Isolation of a Cu–H Monomer Enabled by Remote Steric Substitution of a N‑Heterocyclic Carbene Ligand: Stoichiometric Insertion and Catalytic Hydroboration of Internal Alkenes

Transient Cu–H monomers have long been invoked in the mechanisms of substrate insertion in Cu–H catalysis. Their role from Cu–H aggregates has been mostly inferred since ligands to stabilize these monomeric intermediates for systematic studies remain limited. Within the last decade, new sterically demanding N-heterocyclic carbene (NHC) ligands have led to isolable Cu–H dimers and, in some cases, spectroscopic characterization of Cu–H monomers in solution. We report an NHC ligand, IPr*R, containing para R groups of CHPh2 and CPh3 on the ligand periphery for the isolation of a Cu–H monomer for insertion of internal alkenes. This reactivity has not been reported for (NHC)­CuH complexes despite their common application in Cu–H-catalyzed hydrofunctionalization. Changing from CHPh2 to CPh3 impacts the relative concentration of Cu–H monomers, rate of alkene insertion, and reaction of a trisubstituted internal alkene. Specifically, for R = CPh3, monomeric (IPr*CPh3)­CuH was isolated and provided >95% monomer (10 mM in C6D6). In contrast, for R = CHPh2, solutions of [(IPr*CHPh2)­CuH]2 are 80% dimer and 20% (IPr*CHPh2)­CuH monomer at 25 °C based on 1H, 13C, and 1H–13C HMBC NMR spectroscopy. Quantitative 1H NMR kinetic studies on cyclopentene insertion into Cu–H complexes to form the corresponding Cu–cyclopentyl complexes demonstrate a strong dependence on the rate of insertion and concentration of the Cu–H monomer. Only (IPr*CPh3)­CuH, which has a high monomer concentration, underwent regioselective insertion of a trisubstituted internal alkene, 1-methylcyclopentene, to give (IPr*CPh3)­Cu­(2-methylcyclopentyl), which has been crystallographically characterized. We also demonstrated that (IPr*CPh3)­CuH catalyzes the hydroboration of cyclopentene and methylcyclopentene with pinacolborane.