Butadiene and Cyclobutene

Figure 11.3 illustrates the classification of the MOs of butadiene and cyclobutene. There are two elements of symmetry that are common to both s-cw-butadiene and cyclobutene. These are a plane of symmetry and a twofold axis of rotation. The plane of symmetry is maintained during a disrotatory transformation of butadiene to cyclobutene. In the conrotatory transformation, the axis of rotation is maintained throughout the process. Therefore, to analyze the disrotatory process, the orbitals must be classified with respect to the plane of symmetry, and to analyze the conrotatory process, they must be classified with respect to the axis of rotation. 

While these orbitals would be easy to identify in butadiene and cyclobutene, it might be considerably more difficult to choose the corresponding orbitals in a TS structure, where syimnetry is lower and mixing of ct and tt character might complicate identification. 

Figure 11.8 Classification of the reacting molecular orbitals of butadiene and cyclobutene for the conrotatory process. Symmetry classifications are with respect to the C2 axis, S indicating symmetric and A antisymmetric orbitals. The correlation lines are obtained by connecting orbitals of the same symmetry.

Example 6.22. Orbital correlation diagrams for the interconversion of butadiene and cyclobutene. 

As we saw in Section 2.B, the interconversion of butadiene and cyclobutene can occur through either a disrotatory or a conrotatory process. As the following diagram shows, the disrotatory opening is symmetrical with respect to a O plane, whereas conrotatory opening is symmetrical with respect to the C axis. 

In summary, for butadienes and cyclobutenes four-electron, thermal, conrotatory four-electron, photochemical, disrotatory. The easiest way to visualize the stereochemical result is to make a fist, use your thumbs to designate substituents at the termini of the 77 system, and rotate your fists to determine the stereochemical result upon disrotatory or conrotatory ring closure or opening. 

Morihashi, K., Kikuchi, O., Suzuki, K., Non adiabatic Coupling Constants for the Disrotatory and Conrotatory Isomerization Paths Between Butadiene and Cyclobutene, Chem. Phys. Lett. 1982, 90, 346 350. 

Fig. 10.23. Elements of symmetry for and classification of orbitals for disrotatory and conrotatory interconversion of 1,3-butadiene and cyclobutene.

Fig. 4.12 The MOs of butadiene and cyclobutene that are interconverted during the ring-closure. The dashed lines indicate the border between occupied and unoccupied MOs...

The orbitals processes described above are quite independent of the electronic state of the reaction system, so can be used without change for the photochemical process. In this case, one electron is promoted from the HOMO to the LUMO of both butadiene and cyclobutene, so that, although the orbitals correlations are not changed, we now have three different types of MOs doubly occupied, singly occupied and unoccupied. This means that, instead of the one border between doubly occupied and unocuppied MOs found for the thermal reaction, we now have two that may not be crossed by an orbital correlation line. This leads to the two diagrams shown in Fig. 4.17. 

The pyrolysis of bicydo[1.1.0]butane, however (Table 9 entry lb), occurs by way of a stepwise, stereoselective, twofold, lateral C—C bond cleavage The 185-nm photolysis of bicyclo[ 1.1.0]butane was described recently by groups of Adam and of Srinivasan (Table 9 entries lc,d). Although both groups found a different ratio of 1,3-butadiene and cyclobutene, they agreed that the dominating primary step is central C—C bond cleavage (equation 34) ... 

Not only must we consider the symmetry properties of 1,3-butadiene orbitals, but we must also consider the s)mtunetry properties of both cyclobutene and the transition structure expected for the conversion of the reactant to product. The two pathways for the closure of 13-butadiene to cyclobutene are illustrated in Figure 11.16. The Cl—C2, C2—C3, and C3—C4 a bonds are shown as solid lines. The p orbitals of 1,3-butadiene and cyclobutene, as well as the sp orbitals of the C3—C4 cr bond of cyclobutene, are represented by the shapes of the atomic p or sp orbitals. This is therefore only a basis set representation, not an illustration of a particular molecular orbital. Although there are many symmetry elements present in the representations of both 1,3-butadiene and cyclobutene, in the conrotatory reaction the only symmetry element that is present continuously from reactant through transition structure to product is the C2 rotation. Similarly, only the a reflection is present from reactant through transition structure to product for the disrotatory pathway. 

This diagram, which is based on quantum-chemical calculations of the butadiene and cyclobutene molecules in the geometries traversed during interconversion, shows... 

Electrocyclic reactions are not really different from cycloadditions. Figure 20.27 compares the equilibration of 1,3-butadiene and cyclobutene with the 2 + 2 dimerization of a pair of ethylenes. The only difference is the extra o bond in butadiene, and this bond is surely not one of the important ones in the reaction—it seems to be just going along for the ride. Why should its presence or absence change the level of detail av able to us through an orbital symmetry analysis It shouldn t, and in fact, it doesn t. 

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