Synthesis Of Ru-Nitrogen-Phosphine Complexes And Highly Efficient Catalytic Hydrogenation Of Esters And Aldehydes

Ester and aldehyde reductions are very important and fundamental reactions in organic synthesis. Traditionally, these reactions usually carried out with stoichiometric metal-hydride reagents, e.g. NaBH4 and LiAlH4. The performance is dangerous due to the high reaction activity and accompanied with mass of wastes. So it can't meet the requirements of atom economy and green chemistry. The tedious workups, e.g. extraction, also increased the cost. Industrially, these reductions usually carried out on heterogeneous catalyst. Advantage of this method is the catalysts can be recovered. As these reactions often carried out under harsh conditions, high temperature and pressure, the requirements of facilities quality is high and also accompanied with side reactions.As hydrogen is a green reducing reagent, studying and developing high efficient homogeneous catalyst for ester and aldehyde reductions is a work with great significance in practical production. Though research on homogeneous catalyst for ester reduction has made great breakthrough in recent years, it's still facing problems of low efficiency, harsh reaction conditions and so on. While aldehyde reduction limited in the basic conditions, in which aldol side-reaction can happen readily. In this work, we mainly focus on the homogeneously catalytic hydrogenation of esters and aldehydes, designed and prepared two efficient ruthenium catalysts for ester reduction and one catalyst for aldehyde reduction.1) Designed and synthesized a pyridin-2-ylmethanamine contained tetradentate nitrogen-phosphine ligand. Product of the coordination reaction of the ligand and ruthenium precursor can be isolated with high stability and the crystal structure also obtained. High efficiency were observed when this catalyst was applied in catalytic hydrogenation of esters under mild conditions(80?,50 atm H2) with wide substrate scope. High chemoselectivity can be achieved in the presence of C=C bond. The TON (turnover numbers) can reach up to 80000. Extremely high catalytic efficiency can be obtained in the hydrogenation of long chain fatty acid esters, which indicate the great potential in industrial production. DFT calculations revealed the intrinsic high catalytic efficiency by built model catalysts.2) A hydrazine group-contained nitrogen-phosphine ligand was synthesized and applied to coordinate with a variety of ruthenium precursors under different conditions. When these catalysts were used for hydrogenation of esters to alcohols, excellent performance was observed (TON up to 17200). A wide substrate scope was suitable for this catalytic system. Aim of this design was the patent breakthrough3) A readily available ruthenium(II) catalyst was developed for the catalytic hydrogenation of aldehydes and exhibited excellent efficiency (TON up to 340 000). This hydrogenation can be performed without additives and even without solvents, showing highly industrial potential. High chemoselectivity can be achieved in the presence of C=C bond and ketone group. Further application of this protocol in reducing glucose to sorbitol showed good efficiency. Theoretical studies revealed that the rate-determining step is the hydrogenation step, not the carboxylate-assisted H2 activation step, which revealed the intrinsic characters of high chemo-selectivities.