Study On Regioselective Halogenation Of Coumarin And The Cycloaddition Reaction Of Dichlorocoumarin Photocatalysis

Halides are important intermediates in organic synthetic chemistry and have important applications in organic synthesis.Many complicated molecules can be synthesized by transformation of halides.The halogenation of aromatic and heteroaromatic compounds is one of the most valuable synthetic transformations.However it is known that electron-deficient coumarins are usually difficult to be halogenated.Thus,a safe,convenient,and regioselective synthesis of3-halocoumarins using a metal halide（Cu X₂alone or Cu X₂combined with Zn X₂）promoted halogenation with N-halosuccinimide（NXS）as halide source is reported.The synthesis involved the steady in situ generation of highly reactive positive halogen（X⁺）by the coordination of copper or zinc with the N-halosuccinimide and subsequent electrophilic aromatic substitution of the electron-deficient coumarins.This procedure works well also for the halogenation of less electron-rich naphthoquinones,chromone,and methoxypsoralen in moderate to quantitative yields.This protocol features simple experimental conditions using readily available inexpensive reagents and provides a convenient approach to the chlorination or bromination of some useful heteroaromatic compounds.The organic reaction catalyzed by visible light has the advantages of mild conditions and no environmental pollution,which makes it an organic synthesis strategy,applied to halogenation,acylation,cycloaddition and other reactions.New compounds are synthesized by photocatalytic cycloaddition of coumarin halide.Therefore,in chapter three,an efficient method for construction of halogenated cyclobutene and polycyclic aromatic hydrocarbons is intrudced,without any additives and accessible starting material.The intermolecular[2+2]or[4+2]cycloaddition reaction between 3,4-dichlorocoumarin and phenylbenzofuran promoted by visible light,by using 2 mmol%of organic dye4Cz IPN as photocatalysis and in non-polar solvent benzene.The reaction provides a more environmentally friendly and convenient strategy for the synthesis of complex cyclobutenes and polycyclic aromatic hydrocarbons,avoiding the excess transition-metal catalysts and high temperature conditions.The reaction involves radical cyclization and electrocyclization.And the synthsis of cyclobutene is obtained that the intermediate cyclobutane is cycled by 1,4-diradical,and subsequent one molecule of HCl is quickly removed after the lactone ring hydrolyzed and decarboxylated by adsorption on silica gel.In addition,cyclobutane can be electrocyclized to form hexatriene structure intermediate after eliminating of one HCl under heating condition,which can be afford ring-expanded product vis eliminating of another HCl under the irradiation of 4 W LED blue light.