MPHT-Promoted Bromocyclization of ortho-Substituted Arylalkynes: Application to the Synthesis of 2-Substituted 3-Bromobenzofurans and -Benzo[b]thiophenes

A convenient and general approach to the synthesis of 2-substituted 3-bromobenzofurans and -benzothiophenes was developed. The procedure is based on the cyclization of ortho-substituted arylalkynes in the presence of N-methylpyrrolidin-2-one hydrotribromide (MPHT) as a soft and easy-to-handle electrophilic brominating reagent. Under mild reaction conditions, MPHT promoted the bromocyclization of various enynes and diynes as well as arylalkynes to give 2-substituted 3-bromobenzofurans and -benzothiophenes in high to excellent yields. Subsequent functionalization by palladium-catalyzed coupling reactions at the C-Br bond afforded general access to 2,3-disubstituted benzofurans and benzothiophenes of biological interest.