Hosoya Takashi, Yamamoto Kohei, Miyafuji Hisashi, Yamada Tatsuhiko
Graduate School of Life and Environmental Sciences, Kyoto Prefectural University 1-5 Shimogamo-hangi-cho, Sakyo-ku Kyoto 606-8522 Japan
Forestry and Forest Products Research Institute 1 Matsunosato Tsukuba Ibaragi 305-8687 Japan.
RSC Adv. 2020 May 20;10(33):19199-19210. doi: 10.1039/d0ra03420g.
Aerobic oxidation of native soft wood lignin in an aqueous solution of BuNOH facilitates efficient production of vanillin (4-hydroxy-3-methoxybenzaldehyde), which is one of the platform chemicals in industry. Oxidation of Japanese cedar () wood flour at 120 °C for 4 h under O in BuNOH-based aqueous solutions produced vanillin in 23.2 wt% yield based on the Klason lignin content of the starting material. This yield was comparable to that in alkaline nitrobenzene oxidation of the same material (27.2%), which indicated that our aerobic oxidation exploited the full potential of the wood flour for vanillin production. Further mechanical investigation with lignin model compounds suggested that the vanillin formation occurred mainly through following successive reactions: alkaline-catalyzed degradation of β-ether linkages in middle units of lignin polymer to form a glycerol end group, oxidation of the glycerol end group by O to a HC[double bond, length as m-dash]O moiety, and release of vanillin from the HC[double bond, length as m-dash]O end. One of the reasons for the high performance of BuNOH for the vanillin production was explained by the general understanding in organic chemistry that BuOH is a stronger base than simple alkali, NaOH. The other more fundamental mechanical aspect was that BuN suppressed disproportionation of the vanillin precursor (the CHO end group) probably due to strong interaction between the cation and the HC[double bond, length as m-dash]O end group.
在BuNOH水溶液中对天然软木木质素进行有氧氧化有助于高效生产香草醛（4-羟基-3-甲氧基苯甲醛），它是工业中的一种平台化学品。在基于BuNOH的水溶液中，于120℃、O₂气氛下对日本雪松木材粉氧化4小时，基于起始原料的克拉森木质素含量，香草醛的产率为23.2 wt%。该产率与相同材料在碱性硝基苯氧化中的产率（27.2%）相当，这表明我们的有氧氧化充分发挥了木材粉生产香草醛的潜力。对木质素模型化合物的进一步机理研究表明，香草醛的形成主要通过以下连续反应：木质素聚合物中间单元中β-醚键的碱催化降解形成甘油端基，甘油端基被O₂氧化为HC=O部分，以及从HC=O端释放香草醛。BuNOH在香草醛生产中表现优异的原因之一可通过有机化学中的一般认识来解释，即BuOH是比简单碱NaOH更强的碱。另一个更基本的机理方面是，Bu⁺可能由于阳离子与HC=O端基之间的强相互作用而抑制了香草醛前体（CHO端基）的歧化反应。
