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.