The Bond-Bisection Requirement Benzvalene

The question posed in the preceding paragraphs as to the need for a reevaluation of the concept of allowedness , can be dismissed as a non-problem as long as it is taken as axiomatic that, for an orbital symmetry analysis to be of any use, the symmetry elements [retained along the pathway] must bisect bonds made or broken in the process . In contrast to the allowed conrotatory cyclization of butadiene to cyclobutene, in which the C2 axis bisects a newly formed cr bond, the only bond bisected by the axis in its conversion to bicyclobutane is the one between C2 and C3, which is essentially single in both the reactant and the product. 

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 role of the CH-bonding orbitals in specifying these geometric requirements had not yet been recognized, so they were not included in the correspondence diagram. They are included in Fig. 5.9, which is completely analogous to Fig. 5.8 and leads to identical conclusions. The orbitals of the bicyclobutane 

Woodward and Hoffmann s original statement, that refers to the symmetry elements chosen for the analysis , has been broadened slightly. In OCAMS, the symmetry elements are not chosen , but comprise all of those common to the reactant and product, some of which are not retained along the reaction path. In the classic WH-LHA correlation procedure [1, p. 31], the elements are necessarily chosen from among those retained . 

The 7 bond between Ci and Ce is omitted its inclusion would merely have added an additional ui orbital on each side of the diagram. 

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