PNNL

Abstract

Transient Cu-H monomers have long been invoked in the mechanism of substrate insertion in Cu-H catalysis. However, the isolation, kinetic studies and reactivity profile of these reactive and unstable intermediates are rare. Using Nheterocyclic carbene (NHC) ligands, IPr*R, in which the para-substituted R groups of -CHPh2 and -CPh3 on the ligand periphery have enabled the isolation of Cu-H monomers and insertion of internal alkenes, which has not been reported for (NHC)CuH complexes despite their common use in CuH-catalyzed hydrofunctionalization. A minor change from para-CHPh2 to para-CPh3 significantly impacts the relative concentration of Cu-H monomers, rate of alkene insertion, and reaction of trisubstituted internal alkene. Specifically, changing para-CHPh2 to para-CPh3 produced 95%+ [(IPr*-CPh3)CuH] monomer compared to 14% [(IPr*-CHPh2)CuH] monomer at 25 oC based on 1H, 13C and 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 Cu-H monomer. Additionally, only [(IPr*-CPh3)CuH], at high monomer concentration, underwent regioselective insertion of a trisubstituted internal alkene, 1-methylcyclopentene, to [(IPr*-CPh3)Cu-(2-methylcyclopentyl)], which has been crystallographically characterized.