Bond-bisection requirement

It has already been pointed out that the bond-bisection requirement is imprecise.Moreover, even if this restriction is accepted provisionally, the problem reappears as soon as we get to the higher homologs of bicyclobutane benzvalene, in which the bicyclobutane moiety is fused onto ethylene, and naphth-valene, in which it is fused to a benzene ring. Both of these molecules are much less stable thermodynamically than their respective aromatic valence isomers, benzene and naphthalene, but - like cyclobutadiene - are remarkably resistant to thermal isomerization. Naphthvalene will be discussed at some length in a subsequent chapter, in connection with its unusual photochemical properties. We will here restrict our attention to the reluctance of benzvalene to undergo thermal isomerization to benzene. 

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 . 

Cyclopropylchlorocarbene (8) should have 0-73 as calculated from Eq. 4. However, as determined experimentally from the measured relative reactivities, w xy 0.41. [34] Close analysis, including ab initio calculations, reveal that in order to add to alkenes, steric demands require the cyclopropyl unit of carbene 8 to twist away from the bisected conformation shown in 8 (where stabilizing electronic interactions of cyclopropyl a bonds and the vacant... 

The essential difference between a homoallylic ion of type 455 and an a-cyclopropylcarbinyl ion of type 461 is that in the latter case the electron delocalization involves two bonds of the cyclopropyl group while in the former, only one. The available data indicate that the conversion of a bisected ion of type 461 into a homoallylic ion requires large energy expenditure and proceeds with difficulty this is also corroborated by quantum chemical calculations. But in the ion 455 these losses are compensated by the energy gain resulting from the formation of a homoaromatic system. 

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