(2) CYCLOADDITION REACTIONS/CYCLOREVERSION REACTIONS:
A cycloaddition reaction is the union of two smaller, independent pi systems. Sigma bonds are created at the expense of pi bonds. A cycloaddition can occur in an intramolecular sense, but it must be between two independent pi systems. Cycloaddition reactions are referred to as [m + n] additions when a system of m conjugated atoms combines with a system of n conjugated atoms. A cycloreversion is simply the reverse of a cycloaddition. Examples:
s Woodward-Hoffmann Theory R. B. Woodward and R. Hoffmann, The Conservation of Orbital Symmetry, Verlag Chemie, Weinheim, 1970. s Frontier Molecular Orbital Theory I. Fleming, Frontier Orbitals and Organic Chemical Reactions, John-Wiley and Sons, New York, 1976. s Dewar-Zimmerman Theory T. H. Lowry and K. S. Richardson, Mechanism and Theory in Organic Chemistry, 3rd Ed., Harper & Row, New York, 1987. s General Reference R. E. Lehr and A. P. Marchand, Orbital Symmetry: A Problem Solving Approach, Academic Press, New York, 1972.