Esterification of Terpenes

The esterification of camphene (2) with acetic acid at a reaction temperature of 80 °C showed a conversion of 88% with a selectivity of 87% with the catalyst SAC 13 and a selectivity of 88% with the Amberlyst A 15. The carboxylic acid was in four-fold molar excess with 10 wt% catalyst. Both, the SAC 13 and the A 15 show high activity (each 88% conversion) and high selectivity in this reaction. This olefinic compound seems to be so reactive as to render high conversion independently of the acid strength of the catalyst. This could be due to the terminal double bond and to the very stable carbocation formed by protonation. 

Recently, selective synthesis of / -cymene from toluene and propane or isopropanol over zeolite catalysts has been thoroughly investigated. Zeolite-based processes avoid the disposal of spent catalyst, product contamination by the catalyst, separation of the catalyst from the product and corrosion of the reactor and tubes. The results using various zeolite types were promising. However, the formation of undesired -propyl toluene is observed particularly in the presence of MFI type zeolites, whereas large pore zeolites yield significant amounts of m-and o-cymene besides the desired / -cymene. However, low conversion and relatively low selectivity are the drawbacks of these investigations. 

Using zeolite-supported, Ce-promoted Pd catalysts brought enhanced dehydrogenation activity during a-limonene conversion. Thereby, they used preferably weakly acidic boron-pentasil zeolites and obtained p-cymene yields around 85%. 

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Esterification of Terpenes - Under standard conditions and temperatures from 40 to 100°C limonene and a-pinene were esterificated with acetic and acrylic acid, respectively, over the Amberlyst resin as well as the Nation composite. The conversion is high, however, the reaction is very unselective and no distinct main product is obtained. It seems that both terpenes rearrange to several different products before the final esterification takes place. Using camphene (2) as a starting material, much better results are obtained (Scheme 3). 

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