Pericyclic reactions correlation diagram

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

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. 

Having described by means of correlation diagrams the nature of forbidden and allowed reactions of two particular pericyclic types, we wish to develop a notation that will permit us to state a generalized selection rule summarizing the conclusions of the pericyclic theory. 

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... 

Fortunately, all the conclusions that can be drawn laboriously from correlation diagrams can be drawn more easily from a pair of rules, known as the Woodward-Hoffmann rules, which distil the essence of the idea into two statements governing all pericyclic reactions, one rule for thermal reactions... 

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,... 

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. 

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... 

Figure 6.11. Schematic correlation diagrams for ground-state-forbidden pericyclic reactions a) HMO model of Zimmerman (1966), b) PPP model of van der Lugt and Oosterhoff (1969), and c) real conical intersection resulting from diagonal interactions. The two planes shown correspond to the homosymmetric (y) and heterosym-metric (6) case. Cf. Figure 4.20.

Photudimerizations and photocycloadditions are important examples of bi-molecular reactions. For such reactions an encounter complex has to be first formed, which in the following will be treated as a supermolecule. Correlation diagrams can be constructed for this supermolecule in the usual manner and can be utilized to discuss the course of the reaction. This was demonstrated in Chapter 4 for the exploration of pericyclic minima using H4 as an example. 

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 difftcult, 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... 

Photodimerization often involves an excimer that can be treated as a su-permolecule. (Cf. Section 6.2.3.) Then, the state correlation diagram for the singlet process (Figure 7.27a) ordinarily calls for a two-step return from S, to So along the concerted reaction path. First, an excimer intermediate E is formed. Second, a thermally activated step takes the system to the diagonally distorted pericyclic funnel P" (cf. Section 4.4.1), and the return to So that follows is essentially immediate. The reaction will be stereospecific and concerted in the sense that the new bonds form in concert. However, it will not be concerted in the other sense of the word, in that it involves an intermediate E. ... 

Figure 7.27. Schematic representation of the state correlation diagram for a ground-state-forbidden pericyclic reaction with an excimer minimum E a) at geometries well before the pericyclic funnel P is reached, and b) at geometries similar to those of P. ...

While the Oosterhoff model that follows from the state correlation diagrams discussed in Section 4.2.3 describes the stereochemistry of electro-cyclic reactions correctly and in agreement with the Woodward-Hoffmann rules, it is oversimplified in that it does not attempt to actually locate the bottom of the pericyclic minimum and simply assumes a planar carbon framework. It therefore predicts a nonzero S -So gap at perfect biradicaloid geometry. 

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