Copper-Catalyzed Recycling of Halogen Activating Groups via 1,3-Halogen Migration

A Cu(I)-catalyzed 1,3-halogen migration reaction effectively recycles an activating group by transferring bromine or iodine from a sp2 to a benzylic carbon with concomitant borylation of the Ar–X bond. The resulting benzyl halide can be reacted in the same vessel under a variety of conditions to form an additional carbon–heteroatom bond. Cross-over experiments using an isotopically enriched bromide source support intramolecular transfer of Br. The reaction is postulated to proceed via a Markovnikov hydrocupration of the o-halostyrene, oxidative addition of the resulting Cu(I) complex into the Ar–X bond, reductive elimination of the new sp3 C–X bond, and final borylation of an Ar–Cu(I) species to turn over the catalytic cycle.