Electrocyclization orbital correlation diagram

We have now considered three viewpoints from which thermal electrocyclic processes can be analyzed symmetry characteristics of the frontier orbitals, orbital correlation diagrams, and transition-state aromaticity. All arrive at the same conclusions about stereochemistiy of electrocyclic reactions. Reactions involving 4n + 2 electrons will be disrotatory and involve a Hiickel-type transition state, whereas those involving 4n electrons will be conrotatory and the orbital array will be of the Mobius type. These general principles serve to explain and correlate many specific experimental observations made both before and after the orbital symmetry mles were formulated. We will discuss a few representative examples in the following paragraphs. 

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. 

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 B14.2. Orbital correlation diagram for electrocyclic opening of cyclopropyl cation S, A and solid lines indicate disrotatory opening S, A and dotted lines indicate conrotatory opening.

Orbital Correlation Diagrams—The Conrotatory Electrocyclic Reaction... 

On the basis of state correlation diagrams we may occasionally make predictions at variance with those based on orbital correlation diagrams, especially where radical species are involved. Consider the three atom electrocyclic process (34) with reference to Table 3 and Fig. 9. The most... 

With respect to the electrocyclic reactions of spin doublets (such as cation and anion radicals), the situation is more complex and an extended orbital correlation diagram approach or the Zimmermann orbital... 

Figure 3 Comparison of the orbital correlation diagrams for the thennal dis (a) and con (b) 6e electrocyclic ring closures. Note that the electronic configuration is shown in each case only for the ground state of the hexatriene revening to the cyclohexadiene via the dis and con modes.

Figure 33 shows molecular orbital correlation diagrams for the disrotatory (51) and conrotatory (52) electrocyclic ring opening pathways for cyclobutene. With a total of four... 

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

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). 

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... 

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