High regioselective Diels-Alder reaction of myrcene with acrolein catalyzed by zinc-containing ionic liquids

The ambient zinc-containing ionic liquids, MX-ZnCl2, functioning as both Lewis acid catalyst and green solvent, are employed for a high regioselective Diels-Alder reaction of myrcene with acrolein for the first time, where MX is either 1-butyl-3-methylimidazolium chloride (BmimCl), 1-ethyl-3-methylimidazolium bromide (EmimBr), N-butylpyridinium bromide (BPyBr), or N-ethylpyridinium bromide (EtPyBr). Compared with the analogous reaction performed over a ZnCl2 catalyst in the conventional solvent dichloromethane, higher regioselectivity of the 'para' cycloadduct and excellent yield were achieved at shorter reaction time in these ionic liquids with optimized molar compositions of MX and ZnCl2. These moisture-insensitive ionic liquids can be easily separated from reaction products after simple washing with hexane, allowing their reuse with no obvious loss in activity.