Alkynylboronates and -boramides in CoI- and RhI-Catalyzed [2+2+2] Cycloadditions: Construction of Oligoaryls through Selective Suzuki Couplings

Oligoaryls were prepared through transition-metal-catalyzed [2+2+2] cycloadditions of borylated alkynes, followed by Suzuki cross-couplings with aryl halides. Ethynylboryl pinacolate took part in cobalt-catalyzed [2+2+2] cycloadditions of all types investigated (i.e., all-intermolecular cyclotrimerization, diyne-yne cocyclization, and all-intramolecular triyne cyclo-isomerization). The resulting platforms gave rise to linear and angular ter- and quateraryls after Pd-catalyzed cross-coupling. In addition, the first [2+2+2] cycloadditions involving alkynyl naphthaloboramides were developed. These derivatives could be cocyclized with diynes in the presence of a rhodium complex as catalyst. Because the boramide group is inactive under Suzuki coupling conditions, but can be readily deprotected to afford an active one, an iterative functionalization of a boronate-/boramide-substituted benzene could be achieved.