Frontier orbital theory.
According to frontier orbital theory, similar to the valence electron interaction of a single atom, there are also valence electrons, that is, the electrons in the frontier orbitals, in a molecular reaction. During bonding, the frontier orbital will take precedence.32During 1,3-dipolar cycloaddition, the H and Cl atoms leave reactant1a to form intermediate 1a’ , while the H atom leaves2a to form intermediate 2a’ , The frontier orbitals involved in cyclization are those of intermediates 1a’ and2a’ . Therefore, we determined the frontier orbital energy difference between these intermediates. The lowest unoccupied molecular orbital 1 (LUMO1)-highest occupied molecular orbital 2 (HOMO2) and LUMO2-HOMO1energy differences between isatin substituted with -H, -F, -OCH, -CH3, or -Br and chloroxime compounds with different substituents are shown in Tables 1 and Table S2.1-S2.4 of supporting information.
Table 1. Energy level differences between the lowest unoccupied molecular orbitals (LUMOs) and highest occupied molecular orbitals (HOMOs) of 1a’1o’ and 2a’ .