Transfer Hydrogenation Of α,β－Unsaturated Aldehydes And Ketones With Functionalized Iridium Complexes

The allyl alcohols generated from selective reduction of α,β-unsaturated aldehydes and ketones is a kind of important fine chemical raw materials. It is widely used in sythesis of medicine, perfume, food additives, and advanced materials etc. At present, selective reduction of α,β-unsaturated aldehydes and ketones usually require a certain pressure of hydrogen for selective hydrogenation while, however, suffering from security problem. Catalytic transfer hydrogenation (TH) using non-hydrogen reductant is safe and easy to operate, have recently attracted much more attention. Compared with TH in organic phase, the catalytic TH in aqueous media is more attractive because water is cheap, abundant, and environmentally friendly. In addition, efficiency and selectivity of most catalyst are generally low for TH of α,β-unsaturated aldehydes and ketones in water. Therefore, development of highly efficient and selective TH catalysis system in aqueous media is extremely important for both academic research and practical green chemical production.In this thesis, we synthesized a series of water-soluble functionalized iridium complexes, and studied their catalytic activity in selective TH of α,β-unsaturated aldehydes and ketones in the aqueous solution using formic acid/sodium formate as hydrogen source.First, we synthesized bipyridine, pyrimidine, and N-pyridylpyrazole bidentate ligands containing hydroxyl groups which then reacted with [Cp*Ir(OH2)3]SO4 to prepare the corresponding functionalized iridium complexes. We used [Cp*Ir(6,6’-(OH)2-bpy)(OH2)]SO4 in catalytic TH of cinnamaldehyde as a model, and optimized reaction conditions including temperature, solution pH, and catalyst loading. The optimal reaction conditions were determined as:50 ℃, pH 2.6,2 mol/L formic acid/sodium formate (v/v= 7/1) aqueous solution as hydrogen source, catalyst loading of 1 μmol. Under the optimal reaction conditions, we examined the activity of the four iridium complexes and determined the most active catalyst as [Cp*Ir(6-OH-py-pz)(OH2)]SO4 and [(Cp*IrCl)2(THBPM)]Cl2, which achieved the highest conversion rate up to 100% and 88% yield of allyl alcohol. Then it was used for TH of a series of aromatic α,β-unsaturated aldehydes and ketones. We found that:(1) the aromatic a,(3-unsaturated aldehydes gave higher yield than aromatic α,β-unsaturated ketones. (2) electronic effect of substituents on the benzene ring of cinnamic aldehyde significantly influence the reaction rate and selectivity. When substituent is electron withdrawing, the reaction is fast, the yield of allyl alcohol products is high (up to 88%); In contrast, when the substituent is electron donating, the reaction is slow, the yield of allyl alcohol products is low (minimum 55%). (3) when aromatic a,p-unsaturated ketones substrate was used, electronic and steric effect of substituents (R2) connected to the C=O affected the reaction rate markedly. With increasing of electron donating ability and steric hindrance of R2, the reduction selectivity for C=O was reduced, and the selective reduction of C=C to yield saturated ketones increased (up to 71% yield).Based on our experimental results and previous reports, we proposed a mechanism for TH of α,β-unsaturated aldehydes and ketones with the functionalized iridium complexes. Hydroxyl groups has played a vital role in the process of catalytic reaction. It forms hydrogen bonds with substrate and exhibits synergistic effect with the metal center in the catalysis, therefore improves the catalyst activity significantly.