Orbital correlation diagram for electrocyclic reactions

Figure 14.3. (a) Orbital correlation diagram for electrocyclic reaction of butadienes (b) Orbital correlation diagram for electrocyclic reaction of hexatrienes. Solid lines and S, A denote correlation for conrotatory motion dashed lines and S, A denote correlation for disrotatory motion. 

Fig. 4.18 Generic orbital correlation diagram for electrocyclic ring-closure reactions...

Fig. 33 a, b. Orbital correlation diagrams for an electrocyclic ring opening reaction proceeding by a) the disrotatory or b) the conrotatory route... 

An orbital correlation diagram can be constructed by examining the symmetry of the reactant and product orbitals with respect to this plane. The orbitals are classified by symmetry with respect to this plane in Fig. 11.9. For the reactants ethylene and butadiene, the classifications are the same as for the consideration of electrocyclic reactions on p. 610. An additional feature must be taken into account in the case of cyclohexene. The cyclohexene orbitals tr, t72. < i> and are called symmetry-adapted orbitals. We might be inclined to think of the a and a orbitals as localized between specific pairs of carbon... 

Orbital correlation diagrams are useful for cycloadditions and electrocyclic reactions but not for sigmatropic rearrangements since no element of symmetry is preserved. 

The orbital correlation diagrams put forth by Woodward and Hoffmann336 for electrocyclic and sigmatropic reactions (Section 6.1.2) and for cycloaddition reactions (Section 6.1.5) are well known and the details of their construction are not reiterated here. We show only the case of the [2S + 2S] cycloaddition of two ethene molecules to cyclobutane as an example (Figure 4.34). 

Thus, we reach the same conclusions as described earlier by using the orbital correlation diagram method. For convenience, the selection rules by this approach to electrocyclic reactions are tabulated in Table 2.2. 

Although it is more fruitfiil to constmct a correlation diagram for the detailed analysis of an electrocyclic reaction, there is, nevertheless, an alternative method that also enables us to reach similar conclusions. In this approach, which is extremely simple, our only guide is the symmetry of the highest occupied molecular orbital (HOMO) of the open-chain partner in an electrocyclic reaction. If this orbital has a C2 symmetry, then the reaction follows a conrotatory path, and if it has a mirror plane symmetry, a disrotatory mode is observed. The explanation for this alternative approach is based on the fact that overlapping of wave functions of the same sign is essential for bond formation. 

Discuss Frontier Molecular Orbital (F.M.O.) method for pericyclic reactions. What are electrocyclic reactions Drawing correlation diagram, describe the comrotatoiy and disrotatory interconversion of cyclobutene and butadiene. Discuss Frontier Molecular Orbital (F.M.O.) method of analysing electrocyclic reactions. Derive selection rules for electrocyclic reactions. What are electrocyclic reactions Drawing correlation diagram discuss disrotatory and conrotatory interconversion of cyclobutene and butadiene. Support the results of correlation diagram by F.M.O. theory. 

Construct the orbital and state correlation diagrams for the conrotatory electrocyclic reaction of c/s-1,3-butadiene to cyclobutene assuming that the twofold rotation symmetry Cj is conserved throughout the reaction, and discuss whether the reaction is symmetry-aiiowed or symmetry forbidden. 

Correlation diagrams can be constructed in an analogous fashion for the disrotatory and conrotatory modes for interconversion of hexatriene and cyclohexadiene. They lead to the prediction that the disrotatory mode is an allowed process whereas the conrotatory reaction is forbidden. This is in agreement with the experimental results on this reaction. Other electrocyclizations can be analyzed by the same method. Substituted derivatives of polyenes obey the orbital symmetry rules, even in cases in which the substitution pattern does not correspond in symmetiy to the orbital system. It is the symmetry of the participating orbitals, not of the molecule as a whole, that is crucial to the analysis. 

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. 

When considering a possible role for a catalyst in converting a symmetry-forbidden to a symmetry-allowed electrocyclic transformation, it is best to examine the correlation diagram of the entire set of transforming molecular orbitals in a manner similar to that described for cycloaddition reactions. The symmetry-forbidden disrotatory transformation of cyclobutene to butadiene is described in Fig. 8. 

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