MCSCF study of singlet oxygen addition to ethenol?a model of photooxidation reactions of unsaturated and aromatic compounds bearing hydroxy groups

The 1Δg Dioxygen Ene Reaction with Propene: A Density Functional and Multireference Perturbation Theory Mechanistic Study

“…2) When the orbitals were relaxed along the instability direction, a new stable solution was found. The new UDFT solution coincides with that obtainable by the orbital switching procedure (see the Methods section), that had already been used when dealing with diradicals, diradicaloid structures,38 homolysis and radical coupling 59. This wavefunction now has acceptable spin densities and a qualitatively proper spin polarization that allows to it to correlate back to the reactants, upon re‐dissociation of the two moieties, in a correct way.…”

“…Information on lower symmetry pathways was not available, and it was not possible to determine whether the C s critical points were authentic transition structures, because of the symmetry constraints. Evidence in favor of the formation of diradical intermediates was derived from CASSCF studies on ethene36, 37 and ethenol,38 by using fully unrestricted geometry optimizations. Peroxirane structures were found to correspond to energy minima on the energy surfaces, but these structures were attainable only by passing first through the open‐chain diradical intermediates.…”

Theoretical Study on the Reactivity and Regioselectivity of the Ene Reaction of 1Δg O2 with α,β‐Unsaturated Carbonyl Compounds

“…A peroxiranic structure does not correspond to an energy minimum, since the delocalization of its unpaired electron on theC–C=O system determines a spontaneous ring opening with formation of the diradical intermediate (see A3 or B3 in Scheme ), as already observed for s‐cis butadiene 18. By contrast, peroxirane intermediates were found both for propene17 and ethenol19 (but ruled out as intermediates in the ene reaction because of their high energy).…”

Mechanistic Significance ofPerepoxide TrappingExperiments, with Epoxide Detection, in1ΔgDioxygen Reactions with Alkenes

“…By contrast, some recent theoretical studies, though carried out undeniably on simple systems, have provided indications in support of open-chain diradical intermediates. Two recent CAS-MCSCF and CAS−PT2 studies on ethene, methyl vinyl ether, and butadiene and on ethenol have found that peroxiranes are energy minima, nonetheless attainable only by passing through the corresponding polar diradicals (a carbon−oxygen diradical has some zwitterionic character) . A diradical intermediate was also found in a recent CAS-MCSCF and CAS-MC-QDPT2 study of the [π4+π2] cycloaddition of singlet oxygen to butadiene .…”

“…13 Then, evidence in favor of its intervention was presented in a kinetic isotope effect (KIE) study. 14 Also more recent stereochemical studies and KIE experiments, [15][16][17][18][19] concerning the ene reaction, 20 studies on ethene, methyl vinyl ether, and butadiene 21 and on ethenol 22 have found that peroxiranes are energy minima, nonetheless attainable only by passing through the corresponding polar diradicals (a carbon-oxygen diradical has some zwitterionic character). 11 A diradical intermediate was also found in a recent CAS-MCSCF and CAS-MC-QDPT2 study of the [π4+π2] cycloaddition of singlet oxygen to butadiene.…”