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Esterification of Terpene Alcohols Catalyzed by Acidic Brønsted Ionic Liquids

The Fisher esterification is an equilibrium reaction and generally requires the removal of water and/or use of excess amounts of the reactants if satisfactory conversion rates are to be obtained.1 The major drawbacks of these common methods are the final neutralization of the homogeneous acid catalyst and/or the separation of a metal catalyst on a solid support. Moreover, the removal of adsorbed products from the catalyst can be difficult and often requires a large excess of organic solvents.2 These issues make this process environmentally questionable. The esterification of primary terpene alcohols is a more difficult task as they are rather prone to isomerization and other untoward reactions under normal acidic conditions. Acidic reagents may promote ring formation, as reported for cyclogeraniol, or allyl rearrangements that isomerize geraniol and nerol. The use of ionic liquids (ILs) as solvents and catalysts in chemical reactions3 offers many advantages from an environmental point of view.4–8 These solvents are nonflammable, thermally stable and exhibit negligible vapor pressure; moreover their physicochemical properties can be easily tuned.9 For these reasons, the replacement of current esterification protocols with environmentally more benign processes involving the use of ILs is an area worthy of investigation. Deng et al. reported the esterification of carboxylic acids with alcohols in acidic chloroaluminate ILs10 and Cole synthesized acidic Brønsted functionalized ILs and used them as solvent-catalysts in the synthesis of ethyl acetate.11 Since then, many Brønsted acidic ILs, including SO3H-functionalized derivatives, including ILs with an acidic counter-anion and protonated N-alkylimidazolium ILs have been used in esterification reactions.12–19 We now report a mild and eco-friendly synthetic procedure to prepare terpene esters using acidic Brønsted ionic liquids (ILs)20 both as catalysts and solvents.