Ionic Liquids as Catalysts for the Acetylation of Camphene

Abstract

[bnmim]HSO4 and [bnpy]HSO4 are active and environmentally friendly catalysts for the acetylation of camphene with acetic acid. The reaction provides isobornyl acetate with 100% selectivity and 72-86% yield. The effect of temperature, molar ratio camphene/acetic acid, and catalyst loading were investigated. The catalyst can be reused four times without loss of activity. Isobornyl acetate is an important fine chemical and has been used in the field of fragrance, medicine, organic synthesis and cosmetics [1]. It is an intermediary in the synthesis of camphor [2]. Usually it is prepared by an acid-catalized reaction of camphene with acetic acid or acetic anhydride. But this process has serious drawbacks such as the corrosion of equipment, non-recyclability of the catalyst and serious environmental pollution. In the face of increasing environmental requirements, the use of such catalysts becomes unacceptable. Therefore many studies have recently focused on the development of "clean" (green) processes for the production of terpene derivatives with high selectivity. For this purpose, heteropolyacids [3, 4], zeolites [5, 6], solid acid catalysts [7, 8], ion-exchange resin [9-11] were used as catalysts for synthesizing terpene esters. However, these catalysts have drawbacks such as a large ratio of catalyst/substrate, fast deactivation and a selectivity that leaves much to be desired. In the recent years ionic liquids (IL) have been investigated by many researchers as catalysts for different reactions. Due to its low volatility, negligible vapor pressure, reasonable thermal stability, outstanding recyclability and reusability, ionic liquids may be a viable alternative to widely applicable catalysts in the processes of modern synthetic chemistry, the green chemistry [12]. The improvement of the versatility of ionic liquids was achieved by creating acidic functionalized ionic liquids and combining the properties of a reagent and solvent [13]. A number of such ionic liquids were synthesized and successfully applied in the esterification reaction [14-17]. Received that the structure of the IL cation determines the direction of the rearrangement of terpene, whereas the nature of the anion affects the selectivity of the reaction [18, 19]. In the present work, we report the acetylation of camphene with acetic acid catalyzed by imidazolium and pyridinium ionic liquids (Scheme 1). The influence of various reaction parameters, such as the temperature, the molar ratio of camphene/acetic acid and catalyst loading, on the activity of the most active catalyst is also studied.