Frontier Molecular Orbital Theory
1,3-Dipolar cycloadditions are thermally allowed, π4s + π2s reactions. They can be treated equivalently by the Woodward-Hoffmann rules and the Dewar-Zimmerman theory. In the Dewar-Zimmerman treatment, 1,3-dipolar cycloadditions proceed through a 5-center, zero-node, 6-electron Huckel transition state, which is thermally allowed. In the Woodward-Hoffmann treatment, frontier molecular orbitals (FMO) in the 1,3-dipole must overlap with corresponding FMOs of the same symmetry in the dipolarophile. According to the FMO diagram drawn above, such an symmetry-allowed orbital overlap can be achieved in three ways, which are termed types I, II and III, the explanations of which are given in the following section. Whichever FMO pair has the smallest energy gap will dominate the reaction.
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