Transfer hydrogenation of butyl sorbate: optimization of reaction conditions

Abstract The transfer hydrogenation (TH) of butyl sorbate was investigated with the aim of determining optimal reaction conditions regarding conversion and selectivity towards desired hydrogenated products. Despite TH being widely used for carbonyls and simple olefins, its selective application to conjugated dienes remains virtually untested; nevertheless, the TH products of butyl sorbate are of significant industrial relevance as flavor and fragrance ingredients. A systematic study was therefore performed to evaluate the effect of catalyst type and loading, ligand concentration, solvent, temperature, hydrogen donor, and the ratio of formic acid to triethylamine. The most effective system in terms of conversion employed 0.1 mmol palladium acetate as the catalyst, 0.1 mmol 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos) as the ligand, and 4 mmol formic acid/triethylamine mixture (5:2) as the hydrogen donor in tetrahydrofuran at 90 °C. Under these conditions, complete conversion of butyl sorbate was achieved within the first hour of the reaction. However, when selectivity was considered, the most favorable conditions were obtained with 0.1 mmol palladium acetate, 0.1 mmol Xantphos, and a 4 mmol ammonium formate as the hydrogen donor in tetrahydrofuran at 90 °C, affording 87% conversion after 3 h and an excellent 88% selectivity towards butyl ( E )-2-hexenoate.