Orbital correlation diagrams butadienes

The cyclobutene-butadiene interconversion can serve as an example of the reasoning employed in construction of an orbital correlation diagram. For this reaction, the four n orbitals of butadiene are converted smoothly into the two n and two a orbitals of the ground state of cyclobutene. The analysis is done as shown in Fig. 11.3. The n orbitals of butadiene are ip2, 3, and ij/. For cyclobutene, the four orbitals are a, iz, a, and n. Each of the orbitals is classified with respect to the symmetiy elements that are maintained in the course of the transformation. The relevant symmetry features depend on the structure of the reacting system. The most common elements of symmetiy to be considered are planes of symmetiy and rotation axes. An orbital is classified as symmetric (5) if it is unchanged by reflection in a plane of symmetiy or by rotation about an axis of symmetiy. If the orbital changes sign (phase) at each lobe as a result of the symmetry operation, it is called antisymmetric (A). Proper MOs must be either symmetric or antisymmetric. If an orbital is not sufficiently symmetric to be either S or A, it must be adapted by eombination with other orbitals to meet this requirement. 

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... 

Fig. 11.10. Orbital correlation diagram for ethylene, butadiene, and cyclohexene orbitals.

When the orbitals have been classified with respect to symmetry, they can be arranged according to energy and the correlation lines can be drawn as in Fig. 11.10. From the orbital correlation diagram, it can be concluded that the thermal concerted cycloadditon reaction between butadiene and ethylene is allowed. All bonding levels of the reactants correlate with product ground-state orbitals. Extension of orbital correlation analysis to cycloaddition reactions involving other numbers of n electrons leads to the conclusion that the suprafacial-suprafacial addition is allowed for systems with 4n + 2 n electrons but forbidden for systems with 4n 7t electrons. 

Figure 15.21 Orbital correlation diagram for the disrotatoric ring closure of butadiene...

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 14.3. (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. 

Figure 7.15 An orbital correlation diagram for the Diels-Alder reaction. The if/A and y/n orbitals at the left are for ethylene, while the others at the left are for butadiene. The orbitals on the right are for the product.

Diels-Alder reaction, 169-170 aromatic TS, 151 benzyne, 160 butadiene + ethylene, 169 diastereoselectivity, 292 interaction diagram, 169 orbital analysis, 169-170 orbital correlation diagram, 198, 201 reverse demand, 169 substituent effects, 169-170 Diethyl tartrate, 11 Difluorocarbene ( CF2), 115... 

Figure 4 Orbital correlation diagram for the Diels-Alder addition of ethylene and butadiene to form cyclohexene...

The dimerization of acyclic polyenes in which all n bonds are lost would lead to the open structures of (54) and (55). A schematic orbital correlation diagram (Fig. 15) for process (54) shows that allyl dimerization is improbable. The cyclization of higher acyclic polyenes, e.g. to cis-or trans-7 in (55), is subject toa similar prohibition, but the formation of 8 is allowed. In general, processes in which the products retain elements of symmetry inherent in the reactants are symmetry-forbidden the argument used to demonstrate this is analogous to that used for ethylene. One dimerization of 1,3-butadiene, namely to 9, is unique this... 

Figure 7-17. Orbital correlation diagram for the ethylene-butadiene cycloaddition. Adaptation of Figure 10.20 from reference [74] with permission.

The following sections present an empirical approach to applying the selection rules. The chapter continues with a basic introduction to the analysis of symmetry properties of orbitals and the application of orbital correlation diagrams to the relatively simply cyclobutene-butadiene interconversion it concludes with some examples of the frontier orbital approach to pericyclic reactions. 

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

Figure 4.7. Disrotatory ring opening of cyclobutene to butadiene a) orbital correlation diagram, b) configuration correlation diagram (dotted lines) and effect of configuration interaction, which converts the diagram into a state correlation diagram (solid lines). The triplet state is indicated by a broken line.

Figure 4.11. Derivation of the orbital correlation diagram for the conrotatory ring opening of cyclobutene a) intended correlation, b) correlation including interaction between a and n and between and o MO s, respectively. Orbiial symmetry labels n and a apply strictly only at the planar cyclobutene and butadiene geometries. Labels S and A, or solid and broken correlation lines respectively, indicate the symmetry behavior with respect to the twofold-symmetry axis (by permission from Michl, 1974b).

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