Oxidative Coupling/Cyclization Reactions Involving Radicals

Nowadays,the concept of sustainable development and green chemistry is getting more and more popular.Constructing compounds with high-atom and high-step economy have been an important goal of modern organic synthesis.As the first-row transition-metal catalysts and numerous oxidative coupling reagents were discovered and applied to the single electron transfer(SET)processes,which made the radical type C-H activation become a promising method for the construction of compound molecules.Due to the developments of photochemistry,the organic photopigments have been gradually replacing those transition metal catalysts in organic synthesis,which made the reaction conditions much milder.Moreover,the developments of electrochemistry have been shown a more efficient bonding ability under catalyst-and external oxidant-free conditions.Both two techniques make radical-type C-H activation much greener,more gentle and efficient.The first part of this thesis is a brief review of recent developments of radical chemistry including the introduction of different ways to generate the radicals and various reactions that radicals participate in.Different types of reactions where radical intermediates produce in photochemistry and electrochemistry are also demonstrated in this part.Our research mainly focuses on the developments of a trail of green,mild and efficient cyclization reactions and oxidative-coupling reactions under photochemical or electrochemical conditions.The furans,benzo 3,4-coumarins and thiocyanide compounds were successfully synthesized from simple materials.Meanwhile,the radical trapping experiments,cyclic voltammetry,and density functional theory were used to study the mechanism of the reaction deeply.The main results of the thesis are as follows:(1)A visible light-mediated oxidative[3+2]cycloaddition of 1,3-diketones and terminal alkyne was achieved to synthesize a series of multi-substituted furans by using methylene blue as a photocatalyst.Compared with the previous work,starting materials of this reaction are simple,easy obtain and any preactivation is not required,which has had a great importance in practical and research significance for the construction of multi-substituted furans in one step.From the view point of mechanism,the capture of?-carbonyl C(sp3)radical using 1,1-diphenyiethycene as a radical scavenger provides a strong evidence for this mechanism.(2)The co-catalysis of acridine as photocatalyst and cobaloxime as hydrogen-producing catalyst,which achieved an external oxidant-free dehydrogenative lactonization of 2-arylbenzoic acids for constructing benzo-3,4-coumarins,accompanied by H2 release.Moreover,based on the results of radical trapping experiments,CV experiments and DFT calculations,the mechanism of the reaction might proceed through a carboxyl radical intermediate.(3)A synthetic protocol of lactones by the electro-oxidative induced C-H bond activition of 2-arylbenzoic acids was developed.Under the catalyst-and oxidant-free conditions,a series of benzo-3,4-coumarins have been synthesized by using Na2SO4 aqueous as a cheap and green supporting electrolyte.Furthermore,radical trapping experiments and CV experiments could provide some evidence for the mechanism of this reaction.(4)An electrochemical oxidation protocol for thiocyanation of electron-rich aromatics has been achieved.This method enabled the synthesis of a series of useful thiocyanide-containing intermediates under high-efficiency,catalyst-and external oxidant-free conditions.In this reaction,ammonium thiocyanate acts as both electrolyte and reactant.Moreover,this transformation could not only tolerate the unprotected aniline and nitrogen-substituted aniline,but also the indole ring,pyrrole ring,and the methoxybenzene.