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

Abstract

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 <i>N</i>-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 <i>para</i> 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.