Application Of Visible Light-catalyzed[2+2]Cycloaddition To Dearomatization Of Benzothiophene Derivatives

Benzothiophene derivatives are widely used in chemistry,biology,pharmacy and agronomy due to their special conjugated structure and unique biological activity.They are also important intermediates for organic synthesis and pharmaceutical synthesis,and can be used as dyes and new types of the basic skeleton of polymers and luminescent materials.Therefore,the research related to benzothiophene derivatives has attracted more and more interest from organic chemists.Dearomatization is a chemical process that destroys the aromatic system of aromatic compounds by chemical means,making it not conform to the Huckel rule.The dearomatization process uses simple and easily available planar aromatic compounds as raw materials to realize the synthesis of a variety of chiral molecules with complex structures,in particular,demonstrating unique advantages in the application of natural products and analogs of drug molecules.The[2+2]cycloaddition reaction represents as an effective way of dearomatization.As for the[2+2]cycloaddition dearomatization reaction of benzothiophene derivatives,however,there are few reports up to now.Considering the concept of green chemistry,using visible light to promote the[2+2]cycloaddition of benzothiophene derivatives would be more meaningful.In this paper,2-carbonyl/phosphono-3-enylether benzothiophene derivatives were designed and synthesized,and their[2+2]cycloaddition dearomatization reactions were investigated under the promotion of visible light.The following two aspects are involved:1.Iridium-catalyzed photocatalytic[2+2]cyloaddition of 2-carbonyl-3-enylether benzothiophene compounds.Experimental results show that under the irradiation of blue LED with fac-Ir(ppy)3 as photocatalyst,[2+2]cycloaddition of 2-carbonyl-3-enylether benzothiophene derivatives could be achieved at room temperature under argon atmosphere.Good functional group tolerance was witnessed with acyl-substituted cyclobutane-fused poly-heterocyclic compounds obtained in moderate to good yields.2.4CzIPN-catalyzed photocatalytic[2+2]cycloaddition reaction of 2-phosphono-3-enyl benzothiophene compounds.It is found that under the irradiation of blue LED with 4CzIPN as photocatalyst,[2+2]cycloaddition reaction of 2-phosphono-3-enylether benzothiophene derivatives could be achieved at room temperature under nitrogen atmosphere.Both electron-withdrawing group and electron-donating group attached to the phosphono group work well in the system,accessing phosphono-substituted cyclobutane-fused poly-heterocyclic compound with good to excellent yields.In this paper,the intramolecular[2+2]cycloaddition dearomatization of 2-carbonyl/phosphono-3-enyl benzothiophene derivatives promoted by visible light was successfully achieved,and a possible reaction mechanism was proposed.In this system,not only the heterocyclic ring was retained,but also a four-ring fused ring structure was constructed combining a sulfur-containing five-membered ring and an oxygen-containing five-membered ring.Good to excellent yields were obtained under mild reaction conditions,with a broad substrate scope tolerated.A total of 29 compounds were synthesized in this paper,all of which are new compounds that have not been reported in the literature.The compounds were fully characterized by nuclear magnetic resonance(1H NMR,13C NMR,31P NMR,19F NMR),mass spectrometry,and single crystal diffraction.