New efficient catalysts based on hydrotalcite for synthesis of sandalwood-type fragrances

Abstract

The presented study focuses on the development of metal-modified bifunctional catalysts based on hydrotalcites (HT) and their utilization for the „one-pot“ synthesis of 2-methyl-4-(2,2,3-trimethylcyclopent-3-en-1-yl)but-2-enol, also known as Santalinol. Such fragrance has a strong scent of sandalwood and is used in the perfume and cosmetics industry. The synthesis of Santalinol involves two steps: the aldol condensation of campholenic aldehyde with propanal to form 2-methyl-4-(2,2,3-trimethylcyclopent-3-en-1-yl)but-2-enal (santalinal), followed by selective hydrogenation. Three hydrotalcites with different magnesium-aluminium ratios ranging from 2 to 3 were prepared and tested for aldol condensation. Among them, the most active catalyst was modified with a metal using the impregnation method and various metal precursors. The modification using palladium (II) acetylacetonate solution in toluene proved to be the most advantageous. An efficient bifunctional catalyst was prepared, consisting of a hydrotalcite with a magnesium-aluminium ratio of 2.5 and 0.1 wt% of palladium for Santalinol synthesis. Under optimal conditions in a one-pot reaction, a 40% conversion of campholenic aldehyde was achieved after aldol condensation, with a selectivity of 86% towards santalinal. During the hydrogenation step, a selectivity of up to 75% towards Santalinol, relative to the formed santalinal, was observed.