A new approach towards the yellowing inhibition of mechanical pulps.: Part I:: Selective removal of α-hydroxyl and α-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)ethanol (3), and the alpha-carbonyl group in 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)ethan-1-one (5) are removed by treatment of the model compounds with trifluoroacetic acid (CF3COOH) and triethylsilane (Et3SiH) in methylene chloride. The reaction is highly chemoselective and it tolerates the lignin gamma-hydroxyl group, phenol and beta-O-4-aryl ether linkage. The alpha-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 triacetoxyborohydride, is also effective in removing the alpha-carbonyl group in 5 and the alpha-hydroxyl group in lignin model compound 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)ethanol (7) and in milled wood lignin. The selective removal of the lignin alpha-hydroxyl and alpha-carbonyl groups may potentially be used, in conjunction with the protection of the lignin phenol, to inhibit the Light and/or heat-induced yellowing of mechanical pulps.