Mini-Review: Organic Reactions in Ionic Liquids

Ionic liquids are organic salts that are liquid at or near room temperature. It has been found recently that such liquids can be useful solvents for organic reactions. Often, the organic products can be removed from the ionic liquid by extraction with, e.g., ether, without resorting to an aqueous workup. This can be particularly useful when a precious metal catalyst is used in the reaction. The catalyst often remains in the ionic liquid, so that the catalyst solution can be directly reused.

Roberta Bernini of the Univ. of Tuscia in Viterbo illustrated (Tetrahedron Lett. 2003, 44, 8991. ) the power of this approach with the MeReO₃-catalyzed oxidation of 1 to 2 in [bmim]BF₄. The product could be extracted with ether and the catalyst-containing ionic liquid recharged with substrate and H₂O₂ for four cycles before the conversion and yield started to drop off. This drop off may be due to the accumulation of water in the ionic liquid, which could be removed by distillation.

Alternative purification protocols are available. Zhaolin Sun of Lanzhou University reports (Tetrahedron Lett. 2004, 45, 2681. ) that the ionic liquid TISC was specifically designed to promote Beckmann rearrangement. TISC is not soluble in water, so the product caprolactam was easily removed from the ionic liquid by extraction with water.

The counterion of the ionic liquid can be tuned to achieve one desired reactivity or another. Martyn Earle of Queen's University, Belfast has observed (Org. Lett. 2004, 6, 707. ) that the reaction of toluene can be directed toward any of the three products 6, 7, or 8, depending on the ionic liquid used.

Ionic liquids have been used for many other reactions. Examples include Friedel-Crafts acylation (Tetrahedron Lett. 2002, 43, 5793. ) , osmylation (Tetrahedron Lett. 2002, 43, 6849.) , Heck coupling (Tetrahedron Lett. 2003, 44, 8395. ) , and Henry reaction (Tetrahedron Lett. 2004, 45, 2699. ) .