Photoreactivity of lignin model compounds in the photobleaching of chemical pulps .1. Irradiation of 1-(3,4-dimethoxyphenyl)-2-(3'-methoxyphenoxy)-1,3-dihydroxypropane in the presence of singlet oxygen sensitizer or hydrogen peroxide in basic methanol solution

The irradiation of 1-(veratryl)-2-(3'-methoxyphenoxy)glycerol in alkaline methanol solution was performed in the presence of hydrogen peroxide (UV and visible light) and sensitized by methylene blue (MB) in the presence of O-2 (visible light). The photoproducts were analysed by gas chromatography-mass spectrometry (GC-MS). The beta-O-4 ether bond cleavage, which usually occurs in reactions involving singlet oxygen, was not operative, since the corresponding phenol 3 was detected in trace amounts only and the carbonyl derivative 4 was not detected at all. Irradiation in the presence of MB/O-2, induced the oxidation of the benzyl group into a carbonyl function and produced the hydroxymethylation of one benzene ring of compound 1. In contrast, such reactivity was not observed when 1 was irradiated in the presence of H2O2; nevertheless, hydroxylation of one benzene ring was detected. For both types of irradiation, four stereoisomers were formed. The mass spectrum showed the addition of hydrogen peroxide to the B benzene ring of compound 1. This is the most important reaction, and was ascribed to the radical addition of a superoxide anion assisted by the hydroxyl groups in the beta or gamma positions of the glycerol moiety. (C) 1997 Elsevier Science S.A.