Rudimentary Analysis of Polyene Cyclization

Woodward and Hoffmann s analysis of this set of reactions [1, pp. 38-45] is too familiar to require more than a brief recapitulation The first member of the series, s-cz5-butadiene cyclobutene, will be discussed in some detail, and the Rules for its higher homologs will be shown to follow directly from the nodal properties of the polyenes. 

The overall symmetry of both the reactant and product is C2v] each has a two-fold rotational axis and a mirror plane, both of which bisect the newly formed cr bond. However, in order for this bond to be formed, the methylene groups have to rotate out of the molecular plane, so only one of the two symmetry elements can be retained along the reaction pathway the two-fold rotational axis during the conrotation or the mirror plane during the disrotation. 

OCAMS departs from the WH-LHA procedure by carrying out the analysis in the symmetry point group conunon to the reactant and the product, here C2v The subgroup into which the reacting molecule has to be desymmetrized along the reaction path, or - equivalently - the irreducible representation of the non-totally symmetric symmetry coordinate(s) that has(have) to be incorporated into the reaction coordinate, is determined by a Correspondence Diagram, In its most rudimentary form, we take into account only the occupied orbitals on each side that are considered to be directly involved in the isomerization process. 

---