Insight into Transmetalation Enables Cobalt-Catalyzed Suzuki–Miyaura Cross Coupling

Among the fundamental transformations that comprise a catalytic cycle for cross coupling, transmetalation from the nucleophile to the metal catalyst is perhaps the least understood. Optimizing this elementary step has enabled the first example of a cobalt-catalyzed Suzuki–Miyaura cross coupling between aryl triflate electrophiles and heteroaryl boron nucleophiles. Key to this discovery was the preparation and characterization of a new class of tetrahedral, high-spin bis­(phosphino)­pyridine cobalt­(I) alkoxide and aryloxide complexes, (^iPrPNP)­CoOR, and optimizing their reactivity with 2-benzofuranylBPin (Pin = pinacolate). Cobalt compounds with small alkoxide substituents such as R = methyl and ethyl underwent swift transmetalation at 23 °C but also proved kinetically unstable toward β–H elimination. Secondary alkoxides such as R = ⁱPr or CH­(Ph)­Me balanced stability and reactivity. Isolation and structural characterization of the product following transmetalation, (^iPrPNP)­Co­(2-benzofuranyl), established a planar, diamagnetic cobalt­(I) complex, demonstrating the high- and low-spin states of cobalt­(I) rapidly interconvert during this reaction. The insights from the studies in this elementary step guided selection of appropriate reaction conditions to enable the first examples of cobalt-catalyzed C–C bond formation between neutral boron nucleophiles and aryl triflate electrophiles, and a model for the successful transmetalation reactivity is proposed.