observed angle is 25-30°. This should permit effective benzylic stabilization. The planes of the cyclopropyl groups in both structures are at 85° to file plane of file trigonal carbon, in agreement with expectation for the bisected... 

Solvolysis rate studies also indicate that there is greater stabilization by a cyclopropyl group in a bisected geomeby. In tosylate 1, the cyclopropane ring is locked into an orientation which affords a perpendicular arrangement. It reacts 300 times more slowly than the model compound 2. Tosylate 3, which corresponds to the bisected geomeby, undergoes acetolysis at least 10 times faster than the model 2-adamantyl tosylate 4. ... 

Cyclopropyl carbinyl cation1". The most dramatic example for a 7i-acceptor/7i-attractor substituent effect is provided by the cyclopropyl carbinyl cation. In its bisected conformation, the vicinal CC bonds are considerably longer (1.65 A) and the distal CC bond considerably shorter (1.41 A) than the bonds in 1 (Table 13). The CH2+ group stabilizes 1 considerably more in the bisected form than in the perpendicular form (13 kcal mol1). Since the CC+ bond possesses partial double bond character, it is shorter (1.35-1.36 A, Table 13) than a normal CC+ bond (1.51 A). 

The double bond is longer than in ethene, 1.313 A from XD84, and rg 1.337 (2) A from ED85, and in 37, and the C—C(=) single bonds are also longer than in 37 (Table 3). The cyclopropyl groups are rotated by 66.5° from the syn bisected form, which would be... 

The participation of the cyclopropyl group is at its maximum in its bisected conformation. Hence, 3-nortricyclyl cations 28 would be expected to be highly stabilized, as the... 

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 nmr spectrum of the 4,4-dimethylcyclopropylcarbinyl cation in superacid is only in agreement with the bisected structure of the ion (78). In Structure 78 the two methyl groups are not equivalent—one is cis to the cyclopropyl ring... 

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