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 α-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 α-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 γ-hydroxyl group, phenol and β-O-4-aryl ether linkage. The α-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 α-carbonyl group in 5 and the α-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 α-hydroxyl and α-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.