BCl3-Induced Annulative Oxo-and Thioboration for the Formation of C3-Borylated Benzofurans and Benzothiophenes

BCl3-induced borylative cyclization of aryl-alkynes possessing ortho-EMe (E = S, O) groups represents a simple, metal-free method for the formation of C3-borylated benzothiophenes and benzofurans. The dichloro(heteroaryl) borane primary products can be protected to form synthetically ubiquitous pinacol boronate esters or used in situ in SuzukiMiyaura cross couplings to generate 2,3-disubstituted heteroarenes from simple alkyne precursors in one pot. In a number of cases alkyne trans-haloboration occurs alongside, or instead of, borylative cyclization and the factors controlling the reaction outcome are determined.