Cobalt-Catalyzed C(sp²)–C(sp³) Suzuki–Miyaura Cross-Coupling Enabled by Well-Defined Precatalysts with L,X-Type Ligands

Abstract

Cobalt­(II) halides in combination with phenoxyimine (FI) ligands generated efficient precatalysts in situ for the C­(sp2)–C­(sp3) Suzuki–Miyaura cross-coupling between alkyl bromides and neopentylglycol (hetero)­arylboronic esters. The protocol enabled efficient C–C bond formation with a host of nucleophiles and electrophiles (36 examples, 34–95%) with precatalyst loadings of 5 mol %. Studies with alkyl halide electrophiles that function as radical clocks support the intermediacy of alkyl radicals during the course of the catalytic reaction. The improved performance of the FI–cobalt catalyst was correlated with decreased lifetimes of cage-escaped radicals as compared to those of diamine-type ligands. Studies of the phenoxyimine–cobalt coordination chemistry validate the L,X interaction leading to the discovery of an optimal, well-defined, air-stable mono-FI–cobalt­(II) precatalyst structure.

Iron-Catalyzed C(sp2)–C(sp3) Suzuki–Miyaura Cross-Coupling Using an Alkoxide Base

“…The role of boronate intermediates in catalysis was also explored. In both cobalt and palladium-catalyzed cross-coupling, potassium aryl boronates have been shown to be competent nucleophiles. , Employing the potassium aryl boronate 5 in cross-coupling resulted in an 89% yield of the cross-coupled product, demonstrating that boronate nucelophiles are competent, if not preferred, in catalysis (Scheme a). A possible pathway of transmetalation is shown in Scheme b, whereby an iron halide intermediate reacts with an aryl boronate to form an iron aryl complex.…”

“…Our laboratory recently reported a cobalt-catalyzed C­(sp 2 )–C­(sp 3 ) Suzuki coupling method of aryl boron neopentyl glycol esters (ArBNeo) with alkyl bromides using the alkoxide base KOMe . Phenoxyimine (FI) ligands provided optimal performance, allowing reduced cobalt precatalyst loadings of 5 mol % and generating minimal side products arising from competing elimination and reduction side products.…”

“…With the series of iron alkyl compounds 1a – 1f in hand, their performance in cross-coupling catalysis was evaluated. In cobalt-catalyzed cross-coupling, aryl BNeo reagents have been effective for both C­(sp 2 )-C­(sp 2 ) and C­(sp 2 )-C­(sp 3 ) coupling, , likely owing to more facile formation of boronate salts and subsequent transmetalation. Accordingly, aryl BNeo nucleophiles were explored.…”

Enabling Suzuki–Miyaura coupling of Lewis-basic arylboronic esters with a nonprecious metal catalyst

“…Complexes of other nonprecious metals besides Ni have also been reported as catalysts for the SMC. Fe 16,17 and Co-catalyzed 18,19 couplings of arylboronic esters with alkyl halides have recently been developed. Co catalysts have also been reported for the coupling of arylboronic esters with aryl halides.…”

“…catalyzed 18,19 couplings of arylboronic esters with alkyl halides have recently been developed. Co catalysts have also been reported for the coupling of arylboronic esters with aryl halides.…”