Studies On Synthesis And C-H Functionalization Of Thiophene Compounds

The studies on the synthesis of thiophene derivatives and the electrocatalytic C-H oxidative coupling reaction have extremely important significance in both academic and industrial fields owing to its economic and environmentally friendly characteristics.Thienopyrroles containing thiophene and pyrrole rings are structural cores for important components of natural products,organic optoelectronic materials,agrochemicals and pharmaceuticals.Arylated benzo[b]thiophenes are important structural features in many drug candidates that exhibit promising biological and pharmaceutical properties.Furthermore,they play a key role as precursors for high-performance electronic and optoelectronic devices.Hence,there have been attracted great interest in developing new strategies for thieno[3,2-b]pyrrole and arylated benzo[b]thiophenes synthesis in the past decades.Electrochemical C-H functionalization has been identified as a efficient strategy that exploits storable electricity in place of by product-generating chemical reagents.Firstly,we have reported a new strategy for thieno[3,2-b]pyrroles synthesis via elemental sulfur incorporated pyrrolidine ring.Then,C-H/C-H oxidative coupling reaction of 3-phenylbenzothiophene and p-methoxyphenol has been achieved by an electrochemical reaction protocol.This paper includes two major research works:1:Elemental sulfur-incorporated cyclization of pyrrolidine leading to thienopyrroleIn order to explore the possibility of efficient and convenient synthesis of functionalized thiophene compounds,we have envisioned that elemental sulfur incorporated synthesis of thieno[3,2-b]pyrroles from alkynylated pyrrolidine.The first step is to synthesize thiophene via oxidative annulation using elemental sulfur as the oxidant.After the thienannulation reaction,further dehydrogenative aromatization delivers the final product.This process could be achieved by the elemental sulfur,available substrate and no other additives.The reaction can also be performed at 120 ? for 24 h to deliver excellent yields.In this route,elemental sulfur not only serves as an electrophile but also functions as an oxidant.Mechanistic investigation suggested that the present reaction involved the formation of dihydrothieno[3,2-b]pyrrole as the intermediate.This strategy has easy access to raw materials,simple operation,and has the advantages of wide substrate applicability and excellent yield.The smooth progress of the gram-level experiment proves that it could be the possibility of being put into large scale.2:External oxidant-free electrooxidative coupling of 3-phenylbenzothiophene and p-methoxyphenolIn order to explore the possibility of the C-H functionalization of the thiophene compounds by electrocatalysis,we have realized a very efficient route for the dehydrogenative C-C cross-coupling of3-phenylbenzothiophene with p-methoxyphenol.The reported electrolysis is well performed,since only an undivided cell and a two electrode arrangement are necessary.The reaction conditions selects n Bu₄NBF₄ as electrolyte,10 m L HFIP and CH₂Cl₂(V_HFIP:V_CH2Cl2=3:2)as mixed solvent,RVC as anode and platinum electrode as cathode.At a constant current of 10 m A,the reaction can also be performed at 25 ? for3.0-5.0 h to deliver the desired products.The C(sp²)-H/C(sp²)-H oxidative coupling in this process occurs at the anode.The results of free radical inhibition experiments show that the reaction involves the free radical process.Here,reagent-and metal-free reaction conditions offer asustainable electrochemical pathway,providing an attractive synthetic method to a broad variety of biand terarylic products from thiophenes and phenols.The use of excess 3-phenylbenzothiophene affords the desired single cross-coupling products.