Orbital correlation diagrams Woodward-Hoffmann

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. 

A key step in one route to the synthesis of hexamethyl Dewar benzene is the cycloaddition of 2-butyne to tetramethylcyclobutadiene (stabilized by A1 cation). Using the parent compounds (no methyls), develop a Woodward-Hoffmann orbital correlation diagram for the reaction and determine whether the reaction is thermally allowed. 

This question requires you to construct an orbital correlation diagram of the Woodward-Hoffmann type. 

Chapter 14 deals with orbital correlation diagrams following Woodward and Hoffmann [3]. State wave functions and properties of electronic states are deduced from the orbital picture, and rules for state correlation diagrams are reviewed, as a prelude to an introduction to the field of organic photochemistry in Chapter 15. 

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 original explanation of Woodward and Hoffmann involved construction of an orbital correlation diagram for the reaction under consideration, and then carrying out the reaction in such a manner so that the symmetries of the reactant and product orbitals matched exactly. If the correlation diagram indicates that the reaction may occur without encountering a symmetry-imposed barrier, it is termed symmetry-allowed. If a symmetry is present, the reaction is designated symmetry-forbidden. 

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 ... 

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