Correspondence diagram for the Cope rearrangement of hexadiene

The two stable conformations of hexadiene, syn and anti are nearly isoen-ergetic and are separated by a sufficiently low barrier ( 20 kcal/mol) that they can be assumed to be in rapid equilibrium at the temperatures required for the rearrangement. It was confirmed computationally that their electron configurations correlate smoothly with that of the planar molecule, so both conformers are represented on the left side of Fig. 8.4 by their superposition in The four out-of-plane CH orbitals span all four irreps of whereas two of the four in-plane combinations are aj and the other two are 62. 

The CH orbitals correlate along either a conrotatory (02) or a disrotatory (61) pathway. The totally symmetric (Jqc correlates with one a orbital and the symmetric tt combination with the HOMO (61), but the antisymmetric 02 combination has to be induced to correlate with the second ai a orbital by displacement along a conrotatory (02) pathway via the chair transition state, in agreement with experiment. It follows that the HOMO and LUMO in the chair TS remain those that were specified in Fig. 8.4 for the STS.  

In semibullvalene, Ci and Ce of hexadiene are bonded to one carbon atom, C3 and C4 are bonded to another, and the two additional C-atoms (C7 and Cg respectively) are linked to one another. Barbaralane differs from it only in that C7 and Cs are linked by a third aliphatic carbon atom, C9. Both molecules have C5 symmetry, with the sole mirror plane passing through C7 and Cg, whereas a perpendicular mirror plane and a two-fold rotational axis appear at the TS, that necessarily has the boat (C y) geometry. 

The a frame and the CH-bonding orbitals retain their identity across the diagram in Fig. 8.4, so only three occupied orbitals on each side of the diagram need be considered explicitly. On the left they are the three uppermost MOs the acc orbital of the bond broken (a in and the symmetric (a ) 

---

Figure 8.4. Correspondence diagram for the Cope rearrangement of hexadiene...

---