Selective Cleavage of Ether Bonds in Lignin by Trimethylsilyl Iodide

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Lignin is a kind of polymeric phenols composed of three different types of structural units, namely guaiacyl, syringyl and phydroxyphenyl units. Major bonding structures between structural units are ether and carbon-carbon bonds, and their distributions are quite characteristic depending on the origins of lignins in the cell wall. Therefore, the authors have been trying to see the contribution of carbon-carbon bonds in various lignin fractions by the selective cleavage of ether bonds. Trimethylsilyl iodide was found to be able to cleave so-called beta-ether bonds in a lignin model compound, guaiacylglycerol-beta-guaiacylether, almost quantitatively without noticeable side-reactions. The ether cleavage by this reagent was very successful also for an isolated lignin, milled wood lignin, if it is previously acetylated, and a milled wood lignin was totally converted to the chloroform soluble low molecular weight compounds. Although the exact molecular weight of the reaction products are not clear yet, most of them seems to be lower than 500 which is equivalent to dimer or trimer of structural units. In other words, most of lignin fraction is not connected by more than three successive carbon-carbon bond. We also applied this reagent to soda pulps prepared under different cooking conditions. It was found that lignin in a pulp was degraded by this reagent to the very low molecular weight compounds which was as low as the case of the original wood lignin. This was true even for the Pulp after a prolonged cooking. On the other hand, the dissolved lignins found to have remarkably condensed structures even at the beginning of the cooking. In other words, a dissolved lignin still keep a considerably high molecular weight after the treatment with this reagent. Furthermore, structural changes of the dissolved lignin was found to complete at the rather initial stage of cooking. These information is very useful to understand the structural changes of lignin during alkali-cooking.