Ab initio study of the mechanism of singlet-dioxygen addition to hydroxyaromatic compounds: negative evidence for the involvement of peroxa and endoperoxide intermediates

In this article we report our study of two possible mechanisms of photooxidation of hydroxyaromatic compounds, involving the intermediacy of zwitterionic peroxa intermediates or 1,4-endoperoxides. To study the pathway of the first of them, as yet unexplored by theoretical methods, a simpler system composed of 1,3-butadiene-1-ol and singlet ((1)Delta(g)) dioxygen was considered first, for which calculations were carried out at the CASSCF/MCQDPT2 ab initio level, mostly with the 6-31G* basis set. The cumulative activation barrier to this reaction was found to be 20 kcal/mol and corresponded to a proton transfer (from the hydroxy oxygen atom to the attached oxygen molecule) in the cyclic zwitterionic peroxacyclopenta-3-ene-2-ol intermediate. This intermediate and the proton-transfer transition state were found to have a closed-shell character, which enabled us to estimate the corresponding activation barrier for the phenol-dioxygen system by carrying out optimization at the RHF level and single-point calculations at the MP2, CASSCF, and MCQDPT2 levels of theory. The energy barrier to the reaction was estimated to at least about 40 kcal/mol, rendering this mechanism for the phenol-oxygen system unlikely for nonpolar solvents. Similarly, calculations of the barrier to proton transfer from the 1,4-endoperoxide of phenol to its hydroperoxide were found to exceed 60 kcal/mol, eliminating such a mechanism too, which leaves only the earlier postulated mechanisms involving an initial charge or hydrogen-atom transfer to dioxygen as probable.