Electrocyclic transformations selection rules

The situation is reversed for the tt2s + n4s addition. Figure 11.16 illustrates this case now the bonding orbitals all transform directly to bonding orbitals of the product and there is no symmetry-imposed barrier. As with the electrocyclic processes, the correlation diagrams illustrate clearly the reason for the striking difference observed experimentally when the number of electrons is increased from four to six. The reader may verify that the 4s + 4s reaction will be forbidden. Each change of the total number of electrons by two reverses the selection rule. 

The photochemical transformation is a 6tt electron (4n + 2) electrocyclic ring opening. The selection rules predict a conrotatory process as illustrated ... 

Electrocyclic transformations, in fact, proceed with high stereospecificity dictated by the number of n electrons in the open-chain it system (XIX). The reaction course taken by an electrocyclic transformation follows that in which the highest occupied molecular orbital in XIX has maximum bonding character throughout the transformation. The symmetry of this orbital, therefore, dictates the course of transformation and is the basis of the Woodward-HoiFmann selection rules (i). Consider, for example, the interconversion of butadiene (XXI) and cyclobutene (XXII). 

Because of the alternating symmetry properties of the HOMOs of linear polyene systems (see Chapter 4), Woodward and Hoffmann were able to deduce the following selection rule for electrocyclic transformations ... 

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