A Visible Light Photoredox Catalyzed Generation Of Ortho-quinone Methides For 2,3-dihydrobenzofuran Synthesis

Ortho-quinone methides(o-QMs)are a versatile class of reactive organic intermediates involved in a wide range of chemical and biological transformations.They are also widely used to construct various nitrogen,and oxygen heterocyclic compounds in organic synthesis.Therefore,the development of methods for the in situ-formation of o-QMs is a very important research area in organic chemistry.On the other hand,as a mild and efficient synthesis method,visible light photoredox catalysis is widely used in the field of organic synthesis.In this paper,we used o-hydroxystyrene,sulfur-(trifluoromethyl)dibenzothiophene triflate(Umemoto reagent)and sulfur ylide to generate o-QMs in situ under visible light photocatalysis and developed[4+1]cyclization,providing a series of 2,3-benzodihydrofuran in good yields.First,we screened a series of conditions(photocatalyst,solvent,additive,ratio of reactants)and then determined the optimal conditions:fac-Ir(4'-CF3-ppy)3 as photocatalyst,CF3CH2OH as solvent,CF3CO2H as additive at room temperature,the ratio of o-hydroxystyrene,Umemoto reagent,sulfur ylide is 1:1.1:2.We could get the best results.A series of substituted 2,3-benzodihydrofuran could be obtained in up to 81%isolated yield.More over,we used radical trapping reagents 2,2,6,6-Tetramethylpiperidinooxy(TEMPO)and butylated hydroxytoluene(BHT)to investigate the reaction mechanism and proposed a possible reaction mechanism.Moreover,we scaled up the reaction to 1.0 mmol to obtain the corresponding 2,3-benzodihydrofuran compound in an isolated yield of 76%.Chiral 2,3-benzodihydrofuran compound was synthesized with moderate ee by using a chiral naphthalene skeleton of sulfur ylide.The reaction can also be achieved in a yield of 57%under sunlight irradiation.All the target compounds were characterized by 1H NMR,13C NMR,19F NMR,HRMS,IR.The spectral properties of the target compounds were studied and their spectral data were assigned and identified.The above research results were published in?Chemical Communications?.