An example of the valorization of terpenes: The selective isomerization of 3-carene to 2-carene in the presence of silica supported nickel catalysts modified by tetrabutyl tin

Abstract

The cyclic olefin, 3-carene, undergoes an isomerization of the carbon-carbon double bond to produce 2-carene, a precursor in the synthesis of (−)-menthol. This isomerization is observed over silica supported nickel, but the selectivity is reduced by side reactions producing trimethylheptenes by hydrogenative ring opening of the cyclopropyl ring. Selectivity for CC bond isomerization is dramatically increased by surface modification of the nickel metal catalyst by reaction with tetra- n-butyl tin. The most selective of these silica supported nickel/tin catalysts produces 2-carene with 91% selectivity at 48% conversion.