Pd-catalyzed halocyclizations of unactivated 1,6-diynes through a formal anti-carbopalladation/bromide radical cascade

We report a Pd-catalyzed halocyclization of unactivated 1,6-diynes with N-bromosuccinimide (NBS). This approach produces stereo-defined dibromo substituted dihydropyrans, tetrahydropyridines, and 3-methylene cyclohexenes with exocyclic double bond appendages in mostly good yields. Copper salt was found to be a useful Lewis acid in this reaction. Mechanistically, a formal anti-carbopalladation and a bromide radical promoted Pd^II-Pd^III-Pd^I-Pd^II catalytic cycles were proposed to be involved in the formation of the dibromo-substituted products. Further functionalization of the dihydropyran derivatives underwent B(C₆F₅)₃-catalyzed ring opening, and reduction afforded dibrominated 1,3-dienes with excellent stereoselectivity.