Correlation Diagrams and Pericyclic Selection Rules

The second method of analysis, developed by Woodward and Hoffmann, traces the molecular orbitals of the starting materials through the entire reaction to the molecular orbitals of the products.16 

Orbital Correlation Diagrams—The Conrotatory Electrocyclic Reaction 

We shall consider first the electrocyclic ring closure of butadiene to cyclobutene 

17 The theory of electrocyclic reactions was first presented in a form that focused on the HOMO by (a) R. B. Woodward and R. Hoffmann, J. Amer. Chem. Soc87, 395 (1965), and later in the form described here (b) R. Hoffmann and R. B. Woodward, Accts. Chem. Res, 1, 17 (1968). See also (c) R. B. Woodward and R. Hoffmann, The Conservation of Orbital Symmetry, Verlag Chemie, Weinheim/ Bergstr., Germany, and Academic Press, New York, 1970. 

Although the noncrossing rule may in many instances be relied upon to determine the correlation pattern where alternatives exist, it is not infallible. In order to avoid difficulties in constructing correlation diagrams, Woodward and Hoffmann cite three precautions that should be observed.19 

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Having described by means of correlation diagrams the nature of forbidden and allowed reactions of two particular pericyclic types, we wish to develop a notation that will permit us to state a generalized selection rule summarizing the conclusions of the pericyclic theory. 

The following sections present an empirical approach to applying the selection rules. The chapter continues with a basic introduction to the analysis of symmetry properties of orbitals and the application of orbital correlation diagrams to the relatively simply cyclobutene-butadiene interconversion it concludes with some examples of the frontier orbital approach to pericyclic reactions. 

Discuss Frontier Molecular Orbital (F.M.O.) method for pericyclic reactions. What are electrocyclic reactions Drawing correlation diagram, describe the comrotatoiy and disrotatory interconversion of cyclobutene and butadiene. Discuss Frontier Molecular Orbital (F.M.O.) method of analysing electrocyclic reactions. Derive selection rules for electrocyclic reactions. What are electrocyclic reactions Drawing correlation diagram discuss disrotatory and conrotatory interconversion of cyclobutene and butadiene. Support the results of correlation diagram by F.M.O. theory. 

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