The Non -Dimerization of CBD to Cubane

The characterization of the MOs of the interacting CBD molecules is straightforward. So is that of the two linear combinations of bond orbitals (LCBOs) of cubane, labeled and cr those labeled and (j+ are inter- 

The ttu twist that induces the first pair of correspondences and the hg glide that induces the second pair are illustrated in (A) of Fig. 7.8. Motion along either coordinate takes the system away from the geometry of cubane this is prima facie evidence that the dimerization is forbidden. 

On second thought, we recognize that if the twist is continued beyond D2, the kernel of the system is taken into D2d, in which the tt bonds of the two are at 90° to each other. Evidently, dimerization to cubane cannot be firmly disallowed before the reaction has also been analyzed in T)2d] this is done in (B) of Fig. 7.7. The two lower MOs of the reactant pair retain their identity and their irreps are determined easily 7t. is totally symmetric, and 7t++, being antisymmetric to 54 and to rotation about the two C2 axes perpendicular to zj is assigned to 62- The two upper tt orbitals are degenerate although D2d does not include 6 4, it does include 54, that also interconverts x and y. As to 

Dimerization of CBD to cubane thus appears to be genuinely forbidden, but the bsg displacement prescribed in (a) of Fig. 7.8 leads to a mutual orientation of the CBD molecules that looks suitable for dimerization to anti-tricyclooctadiene. The disposition of the tt orbitals is, moreover, different from that in the original analysis [5, Fig. 7], which found formation of syn-TCOD 

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Figure 7.7. Correspondence diagram for the (non-)dimerization of CBD to Cubane (A) D2h orientation (B) D2d orientation...

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