Modular assembly of versatile tetrasubstituted alkenyl monohalides from alkynyl tetracoordinate borons

Tetrasubstituted olefins as a pivotal scaffold have been extensively employed in pharmaceuticals, natural products, polymer chemistry, and material sciences and arouse great attention in chemical community. Despite advances on the assembly of fully carbon tetrasubstituted olefins, the synthesis of tetrasubstituted alkenyl monohalides are underdeveloped and elusive. Herein, we report four intriguing migration reactions from alkynyl tetracoordinate boron species, in which various common, inexpensive, and user-friendly halogen electrophiles (Selectfluoro, TCCA, NIS, and NBS) are suitable electrophilic halogen sources. Versatile tetrasubstituted olefins were thus readily accessible from transition-metal-catalyzed transformations of these newly built tetrasubstituted alkenyl monohalides under suitable reaction conditions. The mechanistic investigations suggest that different electrophilic halogen sources promote different aryl migration routes of alkynyl tetracoordinate boron species, rendering various tetrasubstituted alkenyl monohalides, respectively. Distinct reaction modes, unusual mechanism, valuable products, and versatile transformations portray the advantages and significance of our protocol.