Isolation of a Cu-H Monomer Enabled by Remote Steric Substitution of a -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 -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, IPrR, containing R groups of CHPh and CPh 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 CHPh to CPh impacts the relative concentration of Cu-H monomers, rate of alkene insertion, and reaction of a trisubstituted internal alkene. Specifically, for R = CPh, monomeric (IPrCPh)CuH was isolated and provided >95% monomer (10 mM in CD). In contrast, for R = CHPh, solutions of [(IPrCHPh)CuH] are 80% dimer and 20% (IPrCHPh)CuH monomer at 25 °C based on H, C, and H-C HMBC NMR spectroscopy. Quantitative H 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 (IPrCPh)CuH, which has a high monomer concentration, underwent regioselective insertion of a trisubstituted internal alkene, 1-methylcyclopentene, to give (IPrCPh)Cu(2-methylcyclopentyl), which has been crystallographically characterized. We also demonstrated that (IPr*CPh)CuH catalyzes the hydroboration of cyclopentene and methylcyclopentene with pinacolborane.