Photochemical Studies On Asymmetric Synthesis And Single Electron Transfer Reaction

Over the last few years, photochemical asymmetric synthesis has attractedconsiderable attention from organic chemists. Compared with thermal asymmetricreaction, it is generally more difficult to control the stereoselectivity of a moleculein a low activation energy and short-lived photoexcited state in photoreaction.However, several valuable strategies have been developed in the realm ofphotochemical asymmetric synthesis. Among them, the application of chiralauxiliary is the most efficient and practical method, including ionic chiral auxiliary,covalent chiral auxiliary and chiral template connected via H-bonding.Cyclopentene derivatives have emerged as a class of very important organicintermediate, which are widely used in the total synthesis of natural products, suchas prostaglandins and carbocyclic nucleosides etc. Herein, a novel approach for thephotochemical synthesis of cyclopentene derivatives through the Norrish type IIcleavage reaction is described for the first example. In solid state, asymmetricstudies using ionic chiral auxiliary afforded enantiomerically pure cyclopentenescaffold and enantiomeric excess of up to90%.The [2+2] photoaddition reaction is the most straightforward method for theconstruction of cyclobutane skeleton in organic synthesis. In particular, the chiralauxiliary induced asymmetric [2+2] photoaddition reactions have attractedwidespread research interest from organic chemists. In this context, we havesuccessfully carried out the asymmetric [2+2] photoaddition reaction by means of acamphor-derived chiral auxiliary, which provides diastereomeric excess of up to95%. It is a practical and valuable synthetic strategy due to several significantadvantages: Firstly, natural camphor is very cheap and easily available. Secondly,camphor-derived chiral auxiliary can be prepared and removed through simplesynthetic steps, and lead to excellent stereoselectivity.In recent years, the application of Ru(bpy)32+as a photocatalyst to initiatesingle electron transfer process under visible light irradiation, has been rapidlydeveloped. Sevaral elegent work has been well demonstrated, such as asymmetricalkylation reaction of aldehyde,[2+2] cycloaddition of aryl enones, a tin-freereductive dehalogenation etc. Based on this synthetic strategy, we disclosed that thereductive dimerization of chalcones under photocatalytic conditions. The reactionmechanism includes the generation of radical anion from the correspondingchalcone, then biradical coupling to generate a dianion intermediate, followingmono-protonation and intramolecular aldol cyclization access to polysubstitutedcyclopentanol derivatives. Compared with electrochemical or thermochemical reduction, it has some advantages in this photocatalytic process, including easyoperation, mild reaction condition and low loading of photoredox catalyst.Oxidative coupling reaction is an efficient and valuable approach for theconstruction of C-C bonds in organic synthesis. In particular, the directfunctionalization of C-H bonds adjacent to nitrogen atoms can be achieved by theoxidative coupling of tertiary amines with a variety of nucleophiles. Generally, thegeneration of reactive iminium intermediate is required from the oxidation ofamines, which subsequently react with various nucleophiles for furtherfunctionalization. Based on this analysis, we investigate the coupling reaction oftertiary amine with enol silane to provide Mannich-type product underphotocatalytic conditions. Our results show that solvent has a significant effect onthis oxidative coupling process, the side reaction can be successfully suppressedwhen the reaction is performed in CH3OH or wet CH3CN. In addition, the reactioncan rapidly complete under blue light irradiation.