Highly selective synthesis of citronellal in water at multigram scale by Pd nanoparticles-catalyzed hydrogenation of citral

The synthesis of citronellal, an added-value chemical for perfumery, was carried out by selective and green hydrogenation of citral into citronellal in water. Aqueous suspensions of spherical ammonium-capped palladium nanoparticles with sizes around 3nm selectively reduced the conjugated carbon-carbon double bond. An excellent selectivity of 95% in citronellal was achieved at complete conversion under mild reaction conditions on a realistic 2 g scale in water. The presence of potassium hydroxide proved crucial to control the selectivity and avoid other hydrogenation co-products. These optimized results were further extended to a 135 g substrate loading with a relevant turnover number (TON) of 10 000.