Selective Cleavage of Ether Bonds in Lignin by Trimethylsilyl Iodide

木质素是一种由愈创木基、丁香基和羟基苯基三种不同结构单元组成的高分子酚类化合物。结构单元间的键合结构主要是醚键和碳-碳键，它们的分布根据木质素在细胞壁中的来源有很大的特点。因此，作者一直试图通过选择性裂解醚键来观察碳-碳键在不同木质素组分中的作用。在木质素模型化合物愈创木酰基甘油-愈创木酰基醚中，发现三甲基硅基碘化物能够切割所谓的-醚键，几乎没有明显的副作用。该试剂对分离木质素的醚裂解也非常成功，如果木质素已乙酰化，则木质素已完全转化为氯仿可溶性低分子量化合物。虽然反应产物的确切分子量尚不清楚，但大多数反应产物的分子量似乎都低于500，相当于结构单元的二聚体或三聚体。换句话说，大多数木质素组分的碳-碳键连接不超过3个。我们还将该试剂应用于不同蒸煮条件下制备的纯碱浆。结果表明，该试剂可将木浆中的木质素降解为极低分子量的化合物，其分子量与原木质素相当。即使是经过长时间烹调的果肉也是如此。另一方面，即使在烹饪的开始，溶解的木质素也被发现具有显著的浓缩结构。换句话说，溶解后的木质素在用该试剂处理后仍保持相当高的分子量。此外，发现溶解木质素的结构变化在烹饪的初始阶段完成。这些信息对了解碱蒸煮过程中木质素的结构变化具有重要意义。

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.