Disrotatory-conrotatory ring opening

In the simple four-electron systems, a route for cis-trans isomerisation of a diene is made available by the photochemical reaction usually being a disrotatory ring closure and the thermal reaction being a conrotatory ring opening ... 

Answer to 2(d). This question illustrates that it is the number of electrons, not the number of nuclei, that is important. The orbital correlation diagram is shown in Figure 14.2. In disrotatory opening, a mirror plane of symmetry is preserved. This correlation is with bold symmetry labels and solid correlation lines. Italic symmetry labels and dotted correlation lines denote the preserved rotational axis of symmetry for conrotatory ring opening. For the cation, the disrotatory mode is the thermally allowed mode. It corresponds to a a2s + 05 pericyclic reaction. 

For the 1,2-dimethylenecyclobutane rearrangement, Gajewski and Shih 53> have demonstrated preferred conrotatory ring opening and closing. For the allene-allene cycloaddition, then, the sequence would be disrotatory motion as two allenes approach to form the perpendicular biallylene intermediate, followed by conrotatory closure of that species. 

The next step is to test whether the disrotatory ring opening is possible. The cr —tt and tt -> cr transitions obviously cannot be used here, since they correspond to the conrotatory ring opening of A2 symmetry. Let us consider the cr -> cr and the tt —tt transitions ... 

The non-synchronous rotation of the two methylene groups implies a common transition state for both disrotatory and conrotatory ring opening, which may split into different transition states under the impact of a substituent. CASSCF/3-21G and CASSCF/6-31G(d) calculations lead to reaction energies which are far too negative (< -30 kcalmoT ) while UMP2/6-31G(d) predicts for this process a reaction energy of-20.2 kcalmoT in... 

Fig. 7. Disrotatory electrocyclic ring opening of 1,3-cyclohexadiene to 1,3,5-hexatriene. The excited state process, forbidden in the disrotatory mode, as indicated on the right, is allowed in the conrotatory mode (not shown).

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