Insight into Transmetalation Enables Cobalt-CatalyzedSuzuki–Miyaura Cross Coupling

摘要

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, demonstratingthe high- and low-spin states of cobalt(I) rapidly interconvert duringthis reaction. The insights from the studies in this elementary stepguided selection of appropriate reaction conditions to enable thefirst examples of cobalt-catalyzed C–C bond formation betweenneutral boron nucleophiles and aryl triflate electrophiles, and amodel for the successful transmetalation reactivity is proposed.