Selective Cleavage of Ether Bonds in Lignin by Trimethylsilyl Iodide

木质素是一种由愈创木基、胡萝卜基和对羟基苯基三种不同结构单元组成的高分子酚。结构单元之间的主要键合结构是醚键和碳-碳键，并且它们的分布取决于木质素在细胞壁中的来源而具有相当的特征。因此，作者一直试图通过醚键的选择性断裂来了解各种木质素级分中碳-碳键的贡献。发现三甲基甲硅烷基碘能够裂解木质素模型化合物愈创木基甘油-β-愈创木基醚中的所谓的β-醚键，几乎定量地没有明显的副反应。通过该试剂的醚裂解对于分离的木质素、研磨的木材木质素也是非常成功的，如果其预先被乙酰化，并且研磨的木材木质素完全转化为氯仿可溶的低分子量化合物。虽然反应产物的确切分子量尚不清楚，但它们中的大多数似乎低于500，相当于结构单元的二聚体或三聚体。换句话说，大多数木质素级分没有通过超过三个连续的碳-碳键连接。我们还将该试剂应用于在不同蒸煮条件下制备的碱法纸浆。结果表明，该试剂可将纸浆中的木质素降解为分子量很低的化合物，其分子量与原始木质素的分子量一样低。这是真实的，即使是在长时间烹饪后的纸浆。另一方面，发现溶解的木质素即使在蒸煮开始时也具有显著的浓缩结构。换句话说，溶解的木质素在用该试剂处理后仍然保持相当高的分子量。此外，溶解的木质素的结构变化被发现完成在蒸煮的初始阶段。这些信息对于了解碱法蒸煮过程中木质素的结构变化是非常有用的。

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.