Highly Efficient And Selective Transfer Hydrogenation Of α,β-Unsaturated Aldehydes And Ketones Catalyzed By Dihydroimidazolium Iridium Complex

Transfer hydrogenation has been developed rapidly in recent years because of its advantages,such as simple operation and mild reaction conditions.At present,transfer hydrogenation is mainly applied to the reduction of C=O double bond and C=N double bond,and there are few reports on the reduction of C=C double bond.In some existing synthesis methods,there are still some problems such as low reaction efficiency,large amount of catalyst,poor universality of substrate,etc.Therefore,it will be of great significance and prospect to develop a new catalytic system to realize the transfer hydrogenation and asymmetric transfer hydrogenation of C=C double bond.Hydrogen sources of transfer hydrogenation include formate,formic acid,alcohol,hansate,hydrazine and cyclohexene.Formic acid and sodium formate are good hydrogen sources because of their wide sources,easy storage and transportation,and simple post-treatment.Among a series of highly efficient catalysts for the decomposition of formic acid and sodium formate into hydrogen,the complex containing dihydroimidazole ligand and iridium has the highest catalytic activity and can catalyze the transfer hydrogenation of C=O double bond and C=N double bond.In this paper,we have designed and synthesized iridium complexes containing dihydroimidazole skeletons,and realized the selective transfer hydrogenation of α,β-unsaturated ketone C=C double bonds.In addition,chiral groups were introduced into the ligands to design and synthesize the chiral dihydroimidazolium iridium complex,realizing the asymmetric transfer hydrogenation reaction of α,β-unsaturated aldehydes.The details are as follows:(1)Transfer hydrogenation of α,β-unsaturated ketone C=C double bond catalyzed by dihydroimidazolium iridium complex: Selective transfer hydrogenation of α,β-unsaturated ketone C=C double bond was achieved by using formic acid/triethylamine azeotropic acid as hydrogen source,iridium dihydroimidazolium complex as catalyst and water as solvent.The yield was up to 90% with the amount of catalyst 0.1 mol%.At the same time,26 α-benzoenomethyl benzonaphthenone were synthesized.(2)Asymmetric transfer hydrogenation of α,β-unsaturated aldehydes catalyzed by chiral dihydroimidazolium.A series of chiral dihydroimidazolium iridium complexes were designed and synthesized based on the structure of dihydroimidazole.The asymmetric transfer hydrogenation of α-methyl cinnamaldehyde was achieved by using the complex as catalyst,sodium formate as hydrogen source and water as solvent.The yield of α-methyl cinnamaldehyde was 84.7% and the ee value was 90.3%.