Orbital correlation diagrams photochemical

The photochemical disrotatory closure of butadiene to cyclobutene has been described with a state-correlation diagram, like that shown in Figure 21.4. It is based on the familiar orbital-correlation diagram of Woodward and Hoffmann," from which the intended correlations indicated by the dashed lines can readily be deduced. The solid lines indicate that there is an avoided crossing, which is put in as a result of the quantum mechanical noncrossing rule. It says that two states of the same total symmetry cannot cross. Instead, as they approach each other in energy, they will mix and separate, as the solid lines indicate. 

Figure 14.5. a) Orbital correlation diagram for the photochemical n2s + n2s dimerization of two olefins to form a cyclobutane. (b) Orbital correlation diagram for the photochemical n4s + n2s cycloaddition of a diene and an olefin. 

Figure 14.6. (ia) Orbital correlation diagram for the photochemical electrocyclic reaction of butadienes. (b) Orbital correlation diagram for for the photochemical electrocyclic reaction of hexa-trienes. Solid lines and S, A denote correlation for conrotatory motion dashed lines and S, A denote correlation for disrotatory motion. 

Fig. 12.19. Orbital correlation diagram for the states involved in the photochemical interconversion of butadiene and 1,3-butadiene.

Optical purity, by NMR, 13, 14 Orbital correlation diagrams, 196-203 cycloaddition reactions, 197-196 Diels-Alder, 198 ethylene -E ethylene, 198 electrocyclic reactions, 198-200 butadienes, 199 hexatrienes, 199 limitations, 203 photochemical, 201 Woodward-Hoffinann, 197 Orbital energies, see also Energies, orbital degeneracy, 27, 90 Orbital interaction theory, 34-71 diagram, 40, 42, 47 limitations, 69-71 sigma bonds, 72-86 Orbitals... 

Fig. 4.17 Orbital correlation diagrams for the photochemical ring-closure reaction conrotatory (forbidden, left) and disrotatory (allowed, right)...

FIGURE 48.2 Orbital correlation diagram illustrating the a-cleavage of acetone. A linear acyl radical (left) is predicted to be photochemically generated from the nit excited state of acetone. The orbital symmetry is represented by the letters a (asymmetric) and s (symmetric), with respect to the o mirror plane. 

The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the same reaction types discussed in Chapter 11 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2ti + 2ti] cycloaddition of two alkenes can serve as an example. This reaction was classified as a forbidden thermal reaction (Section 11.3) The correlation diagram for cycloaddition of two ethylene molecules (Fig. 13.2) shows that the ground-state molecules would lead to an excited state of cyclobutane and that the cycloaddition would therefore involve a prohibitive thermal activation energy. 

Thus, the frontier-orbital and Hiickel-Mobius methods (and the correlation-diagram method as well) lead to the same conclusions thermal 2 + 4 cycloadditions and photochemical 2 + 2 cycloadditions (and the reverse ring openings) are allowed, while photochemical 2 + 4 and thermal 2 + 2 ring closings (and openings) are forbidden. 

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