Rapid and efficient protic ionic liquid-mediated pinacol rearrangements under microwave irradiation

Abstract

Several protic ionic liquids were tested as potential mediators for pinacol rearrangements employing microwave irradiation. Using hydrobenzoin as a model substrate, the optimal conditions were found to be heating at 80 °C for 5 min using H₂SO₄:triethylamine as the ionic liquid. A key feature of this reaction was to keep the microwave power low (20 W) to avoid ionic liquid degradation. Application of these conditions to triphenylethylene glycol gave rearrangement products in high yield and purity, while phenylethylene glycol and styrene oxide gave pinacol products that underwent a cascade aldol condensation. These conditions represent an efficient means by which pinacol rearrangements can be carried out while avoiding the use of strong Brønsted acids, high temperatures and extended reaction times.