A Characteristic Reaction of Lignin in Ionic Liquids; Glycelol Type Enol-Ether as the Primary Decomposition Product of β-O-4 Model Compound

Abstract

Guaiacylglycerol-β-guaiacyl ether (GG), which contains a predominant inter-unit linkage of lignin, could be converted into a corresponding glycerol type enol-ether (EE), 3-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-2-propenol, by the heat treatment in ionic liquids. EE is believed to be the unstable intermediate of the lignin decomposition process under acidic and alkaline conditions. By contrast, EE could be isolated as a relatively stable compound from the reaction mixture of ionic liquids. EE was formed as a primary reaction product in all ionic liquids used in this research under the temperature conditions of 120°C, although the decomposition rate and secondary decomposition products of GG varied with the ionic liquid used. NMR data suggested that dehydration reaction of GG progressed stereospecifically and [Z] isomer was predominantly formed (stereoselectivety of [Z] is higher than 90%).

Keywords:

• Lignin model compound
• ionic liquid
• enol-ether
• stereospecific dehydration
• hydroboration-oxidation

The authors are grateful to the FFPRI for financial support of this work.