Woodward-Hoffman rules

The Woodward–Hoffmann rules (or the pericyclic selection rules), devised by Robert Burns Woodward and Roald Hoffmann, are a set of rules used to rationalize or predict certain aspects of the stereochemical outcome and activation energy of pericyclic reactions, an important class of reactions in organic chemistry. The Woodward–Hoffmann rules are a consequence of the changes in electronic structure that occur during a pericyclic reaction and are predicated on the phasing of the interacting molecular orbitals. They are applicable to all classes of pericyclic reactions (and their microscopic reverse 'retro' processes), including (1) electrocyclizations, (2) cycloadditions, (3) sigmatropic reactions, (4) group transfer reactions, (5) ene reactions, (6) cheletropic reactions, and (7) dyotropic reactions. Due to their elegance, simplicity, and generality, the Woodward–Hoffmann rules are credited with first exemplifying the power of molecular orbital theory to experimental chemists.

Woodward and Hoffmann developed the pericyclic selection rules by examining correlations between reactant and product orbitals (i.e., how reactant and product orbitals are related to each other by continuous geometric distortions that are functions of the reaction coordinate). They identified the conservation of orbital symmetry as a crucial theoretical principle that dictates the outcome (or feasibility) of a pericyclic process. Other theoretical approaches that lead to the same selection rules have also been advanced. Hoffmann was awarded the 1981 Nobel Prize in Chemistry for elucidating the importance of orbital symmetry in pericyclic reactions, which he shared with Kenichi Fukui. Fukui developed a similar set of ideas within the framework of frontier molecular orbital (FMO) theory. Because Woodward had died two years before, he was not eligible to win what would have been his second Nobel Prize in Chemistry.

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