KOH/DMSO Superbase Or Bu~tOK Promotes The Synthesis Of Benzo[b]thiophenes And Carbonyl Compounds Under Visible Light

Organosulfur compounds containing aryl C-S bond are of great biomedical value due to their outstanding therapeutic effect in bacteriostasis and cancer treatment.Constructing C-S bond with high efficiency and selectivity is not only an important research brand in the field of carbon-heteroatom bond-forming reaction but also an effective approach to organic molecules functionalization.Among organic sulfides,benzo[b]thiophenes are bicyclic heteroaromatic compounds which have typical biomedical function.The construction of benzo[b]thiophene structure is one of breakthrough directions in sulfur chemistry.However,there are only limited literatures about the synthesis of benzo[b]thiophenes by an economical and green way to date.In view of the above,we have establilshed a photochemical reaction for the synthesis of benzo[b]thiophenes by the use of inert 2-halothioanisole as the reaction substrate.Under visible light irradiation,aryl radical intermediate is produced with the promotion of KOH/DMSO superbase and then adds to alkynes to give the target benzo[b]thiophenes.Meanwhile,similar base system can be extended to the construction of the carbonylative cross-coupling reactions.Based on the experimental results,the current paper arrives at the following conclusions:(1)At 25℃,under visible light irradiation(420 nm<λ<780 nm),one-step annulation reaction of 2-halothioanisole and various alkynes have successfully realized by the use of KOH/DMSO superbase as the promoter.In our system,the highest 1H NMR yield was up to 99%.This reaction system had a good functional group tolerance of alkynes and bears little by-products.We synthesized sixteen 2-substituted benzo[b]thiophenes which are sensitive to the electronic effect of the substrates with different substituent groups.(2)The cross-coupling reaction of C-S bond proceeds via a radical process.The yield of benzo[b]thiophenes is linearly proportional to the strength of the excited light,which implies that the annulation reaction of 2-halothioanisole and alkynes is a typical photo-induced reaction.(3)When using aryl halide as the reaction substrate and a certain pressure of CO or a certain amount of Mo(CO)6 as the source of carbonyl group,the insertion reaction of CO was successfully performed for the synthesis of target carbonyl compounds via single electron transfer mechanism induced by ButOK/1,10-Phenanthroline or ButOK/DMSO under visible light irradiation.The main innovation of this paper is as follows:(1)We have established a visible light stimulation pattern to induce electron transfer between KOH/DMSO superbase and inert substates for the formation of aryl radical,followed by the electrophilic addition to form C-S bond through annulation reaction.This reaction paradigm features the use of inert 2-halothioanisole as substrates,abundant visible light energy for one-step synthesis of benzo[b]thiophenes in high efficiency.Different from conventional thermocatalytic processes,we provide a green and environmentally friendly alternative strategy to the construction of C-S bond.(2)The insertion reaction of CO was successfully performed for the synthesis of target carbonyl compounds via single electron transfer mechanism induced by ButOK/1,10-Phenanthroline under visible light irradiation which is a rewarding exploration of CO fixation for the formation of carbonyl compounds.