Correlation diagrams process

Q7 PROCESS CHART. PARETO ANALYSIS, CAUSE AND EFFECT DIAGRAM, HISTOGRAM, CORRELATION DIAGRAMS, PROCESS CONTROL CHARTS, CHECK SHEETS... 

Correlation diagrams can be constructed in an analogous fashion for the disrotatory and conrotatory modes for interconversion of hexatriene and cyclohexadiene. They lead to the prediction that the disrotatory mode is an allowed process whereas the conrotatory reaction is forbidden. This is in agreement with the experimental results on this reaction. Other electrocyclizations can be analyzed by the same method. Substituted derivatives of polyenes obey the orbital symmetry rules, even in cases in which the substitution pattern does not correspond in symmetiy to the orbital system. It is the symmetry of the participating orbitals, not of the molecule as a whole, that is crucial to the analysis. 

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. 

The same arguments can be applied to construct the correlation diagram for the conrotatory process in the transformation of cyclobutene v " butadiene system. In such a case, as we have seen, a C2 symmetry is maintained throughout. 

Scheme 6 Simple orbital correlation diagram depicting the interaction of a bromide ion with an allyl radical. The reverse process represents the intramolecular concerted ET.

The application of correlation diagrams to photochemical processes goes back to the early work of Laidler and Shuler (1951). More recently, Longuet-Higgins and Abrahamson (1965) showed how orbital-symmetry correlations may be converted into state-symmetry correlations. Here the fundamental consideration is that states of the same symmetry do not cross, i.e. the result is an avoided crossing, while states of different symmetry do cross. The principles involved are illustrated in Fig. 19 for the dimerization of... 

While this state-correlation diagram makes the correct predictions concerning thermal and photochemical reactivity it does contain certain flaws. Firstly, the treatment seems to suggest that allowed photochemical processes yield products in an excited state. Experimentally, however, relaxation of the excited products to the ground state is not observed. A second problem has been pointed out by Dauben (1967) for the closely related photochemical ring closure of butadiene to cyclobutene (75). 

The situation is reversed for the tt2s + n4s addition. Figure 11.16 illustrates this case now the bonding orbitals all transform directly to bonding orbitals of the product and there is no symmetry-imposed barrier. As with the electrocyclic processes, the correlation diagrams illustrate clearly the reason for the striking difference observed experimentally when the number of electrons is increased from four to six. The reader may verify that the 4s + 4s reaction will be forbidden. Each change of the total number of electrons by two reverses the selection rule. 

In non-simply connected transformations, it is not possible to construct the interaction diagram by the procedure of simple union of components at their ends. Processes of this type are nevertheless subject to analysis. Orbital correlation diagrams may still be constructed, using the symmetry or, if the symmetry does not offer sufficient guidance, as would be the case if no element bisects bonds being formed or broken, by tracing each individual orbital through the reaction in such a way as to preserve its nodal structure. 

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... 

An isoelectronic analogue of the -component of benzene is the transition state for the degenerate semibullvalene rearrangement in Scheme 41a. The VB correlation diagram,37 in Scheme 41b, for this process is also analogous to that of benzene and involves two curves which correspond to the VB... 

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