The first iron-catalyzed enantioselective cross-coupling reaction between an organometallic compound and an organic electrophile is reported. Synthetically versatile racemic α-chloro- and α-bromoalkanoates were coupled with aryl Grignard reagents in the presence of catalytic amounts of an iron salt and a chiral bisphosphine ligand, giving the products in high yields with acceptable and synthetically useful enantioselectivities (er up to 91:9). The produced α-arylalkanoates were readily converted to the corresponding α-arylalkanoic acids with high optical enrichment (er up to >99:1) via simple deprotections/recrystallizations. The results of radical probe experiments are consistent with a mechanism that involves the formation of an alkyl radical intermediate, which undergoes subsequent enantioconvergent arylation in an intermolecular manner. The developed asymmetric coupling offers not only facile and practical access to various chiral α-arylalkanoic acid derivatives, which are of significant pharmaceutical importance, but also a basis of controlling enantioselectivity in an iron-catalyzed organometallic transformation.

An efficient and high-yielding cross-coupling reaction of various primary, secondary, and tertiary alkyl chlorides with aryl Grignard reagents was achieved by using catalytic amounts of N-heterocyclic carbene ligands and iron salts. This reaction is a simple and efficient arylation method having applicability to a wide range of industrially abundant chloroalkanes, including polychloroalkanes, which are challenging substrates under conventional cross-coupling conditions.

The mechanism of the full catalytic cycle for Fe-chiral-bisphosphine-catalyzed cross-coupling reaction between alkyl halides and Grignard reagents (Nakamura and co-workers, J. Am. Chem. Soc. 2015, 137, 7128) was rationalized by using density functional theory (DFT) and multicomponent artificial force-induced reaction (MC-AFIR) methods. The computed mechanism consists of (a) C-Cl activation, (b) transmetalation, (c) C-Fe bond formation, and (d) C-C bond formation through reductive elimination. Our survey on the prereactant complexes suggested that formation of Fe(BenzP*)Ph and Fe(BenzP*)Ph complexes are thermodynamically feasible. Fe(BenzP*)Cl complex is the active intermediate for C-Cl activation. Fe(BenzP*)Ph complex can be formed if the concentration of Grignard reagent is high. However, it leads to biphenyl (byproduct) instead of the cross-coupling product. This explains why slow addition of Grignard reagent is critical for the cross-coupling reaction. The MC-AFIR method was used for systematic determination of transition states for C-Fe bond formation and C-C bond formation starting from the key intermediate Fe(BenzP*)PhCl. According to our detailed analysis, C-C bond formation is the selectivity-determining step. The computed enantiomeric ratio of 95:5 is in good agreement with the experimental ratio (90:10). Energy decomposition analysis suggested that the origin of the enantioselectivity is the deformation of Ph-ligand in Fe-complex, which is induced by the bulky tert-butyl group of BenzP* ligand. Our study provides important mechanistic insights for the cross-coupling reaction between alkyl halides and Grignard reagents and guides the design of efficient Fe-based catalysts for cross-coupling reactions.