Bisecting conformation

There are two families of conformations available to terminal alkenes. These are the eclipsed and bisected conformations shown below for propene. The eclipsed conformation is more stable by about 2kcal/mol. ... 

The origin of the preference for the eclipsed conformation of propene can be explained in MO terms by focusing attention on the interaction between the double bond and the n component of the orbitals associated with the methyl group. The dominant interaction is a repulsive one between the filled methyl group orbitals and the filled n orbital of the double bond. This repulsive interaction is greater in the bisected conformation than in the eclipsed conformation. ... 

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. 

Cations are by no means the only species where the effects of hyperconjugative delocalization reveal themselves in such a striking manner. Similar effects exist in neutral systems or in anions. For instance, the normal propyl anion should tend to be eclipsed (E) since in this manner the molecule would optimize the 4-electron interactions between the ethyl group t orbital and the p orbital which carries the electron pair. In the bisected conformation, where ttchs and ttchs have both been raised in energy, the four-electron, destabilizing (see Section 1.7, rule 2) p ->7r interaction is stronger than in the eclipsed conformation. At the same time the two-electron, stabilizing p ->ir interaction is weaker than in the eclipsed conformation. Both effects favor the eclipsed conformation. 

The structural parameters for the Cs symmetric bisected conformation obtained at B3LYP/6-311+G are similar to those obtained at B3LYP/6-31G51 level. At B3LYP/6-311+G level, C1-C2 bond slightly elongated (1.579 A) as... 

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 effect of the cyano substituent seems to dominate in 43 the bond opposite to it is the shortest91. The C—C (N) bond has the same length, 1.447 (4) A, as in other cyanocyclo-propanes (Section III.D). The phenyl group adopts the bisected conformation. Crystallization leads to a spontaneous resolution, which is rather rare among phenylcy-clopropanes, and a conglomerate of chiral crystals is formed91. 

The Ph group is not far from the bisecting conformation in 152 the two rings are nearly coplanar in 153. 

Further confirmation of the structure comes from the X-ray crystal structure analysis of the cyclopropylcarbinyl tungsten complex CpW(CO)3(C5FI9) which reveals a tetragonal bipyramidal geometry at the metal, and a typical bisected conformation of the cyclo-propylcarbinyl-M moiety. The sigma C—M bond distance of 234 pm is expectedly much longer than the W—CO bonds (115 pm). 

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... 

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