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

Abstract

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.

On the Generality of Molecular Volcano Plots

“…This ligand environment affects not only the properties and performance of a catalyst strongly but also influences the ultimate product and/or product ratio . In the course of a reaction, a precatalyst is converted into an active catalytic species that may involve a host of changes, which include structural alterations through ligand loss, redox processes that modify the oxidation state, or transitions between low and high spins . Computational analysis based on DFT has made considerable headway in deciphering the myriad energetic pathways to determine the true nature of the active catalyst.…”

“…[1] In the course of ar eaction, a precatalyst is converted into an active catalytic species that may involve ah ost of changes, which include structural alterations through ligand loss, [2] redoxp rocesses that modify the oxidation state, [3] or transitions between low andh igh spins. [4] Computational analysis based on DFT has made considerable headway in deciphering the myriade nergetic pathways to determinet he true nature of the active catalyst. For instance, for Suzuki-Miyaura cross-coupling, [5] computationh as revealed that existence of mono-(ML) and bisligated (ML 2 )r eaction pathways within the conventional low-spin Pd 0 /Pd II catalytic cycle (Scheme 1).…”

Cobalt‐Catalyzed Suzuki Biaryl Coupling of Aryl Halides

“…Iron‐catalyzed Suzuki cross‐coupling can be performed between a variety of organic halides and organoboron reagents,6 but simple biaryl bond formation remains elusive and challenging 6a, 7, 8. Recently, Chirik and co‐workers reported early results in the coupling of aryl triflates with arylboron pinacol esters by using a cobalt PNP‐pincer‐based catalyst 9. We now report the cross‐coupling of aryl chlorides and bromides with activated arylboronic pinacol esters,10, 11 using simple cobalt catalysts prepared in situ from commercially available precursors.…”

Synthesis, characterization and theoretical and fluorescence emission microscopy studies of new Pd/Pt–cyclopropa[60]fullerene complexes: Application of Taguchi method for optimization of parameters in Suzuki–Miyaura reaction

“…The carbene has strong σ‐donor and relatively weak π‐acceptor interactions and it can increase the electron density on the metal centre, which destabilizes the carbon–halogen bond, leading to faster cleavage and eventually oxidative addition to Pd. The catalytic cycle is continued by a transmetallation step, in which an aryl group transfers to Pd centre from the organoboron compound, activated by K 2 CO 3 base ( C ) . Finally, reductive elimination of intermediate C produces the desired coupling product and regenerates the L n Pd 0 active catalyst which starts another catalytic cycle …”