Study On Selective Hydrogenation Of Propiolic Acids To ?,?-Unsaturated Acids In Water

?,?-Unsaturated acids are important intermediates of organic synthesis,which has been widely used in the fields of drug synthesis,flavor processing,and material preparation.The traditional preparation methods of ?,?-unsaturated acids need either high temperature or complicated process,which are difficult to achieve in practical synthesis.The use of organic solvents is potentially harmful to the environment and operators,which does not conform to the principle of green chemistry.Therefore,design and development of catalytic hydrogenation system for synthesis of ?,?-unsaturated acid in water phase with high efficiency and high selectivity not only has the important theoretical research value in the academia,but also has the vital significance to the realization of green industrial production.In this study,a series of water-soluble functionalized metal complexes were designed and synthesized.Using hydrogen as hydrogen source,the catalytic activity of the complexes was investigated for the selective reduction of propiolic acids to ?,?-unsaturated acid in aqueous solution.We synthesized a series of N,N-bidentate ligands including bipyridine,bipyrimidine bearing proton responsive groups,and TPPTS ligands.The corresponding complexes were prepared by coordination with ruthenium,iridium,palladium precursors.[Cp*Ir?6,6'-?OH?₂-bpy??OH₂?]SO₄?Cat.1?was utilized as the catalyst for the reduction of propiolic acid in aqueous solution.Under the preliminary experimental conditions,the catalyst were screened,[??⁶-C₆H₆?Ru?6,6'-?OH?₂-bpy??OH₂?]Cl?Cat.3?was demonstrated to be the best catalyst.Then Cat.3 was employed to catalyze the selective reduction of propiolic acids to ?,?-unsaturated acid,which was adopted as a model reaction.The effects of pH,temperature,reaction time,pressure,and solvent volume on the catalytic reaction were investigated.The optimal conditions of the reaction were determined as: 90 °C,45 h,4 MPa,2 mL solution,and catalyst loading of 2 mol%.Under the optimal conditions,the substrate scope was examined.The results indicated that:?1?Cat.3 bearing pendent base showed better catalytic activity than Cat.4 containing no pendent base.The results suggested that pendent base can enhance the activity of the catalys.?2?Iridium complex Cat.1 showed low activity and gave both lower conversion and yield.Palladium complex Cat.5 exhibited high catalytic activity,while no selectivity was observed.Ruthenium complex Cat.3 provided highest conversion and good selectivity.?3?100% conversion was obtained when hydroxyl pyridine was used as ligand,while the conversion was only 16% when bimetallic ruthenium catalyst Cat.7 was used.Based on experimental results and previous studies,we proposed a mechanism of reduction of propiolic acid to ?,?-unsaturated acids catalyzed by proton responsive metal complexes.The ligands with hydroxyl groups are supposed to be crucial to the improvement of catalyst activity.