Mechanism and Control of Cleavage of the beta-O-4 Bonds in Lignin

在160 ℃下醇-亚硫酸氢盐法制浆过程中，木质素及其模型化合物中的β-omicron-4键被还原裂解，生成丁香酚和/或异丁香酚。用二聚体化合物研究了这种还原断裂的机理和控制方法.机制(1)这种裂解似乎通过醌方法中间体进行，因为非酚、α-羰基和α-亚甲基模型不能产生高产率的异丁香酚。醌法侧链α位的磺化或烷基化或多或少地抑制了醇亚硫酸氢盐制浆条件下的裂解反应。(2)β-双辛基醚的裂解比愈创木基醚的裂解快得多。这有力地表明醌法形成不是裂解反应的速率决定步骤。(3)一个反应机制，涉及单电子转移从亚硫酸氢根阴离子或从亚硫酸氢根醌的方法似乎可以很好地解释这个裂解反应。并讨论了均裂的可能性.控制(1)裂解是广泛的一些伯和仲醇的水溶液中，在二甲基甲酰胺水溶液中，但在许多其他溶剂，包括含水醚，叔醇，酮，羧酸，胺，苯酚，和亚砜少得多广泛。(2)关于溶剂浓度，在50%乙醇中裂解进行得最快。因此，可以通过改变溶剂及其浓度在一定程度上控制裂解。然而，有必要抑制与裂解竞争的磺化。我们今后应该研究解决这个问题。

The beta -omicron-4 bonds in lignin and its model compounds were reductively cleaved to give eugenol and/or isoeugenol during the alcohol-bisulfite pulping at 160 ﾟC . The mechanism and the control method of this reductive cleavage has been studied using some dimeric compounds.1. Mechanism. (1) This cleavage seems to proceed via a quinone method intermediate because non-phenolic, alpha -carbonyl and alpha -methylene models did not produce high yield of isoeugenol. Sulfonation or alkylation at alpha position of the side chain of the quinone method more or less retarded the cleavage reaction under alcohol bisulfite pulping conditions. (2) Cleavage of the beta -syringyl ether was much faster than that of the guaiacyl ether. This strongly suggests that the quinone method formation is not the rate determining step of the cleavage reaction. (3) A reaction mechanism involving single electron transfer from bisulfite anion or from bisulfite radical to the quinone method seems to explain this cleavage reaction well. The possibility of homolytic cleavage is also discussed.2. Control. (1) The cleavage was extensive in aqueous solutions of some primary and secondary alcohols and in aqueous dimethylformamide but much less extensive in many other solvents including aqueous ethers, tertiary alcohols, ketones, carboxylic acids, amines, a phenol, and a sulfoxide. (2) Concerning solvent concentration, the cleavage proceeded most rapidly in 50 % ethanol. Thus, the cleavage can be controlled to some extents by changing solvents and their concentration. However it is necessary to inhibit sulfonation which is competitive with the cleavage. We should study to solve this problem in the future.