Zinc-Free, Scalable Reductive Cross-Electrophile Coupling Driven by Electrochemistry in an Undivided Cell

Nickel-catalyzed reductive cross-electrophile coupling reactions are becoming increasingly important in organic synthesis, but application at scale is limited by three interconnected challenges: a reliance on amide solvents (complicated workup, regulated), the generation of stoichiometric Zn salts (complicated isolation, waste disposal issue), and mixing/activation challenges of zinc powder. We show here an electrochemical approach that addresses these three issues: the reaction works in acetonitrile with diisopropylethylamine as the terminal reductant in a simple undivided cell (graphite(+)/nickel foam(-)). The reaction utilizes a combination of two ligands, 4,4'-di-tert-butyl-2,2'-bipyridine and 4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine. Studies show that, alone, the bipyridine nickel catalyst predominantly forms protodehalogenated aryl and aryl dimer, whereas the terpyridine nickel catalyst predominantly forms bialkyl and product. By combining these two unselective catalysts, a tunable, general system results because excess radical formed by the terpyridine catalyst can be converted to product by the bipyridine catalyst. As the aryl bromide becomes more electron rich, the optimal ratio shifts to have more of the bipyridine nickel catalyst. Lastly, examination of a variety of flow-cell configurations establishes that batch recirculation can achieve higher productivity (mmol product/time/electrode area) than single-pass, that high flow rates are essential to maximizing current, and that two flow cells in parallel can nearly halve the reaction time. The resulting reaction is demonstrated on gram scale and should be scalable to kilogram scale.

Heterotrimetallic Carbon Dioxide Copolymerization and Switchable Catalysts: Sodium is the Key to High Activity and Unusual Selectivity

A challenge in polymer synthesis using CO2 is to precisely control CO2 placement in the backbone and chain end groups. Here, a new catalyst class delivers unusual selectivity and is self-switched between different polymerization cycles to construct specific sequences and desirable chain-end chemistries. The best catalyst is a trinuclear dizinc(II)sodium(I) complex and it functions without additives or co-catalysts. It shows excellent rates across different ring-opening (co)polymerization catalytic cycles and allows precise control of CO2 incorporation within polyesters and polyethers, thereby allowing access to new polymer chemistries without requiring esoteric monomers, multi-reactor processes or complex post-polymerization procedures. The structures, kinetics and mechanisms of the catalysts are investigated, providing evidence for intermediate speciation and uncovering the factors governing structure and composition and thereby guiding future catalyst design.

Evaluating Metal Ion Identity on Catalytic Silylation of Dinitrogen Using a Series of Trimetallic Complexes

We report catalytic silylation of dinitrogen to tris(trimethylsilyl)amine by a series of trinuclear first row transition metal complexes (M = Cr, Mn, Fe, Co, Ni) housed in our tris(β-diketiminate) cyclophane (L ^3- ). Yields are expectedly dependent on metal ion type ranging from 14 to 199 equiv NH₄ ⁺/complex after protonolysis for the Mn to Co congeners, respectively. For the series of complexes, the number of turnovers trend observed is Co > Fe > Cr > Ni > Mn, consistent with prior reports of greater efficacy of Co over Fe in other ligand systems for this reaction.