Woodward-Hoffman orbital correlation rules

The photochemical dimerization of unsaturated hydrocarbons such as olefins and aromatics, cycloaddition reactions including the addition of 02 ( A ) to form endoperoxides and photochemical Diels-Alders reaction can be rationalized by the Woodward-Hoffman Rule. The rule is based on the principle that the symmetry of the reactants must be conserved in the products. From the analysis of the orbital and state symmetries of the initial and final state, a state correlation diagram can be set up which immediately helps to make predictions regarding the feasibility of the reaction. If a reaction is not allowed by the rule for the conservation of symmetry, it may not occur even if thermodynamically allowed. 

The leaving ligand treatment within the AOM has been extended" to the following steps of the photoisomerization process— the isomerization of the resulting five-coordinate species, followed by the nucleophilic attack of the entering ligand. The electronic structure control of the latter two steps was based on state correlation diagrams, but nicely translated into orbital pictures of the Woodward-Hoffman rules of conservation of orbital symmetry. 

As a consequence, cis-attack on the 2 ground state of the trigonal bipyramid leads to the ground state of the hexacoordinated complex without energy barrier a trans-attack on the same state correlates with an excited state of the product. This simple rationalization of the experimentally observed facts can be formulated as a selection rule in the Woodward-Hoffman sense. The principle of conservation of orbital symmetry leads to the conclusion that the cis-attack is an allowed process, while the trans-attack is a forbidden process. 

J. Orbital Symmetry Correlations Woodward-Hoffman Rules... 

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