Journal of Photochemistry and Photobiology A: Chemistry 157 (2003) 1–4 Frontier orbitals control in the reactivity of singlet oxygen with lignin model compounds An ab initio study Maurizio D’Auria ∗ , Rachele Ferri Dipartimento di Chimica, Università della Basilicata, Via N. Sauro 85, 85100 Potenza, Italy Received 14 October 2002; received in revised form 2 December 2002; accepted 20 January 2003 Abstract 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 -O-4-aryl ether derivatives 1 [4-ethoxy-3-methoxy-2-(2-methoxyphenoxy)-acetophenone], 2 [1-(4-ethoxy-3-metho- xyphenyl)-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 -C–O cleavage formation. The reaction occurred in low yields when a 2,6-dimethoxy- phenol 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. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Lignin; Singlet oxygen; Frontier orbitals 1. Introduction Some years ago we reported that several -O-4-aryl ether model compounds (Fig. 1) react with singlet oxygen to give interesting reaction products: in fact, we observed that the re- actions occurred mainly on the phenoxy part of the molecule while the other part was not involved in the oxidation pro- cess [1,2]. Furthermore, when dimethoxy derivatives were used, only in this case the oxidation reactions occurred on the other part of the molecules. This behavior could represent only an unusual chemical reactivity of the sample we used. However, more recently, we reported that the kinetics of singlet oxygen degradation of lignin depends on the presence of syringyl and guaiacyl units, being guaiacyl units more oxidizable than the other one [3]. Furthermore, we showed also that the treatment of lignin with singlet oxygen allowed to obtain variable amounts of some fine chemicals such as trans-sinapyl alcohol, 4-hy- ∗ Corresponding author. Tel.: +39-0971-202240; fax: +39-0971-202223. E-mail address: dauria@unibas.it (M. D’Auria). droxy-3, 5-dimethoxybenzaldehyde, 4-hydroxy-3, 5-dime- thoxyphenylacetone, 4-hydroxy-3-methoxybenzalde-hyde, cis-sinapyl alcohol, and sinapyl aldehyde (Fig. 2) [4]. Most of these compounds derived from syringyl units in lignin, showing that guaiacyl units could be more easily ox- idized than syringyl units. The only difference in the struc- ture of these units is the presence of an additional methoxy group in the structure. On the basis of these results we decide to study the rea- son of the different reactivity of these types of substrates to- wards singlet oxygen. We tested if the reactions of the above described model compounds could be explained assuming a frontier orbitals control. Frontier orbital control has been assumed in the ene reaction of singlet oxygen with alkenes, while it has not been considered in the reaction with pheno- lic compounds [5]. With this purpose we performed this ab initio study. 2. Results and discussion The irradiation of 1 in the presence of Bengal Rose as sensitizer gave 2-methoxyphenol (12) as the main product 1010-6030/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1010-6030(03)00068-6