Visible Light-induced Oxidative Functionalization Of Alkynes Catalyzed By Acetylated Vitamin B

There is great significance in preparing valuable organic molecules via the multi-fuctionalization of C≡C bonds of alkyne as an important and readily available chemical feedstock.Nowadays,numerous progresses have been made in the field of radical addition to alkynes.It is an alternative and promising strategy that the initial one-electron oxidation of C≡C bond to result an alkyne radical cation followed by the reaction with nucleophiles to give versatile products.However,the one-electron oxidation potentials of alkynes are usually very positive due to the high C≡C bond energy.Therefore,the realization of this strategy is challenging under mild conditions.Vitamin B2（Riboflavin,RF）and the most known derivative,such as riboflavin tetraacetate（RFT）is a low toxic,readily available and photoredox organic molecule which is able to catalyze two-electron oxidation reactions of organic substrates in the presence of molecular oxygen and photo irradiation.Meanwhile,the excited RFT also exhibits a high one-electron reduction potential that can be further enhanced by the binding of two scandium ions（[RFT-2Sc],2.45 V vs saturated calomel electrode）which makes it possible to oxidize a variety of difficult substrates.Inspired by these findings,this thesis focuses on the oxyfuctionalization of alkynes with a Sc³⁺-assisted RFT photocatalyst under aerobic and visible-light condition.The detailed work are as follows:Part I:We have disclosed a green catalytic system for the functionalization of alkyne with RFT as the photocatalyst and non-redox Sc³⁺as the co-catalyst.This method enabled the regioselective synthesis ofα-keto ketal with high atom economy from the reaction of diaryl acetylene or aryl alkyl acetylene,aliphatic alcohol as both the solvent and nucleophile,and air as both the terminal oxidant and oxygen source.It has a broad scope of substrates especially including electron-deficient alkynes.For unsymmetrical di-aryl acetylenes and phenyl alkyl acetylenes,the regioselectivity was probably thermodynamically controlled because the more stable final products should come from the ketals having electron poorer conjugations such as EWG-substituted benzene ring or aliphatic chains.Also,from a preliminary DFT study of the negative charge distribution,the EWGs at para-or meta-position of the adjacent arene induced an electron richerα-C which tended to lose one electron and be attacked by alcohol selectively.The di-aryl acetylenes with ortho-substitutions tend to have poor site-selectivity also because of unfavorable steric hindrance during the nucleophilic addition process probably.Moreover,the electron transfer from the alkyne to the excited[RFT-2Sc]complex should be the key step on the basis of a series of control experiments,isotpoical experiments,Hammett analysis of competitive reaction and redox potential for diaryl acetylene and spectroscopy characterization.Furthermore,the gram-scale reaction of the model substrate also delivers the products in an acceptable yield,presenting potential applications in organic synthesis.Part II:We have developed an approach to 1,2-diketone via the oxidation of diaryl acetylene by the photocatalytic system consisting of RFT photocatalyst,Sc³⁺and Cl^-as the co-catalyst at 40 ^oC and aerobic conditions.The ¹⁸O-labelling experiments unveils that the two oxygen atoms in the diketone product should come from molecular oxygen and water,respectively.In addition,control experiments and fluorescence quenching experiments point out the possible mechanism involving single electron transfer steps.Without extra stoichiometric additives,this system employs readily available catalysts and safe,abundant and clean oxygen sources,meeting the requirement of green chemistry.In summary,this thesis not only describes the oxyfunctionalization of alkyne with a vitamin-based photocatalyst to access the efficient preparation ofα-keto ketal and diketone under environmental benign condition,but also provide more information for the establishment of novel bioinspired and sustainable catalytic oxidation systems.