Frontier orbitals control in the reactivity of singlet oxygen with lignin model compounds: An ab initio study

The photochemical oxidation of lignin models in the presence of singlet oxygen was studied by using ab initio calculations. The treatment of the non-phenolic β-O-4-aryl ether derivatives 1 [4-ethoxy-3-methoxy-2-(2-methoxyphenoxy)-acetophenone], 2 [1-(4-ethoxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-3-hydroxy-1-propanone], and 3 [1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-3-hydroxy-1-propanol] gave products deriving from a formal β-CO cleavage formation. The reaction occurred in low yields when a 2,6-dimethoxyphenol derivative 2 is used. When the phenoxy part of the molecule showed a lower reactivity towards singlet oxygen [1-(4-hydroxy-3-methoxyphenyl)-2-(2,6-dimethoxyphenoxy)-3-hydroxy-1-propanol, 5], the oxidation of the phenol moiety can occur. These results can be interpreted assuming a frontier orbitals control between the HOMO of the lignin model and the LUMO of singlet oxygen. In fact, in the case of the compounds 1, 2, and 3 the HOMOs show almost the same value and it is localized mainly on the phenoxy part of the molecule. In the case of compound 4 the HOMO shows higher energy than in the other model compounds. In compound 5 the HOMO is mainly localized on the phenolic part of the molecule.