Nucleophilic Cleavage of Lignin Model Compounds under Acidic Conditions in an Ionic Liquid: A Mechanistic Study.

A range of lignin model compounds were examined for their reactivity with hydrogen bromide in the ionic liquid N-butylpyridinium triflate. It was found that the ionic liquid enabled rapid reaction at both the hydroxy and methyl ether sites of the model compounds at room temperature. Reactions at the phenyl ether moieties were more complex; rather than facilitating cleavage at these sites, alternate breakdown products that had not been seen in previous studies were observed; these products are consistent with functionalisation of the aromatic components of the model compounds.

Cleavage of ethers in an ionic liquid. Enhancement, selectivity and potential application.

The cleavage of a series of ethers was examined in an ionic liquid containing hydrogen bromide. Reactions that did not proceed in either water or DMSO were found to proceed readily in this system, with notable selectivity between the cleavage of the different ether types examined herein. Increasing the proportion of water in the reaction mixture dramatically decreased the rate constant of ether cleavage; this could, in part, be attributed to a decrease in the solvent stabilisation of the transition state. Through analysis of the electronic requirements of the reaction (using substrates containing substituents with different Hammett parameters) and observation of rate enhancements for an ortho substituted system, the importance of the extent of protonation of the ether prior to nucleophilic attack was demonstrated.