PERICYCLIC REACTIONS
For many years, as mechanistic organic chemistry developed, chemists collected examples of peculiar reactions that had no apparent mechanisms. With enough examples identified, some common characteristics could be listed:
- No ionic, radical, or carbenoid intermediates could be detected
- No nucleophilic or electrophilic reagents were required
- Reactions exhibited minimal solvent effects and only rarely could be catalyzed.
- Reactions involved the apparently simultaneous formation/rupture of two or more bonds
- Reactions exhibited very high stereoselectivity
In 1965, Woodward and Hoffman suggested that these reactions were driven by conservation of orbital symmetry: the maintenance of maximum bonding interactions by transferring electrons between molecular orbitals of the same symmetry in reactant and products.
Three principal kinds of pericyclic processes have been described:
During the years since the Woodward-Hoffman proposal, three principal ways of understanding these reactions have developed:
- Frontier orbital analysis, which is the original Woodward-Hoffman method; one examines the HOMO of a single molecule, or a HOMO and a LUMO from a reacting pair. Cf.: Fleming, "Molecular Orbitals and Organic Chemical Reactions", John Wiley and Sons, New York, 2009.
Example analyses for:
- The construction of correlation diagrams, which follow the fate of all electrons involved in a reaction. This method, discussed extensively by Woodward and Hoffman in their Angewandte review, was originally suggested by Longuet-Higgins.
- Identification of aromatic and antiaromatic transition states, with Huckel or Mobius topologies, methodology proposed by Dewar and Zimmerman. Cf.: Dewar and Dougherty, "The PMO Theory of Organic Chemistry", Plenum Press, New York, 1975; and Zimmerman, Acc. Chem. Res., 1971, 4, 272.
Each of these modes of analysis has advantages; however, the Dewar-Zimmerman method is the easiest to apply. We shall use chiefly this methodology in working the assigned problems.
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