Conrotatory transition state, electrocyclic opening

Figure 15.17 B shows the aromatic transition state analysis of these reactions. We draw a picture of an opening pathway with the minimum number of phase changes and examine the number of nodes. The four-electron butadiene-cyclobutene system should follow the Mobius/conrotatory path, and the six-electron hexatriene-cyclohexadiene system should follow the Hiickel/disrotatory path. As such, aromatic transition state theory provides a simple analysis of electrocyclic reactions. The disrotatory motion is always of Hiickel topology, and the conrotatory motion is always of Mobius topology.

FIGURE 5.37. Conversion of cyclobutene to (a) the conrotatory and (b) the disro-tatory transition states for electrocyclic opening to butadiene. The C=C bond has been sketched in the diagram to show the orbital overlap more clearly. Dashed lines indicate the overlap of AOs. 

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