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

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 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.

Distinct Reactivity Modes of a Copper Hydride Enabled by an Intramolecular Lewis Acid

“…The results of both experiments provide key insight into the reactivity of 3 : addition of an oxidant to the Cu­(I)–H oxidizes at the Cu center, which induces reduction of phenylacetylene and reforms Cu­(I). This reactivity is distinct from previous Cu­(I)–H complexes that react directly with alkynes and represents an unusual pathway because the addition of an oxidant is required for alkyne reduction with 3 .…”

Synthesis, Structural Characterization, and CO2Reactivity of a Constitutionally Analogous Series of Tricopper Mono-, Di-, and Trihydrides

“…Addition of 3 equiv of potassium tert -butoxide to tribromide 1 affords the tris­( tert -butoxide) complex, 5 , a viable synthon for the preparation of a tricopper trihydride. ,,− The reaction of 3 equiv of HBPin with 5 in thawing toluene led to a new salient 1 H NMR resonance at 0.07 ppm (C 6 D 6 ) with a relative integration of 3 . XRD confirmed the generation of symmetric trihydride 6 .…”

Azobenzene-Integrated NHC Ligands: A Versatile Platform for Visible-Light-Switchable Metal Catalysis

“…This sterically demanding and readily accessible substituent was selected with the intention of enhancing metal confinement. 95 To improve the visible-light photoswitch performance of the ligand, the respective 2,6-difluorophenylazo derivatives 15a,b were prepared. Based on this fluorinated arylazo tether, we further synthesized 15c incorporating a 4-bromo DIPP group (DIPP Br ), offering a synthetic gateway for late-stage functionalization.…”