Pericyclic reactions orbital correlation diagrams

Answer to 2(d). This question illustrates that it is the number of electrons, not the number of nuclei, that is important. The orbital correlation diagram is shown in Figure 14.2. In disrotatory opening, a mirror plane of symmetry is preserved. This correlation is with bold symmetry labels and solid correlation lines. Italic symmetry labels and dotted correlation lines denote the preserved rotational axis of symmetry for conrotatory ring opening. For the cation, the disrotatory mode is the thermally allowed mode. It corresponds to a a2s + 05 pericyclic reaction. 

The Woodward-Hoffmann pericyclic reaction theory has generated substantial interest in the pathways of forbidden reactions and of excited state processes, beginning with a paper by Longuet-Higgins and Abrahamson,54 which appeared simultaneously with Woodward and Hoffmann s first use of orbital correlation diagrams.55 We have noted in Section 11.3, p. 586, that the orbital correlation diagram predicts that if a forbidden process does take place by a concerted pericyclic mechanism,56 and if electrons were to remain in their original orbitals, an... 

The term orbital correlation diagram describes the theoretical device that Woodward and Hoffmann developed to interpret pericyclic reactions. The Woodward-Hoffmann method for correlating reactant orbitals with product orbitals includes the following ... 

In view of the demonstrated stereospecificity of at least some cation radical Diels-Alder reactions, it is at least possible that these reactions, like the neutral Diels-Alder, are true pericyclic reactions, i.e., they may occur via a concerted cycloaddition. The results of a variety of calculations, however, make clear that the cydoadditions must at least be highly non-synchronous, so that the extent of the formation of the second bond, which completes the cyclic transition state, is no more than slight [55, 56]. If the cation radical Diels-Alder reaction is nevertheless interpreted as pericyclic and the concept of orbital correlation diagrams is applied to them, it emerges that the cycloaddition is symmetry allowed if the ionized (cation radical) component is the dienophile, but forbidden if it is the diene [39, 55], The former mode of reaction has been referred to as the [4-1-1] mode, and the latter as the [3 -t- 2] mode. Interestingly, the great majority of cation radical Diels-Alder reactions thus far observed seem to represent the formally allowed [4-1-1] mode. An interesting case in point is the reaction of l,l -dicyclohexenyl with 2,3-dimethylbutadiene (Scheme 24) [57]. 

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. 

These orbital correlation diagrams subsequently played a very important role in the development of Walsh diagrams and most importantly in the elucidation of the orbital symmetry rules developed by Woodward and Hoffmann which accounted for the stereochemistries of pericyclic reactions of organic molecules [174—184]. 

The orbital correlation diagram was introduced by Longuet-Higgins and Abrahamson to predict the allowedness of a pericyclic reaction [6]. In this method, the orbital symmetry properties of both reactants and products are considered. The symmetry elements of the MOs are evaluated and the MOs of reactants and products are arranged in a diagram in two columns. In an allowed pericyclic reaction, the ground-state MO of the reactants and the products has the same element of symmetry. 

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. 

Fig. 7. Orbital (a), configuration (b), and state (c, d) correlation diagrams for a typical ground-state symmetry-forbidden pericyclic reaction...

Three levels of explanation have been advanced to account for the patterns of reactivity encompassed by the Woodward-Hoffmann rules. The first draws attention to the frequency with which pericyclic reactions have a transition structure with (An + 2) electrons in a cyclic conjugated system, which can be seen as being aromatic. The second makes the point that the interaction of the appropriate frontier orbitals matches the observed stereochemistry. The third is to use orbital and state correlation diagrams in a compellingly satisfying treatment for those cases with identifiable elements of symmetry. Molecular orbital theory is the basis for all these related explanations. 

The Woodward-Hoffmann rules arise fundamentally from the conservation of orbital symmetry seen in the correlation diagrams. These powerful constraints govern which pericyclic reactions can take place and with what stereochemistry. As we have seen, frontier orbital interactions are consistent with these features,... 

When pericyclic reactions are analyzed in terms of correlation diagrams, all of the 77 and cr molecular orbitals taking part in the reaction are analyzed in terms of their symmetry properties with respect to reflection in a mirror... 

Most known photochemical processes are not pericyclic reactions. Even in many of these cases correlation diagrams can be helpful in estimating the location of minima and barriers on excited-state surfaces. (Cf. Section 4.2.2.) The derivation of these correlation diagrams, however, is often more difficult, not only because of lack of symmetry, but also because it may be difficult to identify any one excited state as the characteristic state, particularly in large molecules. For example, many of the excited states of toluene will have some contribution from the bond orbital excitation... 

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