Orbital Symmetry Correlations Woodward-Hoffman Rules

Orbital Symmetry Correlations Woodward-Hoffman Rules 

Both thermal and photochemical isomerization of disubstituted trans-butadienes to disubstituted cyclobutenes are stereospecific however, the products are different  

Similar stereochemical control is found in countless reactions, both uni-molecular and bimolecular. The explanation is based upon three assump- 

To illustrate the molecular basis for this selectivity consider the molecular orbitals of butadiene and cyciobutene which are involved in the electronic rearrangement. These are illustrated in Fig. 6.7 their relative energies are tii 7t2 ti and a n n a. Now consider disrotatory motion around the C—C bonds (rotation in the opposite direction) as shown in Fig. 6.8. The two orbitals and must correlate with two orbitals of cyciobutene. One of these is obviously n since both 7ti and TTg are in phase and therefore bonding between Cg and Cg the other is a since in both 7ti and Jig disrotatory motion creates a bonding (T-orbital between Cj and C4. Similar arguments can be used to establish the pair- 

The orbital structure changes continuously as rotation occurs and the energy levels of a correlated pair cannot cross since the orbitals have the same symmetry properties.Thus the individual orbital correlations are 

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J. Orbital Symmetry Correlations Woodward-Hoffman 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. 

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