N-Heterocyclic Bisphosphine Ligands For Use In Nickel-Catalyzed C(sp2)-N Cross-Coupling
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Notwithstanding the utility of Buchwald-Hartwig Amination (BHA) technology, the cost and scarcity of palladium has prompted a move to base metal catalysis in recent years. Nickel has emerged as a prime candidate in this chemistry to compete with palladium, being both cheap and abundant. Amination chemistry with nickel is relatively unexplored, and the distinct electronic properties of nickel offer reactivity benefits over palladium when employing challenging phenol derived electrophiles. While ligand design requirements for BHA chemistry are well-understood, structure-reactivity trends for nickel counterparts are ill-defined, with few ligands capable of promoting these nickel-catalyzed cross-couplings. Following the establishment of PAd-DalPhos as the state-of-the-art ligand in challenging nickel-catalyzed amination reactions, we sought to build on this ortho phenylene bridged motif and explore new architectures encompassing a bulky, modestly electron-donating bisphosphine framework which would favour reductive elimination, the presumed rate-limiting step in the catalytic cycle.