Woodward-Hoffmann Correlation-diagram Approach

The approximate energies of molecular orbitals can be obtained by adding and subtracting all the bonding and antibonding molecular orbitals of the 

After arranging the m.os. of reactants and products in the increasing order of energies and writting their symmetries we simply match the orbitals of similar symmetries. If matching is possible in G.S. reaction is feasible thermally otherwise photochemically. 

A simple example to predict feasibility of reaction conditions by this method is 1, 3-butadiene cyclobutene interconversion which is discussed below  

The molecular orbitals (m.os.) of butadiene involved in transformation are Vi. 2 3 and i/4 whereas that of cyclobutene are a, 71, jr and a. This statement does not means that other lower energy o-orbitals have no effect upon energy 

For disrotatory ring closure mirror plane (m) symmetry is conserved. Correlation diagram is prepared for it in the same manner as for 

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Most of the theoretical methods used to correlate chemical reactivity trends implicitly (or in some cases explicitly) use a model of interacting diabatic surfaces in which the transition structure is associated with an avoided crossing of a reactant-like or a product-like diabatic surface. In the molecular-orbital correlation diagram approach of Woodward and Hoffmann or the frontier orbital method of Fukui, the molecular orbitals of the fragments are first mixed to form the MO of the supermolecule and then the electrons are assigned to various configurations of these supermolecule MO. We shall refer... 

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 few months after the communication by Woodward and Hoffmann, H. C. Longuet-Higgins and E. W. Abrahamson 3> suggested a different approach to the problem of stereospecificity of electrocyclic reactions, based on correlation diagrams between the orbitals, the electron configurations, and the states of the reactant and the product. 

As can be seen from the formulation, the principle of correspondence is an extension of the Woodward-Hoffmann principle to polymerization. The original Woodward-Hoffmann approach [13] was based on the analysis of correlation diagrams. At the beginning of the 1970s, Pearson [24] greatly simplified the analysis of symmetry by formulating a simple and effective rule a reaction is... 

Recently, Woodward and Hoffmann (1965, 1968, 1969), Longuet-Higgins and Abrahamson (1965), and Fukui (1971) have suggested that the stereochemical courses of these reactions are controlled by the symmetry properties of the orbitals of the reactants and products. Two approaches are employed, the frontier orbital method, and the correlation diagram method. The first approach requires a knowledge of the molecular orbitals of unsaturated hydrocarbons and consideration of the way in which they can interact. 

An antarafacial interaction on one, or both, of the molecules requires that the approach be orthogonal. Because of the high symmetry of the system the supra-antara and antara-supra modes are identical. Correlation diagrams for the [ 2j + 2, ] addition are to be found in the literature (Woodward and Hoffmann, 1965, 1969). We will concentrate here on the other two modes of addition. 

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