Iron-Catalyzed C(sp <sup>2</sup> )–C(sp <sup>3</sup> ) Suzuki–Miyaura Cross-Coupling Using an Alkoxide Base
Paul O. Peterson, Matthew V. Joannou, Eric M. Simmons, Steven R. Wisniewski, Junho Kim, Paul J. Chirik
Abstract
The synthesis and characterization of phenoxy(imine) iron(II) alkyl precatalysts for C(sp 2 )–C(sp 3 ) Suzuki–Miyaura cross-coupling of aryl boronic esters and alkyl bromides is described. Addition of phenoxyimines (FI) to (py) 2 Fe(CH 2 SiMe 3 ) 2 (py = pyridine) afforded the high-spin iron(II) alkyl derivatives, (FI)Fe(CH 2 SiMe 3 )(py) with varying N -imine substituents. With both neopentyl glycol-protected (BNeo) and pinacol-protected boronic ester (BPin) aryl nucleophiles, an iron-catalyzed cross-coupling method was realized that utilizes mild alkoxide bases. Optimal performance was observed in nonpolar solvents with anisole and fluorobenzene identified as more benign alternatives to benzene. The scope of this transformation includes high efficiency C(sp 2 )–C(sp 3 ) bond formation with both primary and secondary alkyl bromides with electron-deficient aryl and heteroaryl nucleophiles. Substrates with base-sensitive functionality including ester and nitrile groups were tolerated, highlighting the broader compatibility with an alkoxide base. Radical clock experiments support the formation of electrophile-derived radicals during catalysis, and experiments with preformed potassium aryl boronates demonstrate the role of boronate intermediates in transmetalation.