Pericyclic reactions disrotatory rotation

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. 

The choice of phase indication in the molecular orbital is arbitrary and it is equally valid to choose to rotate together the unshaded orbitals in Equation 5.14, that is, 90° clockwise rotation at C-2 paired with 90° counterclockwise rotationatC-6. Both choices are equally valid for consideration of allowedness and both indicate disrotatory motion, but each may lead to different stereoisomeric products. Two allowed motions should be considered in all pericyclic reactions. Which of the two disrotatory motions occurs will be determined in most cases by steric or electronic effects among the substituents, or both may occur. Conrotation, in which both orbitals rotate in the same direction, is forbidden for this reaction because it leads to antibonding overlap at one end. 

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