A new approach towards the yellowing inhibition of mechanical pulps. Part I: Selective removal of alpha-hydroxyl and alpha-carbonyl groups in lignin model compounds

The alpha-hydroxyl group in lignin model compounds 1-(3,4-dimethoxyphenyl)- 2-(2-methoxyphenoxy)-1,3-propanediol (1) and 1-(4-hydroxy-3-methoxyphenyl)e thanol (3), and the alpha-carbonyl group in 1-(3,4-dimethoxyphenyl)-2-(2-me thoxyphenoxy)ethan-1-one (5) are removed by treatment of the model compound s with trifluoroacetic acid (CF3COOH) and triethylsilane (Et3SiH) in methyl ene chloride. The reaction is highly chemoselective and it tolerates the li gnin gamma-hydroxyl group, phenol and beta-O-4-aryl ether linkage. The alph a-hydroxyl group in 1 has also been removed with triethylsilane in aqueous trifluoroacetic acid (CF3COOH:H2O = 75 : 25). The combination of trifluoroacetic acid and sodium borohydride, or sodium t riacetoxyborohydride, is also effective in removing the alpha-carbonyl grou p in 5 and the alpha-hydroxyl group in lignin model compound 1-(3,4-dimetho xyphenyl)-2-(2-methoxyphenoxy)ethanol (7) and in milled wood lignin. The se lective removal of the lignin alpha-hydroxyl and alpha-carbonyl groups may potentially be used, in conjunction with the protection of the lignin pheno l, to inhibit the Light and/or heat-induced yellowing of mechanical pulps.