Cleavage of Aromatic Methyl Ethers by Chloroaluminate Ionic Liquid Reagents

We have discovered serendipitously that chloroaluminate ionic liquids can cleave aromatic methyl ethers under surprisingly mild conditions. Three ionic liquids, viz. [TMAH][Al2Cl7], [BMIM][Al2Cl7], and [EMIM][Al2Cl6I], and aluminum chloride were compared in the selective demethylation of 4,5-dimethoxyindanone at the 4-methoxy-function. The ionic liquids exhibited a remarkably high selectivity (96:4) in comparison with aluminum chloride (70:30). In addition, the reaction time was drastically shortened when the ionic liquids were used. Interestingly, the three ionic liquids displayed the same reactivity in the demethylation of 4,5-dimethoxyindanone. Considering the lower cost and the bulk availability of the precursors of [TMAH][Al2Cl7], we conclude that this is the most attractive ionic liquid from an industrial point of view. To make the large-scale application of [TMAH][Al2Cl7] feasible, we have developed a safe upscalable method for its preparation. Furthermore, the scope of ether cleavage by the ionic liquid reagent [TMAH][Al2Cl7] was investigated and it was found that aromatic methyl-, allyl-, and benzyl-ether cleavage is applicable to a variety of heterocyclic compounds. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)