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 degrees C based on H-1, C-13, and H-1-C-13 HMBC NMR spectroscopy. Quantitative H-1 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.

Automated summary

By using new NHC ligands, isolated Cu-H monomers have been obtained and their role in internal alkene insertion reactions has been demonstrated.