Identification of the Active Catalyst for Nickel‐Catalyzed Stereospecific Kumada Coupling Reactions of Ethers

Abstract

A series of nickel complexes in varying oxidation states were evaluated as precatalysts for the stereospecific cross‐coupling of benzylic ethers. These results demonstrate rapid redox reactions of precatalysts, such that the oxidative state of the precatalyst does not dictate the oxidation state of the active catalyst in solution. These data provide the first experimental evidence for a Ni0–NiII catalytic cycle for a stereospecific alkyl–alkyl cross‐coupling reaction, including spectroscopic analysis of the catalyst resting state.

Nickel-Mediated Enantiospecific Silylation via Benzylic C–OMe Bond Cleavage

“…Although alkenes are known to promote nickelmediated cross-coupling reactions, 58,59 it has been reported that 1,5-cyclohexadiene (COD) may form an off-cycle nickel species that is ineffective in C−O bond cleavage. 60 Similarly, we also observed the inactivity of Ni(cod) 2 (Table 1, entry 5); however, employing 0.1 equiv of COD in the standard reaction still afforded 82% of cross-coupled product 3a. We determined that the inactivity of Ni(cod) 2 is due to the unavailability of COD to stabilize the Ni(0) species.…”

“…Unfortunately, alkene 1h did not yield the cross-coupled product. Although alkenes are known to promote nickel-mediated cross-coupling reactions, , it has been reported that 1,5-cyclohexadiene (COD) may form an off-cycle nickel species that is ineffective in C–O bond cleavage . Similarly, we also observed the inactivity of Ni­(cod) 2 (Table , entry 5); however, employing 0.1 equiv of COD in the standard reaction still afforded 82% of cross-coupled product 3a .…”

Nickel-Catalyzed Kumada Cross-Coupling Reactions of Benzylic Sulfonamides

“…We next investigated an alternative precatalyst. Previously, the Jarvo laboratory reported the cross-electrophile coupling (XEC) reaction of benzylic and allylic sulfonamides which employed a BINAP-ligated nickel (II) precatalyst [ 50 , 57 , 58 ]. Utilizing these conditions, with 15 mol % of catalyst, we were delighted to observe the desired product in 54% yield and 40% yield of styrene 3 (entry 8).…”

Transition-Metal-Catalyzed Cross-Coupling Reactions of Grignard Reagents

“…47 The effect of different chiral BINAP ligands on the stereoselectivity of the nickel-catalyzed Kumada-Tamao-Corriucoupling of benzylic ethers was investigated in 2020 by Borovick, Jarvo, and co-workers. 49 They examined the mechanism of the catalytic process and the Ni(0)-Ni(II) catalytic cycle has been evidenced using spectroscopic methods.…”