Product analysis of the photo-oxygenation of tert-butylguaiacylcarbinol sensitized by 4-hydroxy-3-methoxy acetophenone

Abstract

The product formation in the photosensitized reaction of a lignin model compound tert-butylguaiacylcarbinol in the presence of acetoguaiacone (4-hydroxy-3-methoxyacetophenone) or polymer-bound acetoguaiacone was studied in dimethoxyethane solutions saturated with air. Volatile products were carried off and trapped. Peroxidic compounds were quantitatively analysed by iodometric titration and the other products from the reaction mixture were fractionated using chromatographic methods. The following products were identified: formaldehyde, isobutene, pivalaldehyde and the dimer 2,2′-dihydroxy-3,3′-dimethoxy-5,5′-bis(1-hydroxy-2,2-dimethylpropyl)biphenyl. The reaction mixture was found to be very complex but nevertheless the quantitative analysis showed that more than 50% of the products were characterized. It is concluded that at least one-quarter of the products results from the oxidative bond cleavage between the side-chain and the aromatic ring, yielding primarily pivalaldehyde and acidic compounds formed by further cleavage of the aromatic ring. Another one-quarter is a result of a coupling reaction of intermediate phenoxyl radicals, leading to biphenyl structures. The mechanism of the reaction is discussed in terms of the formation of products.