AB-INITIO MOLECULAR-ORBITAL STUDIES OF SINGLET OXYGEN REACTIONS OF OLEFINS, ENOL ETHERS, AND ENAMINES

Ab initio molecular orbital (MO) studies of singlet oxygen reactions o f olefins, enol ethers, and enamines were carried out. The relative st abilities of biradical (BR), the so-called perepoxide-like (PE), perep oxide-like zwitterionic (PEZW) and zwitterionic (ZW) intermediates for singlet oxygen reactions of these species are described theoretically , together with their ionization potentials, molecular structures and solvation energies. The activation energies for the ene and dioxetane formation reactions were found to be determined by the electron-donati ng properties of substituents introduced to these substrates. The foll owing conclusions were drawn from these calculated results: (1) A nonr adical transition state with a PE-like conformation is the most plausi ble for ene reactions of allylic olefins and enol ether with allylic h ydrogens. (2) A BR mechanism is favorable for the (2+2) reactions of s inglet oxygen with simple olefins in the gas phase. (3) A nonconcerted mechanism via polar (PEZW, ZW) intermediates is operative for (2+2) c ycloadditions of unsymmetrical enol ethers in the solution phase, wher eas the (2a+2a) mechanism and a two-step mechanism via the PE intermed iate are conceivable for the (2+2) reactions of symmetrical olefins in the solution phase. The implications of these theoretical results to the mechanisms of singlet oxygen reactions are discussed in relation t o various experimental results, such as the stereochemistry, isotope e ffects, solvent effects and laser-photolysis experiments.