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

CuH-Catalyzed Regio- and Enantioselective Hydroallylation of 1-Trifluoromethylthioalkenes: Leaving Group-Dependent Stereochemistry

A copper-catalyzed regio- and enantioselective hydroallylation of 1-trifluoromethylthioalkenes with hydrosilanes and allylic electrophiles has been developed. The judicious choice of chiral ligands successfully promotes the enantioselective C(sp3)-C(sp3) bond formation at the α-position of trifluoromethylthio (SCF3) group, which is otherwise difficult to perform by other means. Experimental and computational mechanistic studies reveal that leaving groups of allylic electrophiles significantly influence the enantioselectivity as well as chemoselectivity.

Mechanistic Insights into Molecular Copper Hydride Catalysis: the Kinetic Stability of CuH Monomers toward Aggregation is a Critical Parameter for Catalyst Performance

The activity of molecular copper hydride (CuH) complexes toward the selective insertion of unsaturated hydrocarbons under mild conditions has contributed significantly to versatile methodologies for upgrading these feedstocks. However, these catalysts are particularly susceptible to deleterious aggregation, leading to the depletion of active CuH species. Little is known about the mechanisms of CuH aggregation, how it influences overall catalyst performance, and how it can be controlled. We address these challenges with mechanistic studies on a model reaction of unactivated alkene hydroboration catalyzed by (IPr*CPh3)CuH (LCuH). We report a comprehensive mechanistic investigation of this system, identifying an aggregation pathway that continuously depletes catalytically active LCuH to form inactive CuH clusters during turnover. Deactivation of LCuH is controlled primarily by the competition between the kinetics of the initial LCuH dimerization step and that of alkene insertion into LCuH. We therefore propose that a comprehensive understanding of CuH catalyst performance must account for the kinetics of the initial LCuH dimerization step, revising a previously explored thermodynamic understanding of CuH aggregation, where the concentration of active species is controlled by equilibria established between CuH clusters and monomers. With a series of (NHC)CuH congeners (NHC = N-heterocyclic carbene), we demonstrate that ostensibly minor structural modifications to the ligand peripheries can drastically affect the LCuH dimerization kinetics, while maintaining reactivity toward on-cycle alkene insertion. We employed a computational approach based on molecular dynamics simulations to provide an in-depth understanding of how specific structural ligand modifications can substantially increase the kinetic stability of monomeric CuH catalysts. Our combined experimental and computational studies suggest strategies for rational ligand design that can be broadly applied to molecular catalyst systems that are susceptible to deactivation via aggregation pathways.

Asymmetric Construction of a SeCF3-Substituted Stereocenter by CuH-Catalyzed Hydroboration of 1-SeCF3-Alkenes

A copper hydride (CuH)-catalyzed regio- and enantioselective hydroboration of 1-trifluoromethylseleno (SeCF3)-alkenes with H-Bpin has been developed. The regio- and enantioselective hydrocupration of an in situ generated CuH species is followed by a boration reaction to successfully construct a SeCF3- and Bpin-substituted chiral carbon center. The key to success is the appropriate choice of tBu-modified biphosphine ligands, which enables an overwhelming high reaction efficiency.

Harnessing Multi-Center-2-Electron Bonds for Carbene Metal-Hydride Nanocluster Catalysis

N-Heterocyclic carbene (NHC) ligands possess the ability to stabilize metal-based nanomaterials for a broad range of applications. With respect to metal-hydride nanomaterials, however, carbenes are rare, which is surprising if one considers the importance of metal-hydride bonds across the chemical sciences. In this study, we introduce a bottom-up approach that leverages preexisting metal-metal m-center-n-electron (mc-ne) bonds to access a highly stable cyclic(alkyl)amino carbene (CAAC) copper-hydride nanocluster, [(CAAC)6Cu14H12][OTf]2 with superior stability compared to Stryker's reagent, a popular commercial phosphine-based copper hydride catalyst. Density functional theory (DFT) calculations reveal that the enhanced stability stems from hydride-to-ligand backbonding with the π-accepting carbene. This new cluster emerges as an efficient and selective copper-hydride pre-catalyst, thereby providing a bench-stable alternative for catalytic applications.

Mono-β-diketonate Metal Complexes of the First Transition Series

Mono-β-diketonate compounds have been fleetingly observed in base metal catalyzed reactions, which are of current interest as alternatives to precious metal catalyzed reactions. Their isolation has been challenging due to synthetic and structural limitations of acac-type ligands, leading to the development of a related NacNac ligand platform. Herein we report the synthesis of a β-diketone capable of kinetically stabilizing relevant catalytic intermediates. Their efficient synthesis requires isolable acyl triflate and lithium enolate reactants. Further, the syntheses of several transmetalation salts are reported and used in transmetalation reactions with a series of late, first-row transition metal compounds (FeII, CoII, NiII, CuI, CuII) of interest in base metal catalysis. In all, a dozen single-crystal XRD structures are reported, among other methods of characterization (i.e., IR, UV-vis, NMR, HRMS). The majority of the compounds present as mono-β-diketonate small-molecule bridged dimers. They serve as effective precatalysts and are accurately modeled by DFT calculations, validating the use of computational methods for determining structures and mechanisms. Their reactivity with various small molecules and solvents is also described. The utility of bis(2,6-dimesitylbenzoyl)methane (L) as a supporting ancillary ligand and a tool for further rational development of this class of ligands is discussed.

Gold-catalysed amine synthesis by reductive hydroamination of alkynes with nitroarenes

Amines are the most pivotal class of organic motifs in pharmaceutical compounds. Here we provide a blueprint for a general synthesis of amines by catalyst differentiation enabled by triple Au-H/Au+/Au-H relay catalysis. The parent catalyst is differentiated into a set of catalytically active species to enable triple cascade catalysis, where each catalytic species is specifically tuned for one catalytic cycle. This strategy enables the synthesis of biorelevant amine motifs by reductive hydroamination of alkynes with nitroarenes. Using this triple cascade approach, we have achieved exceptional functional group tolerance, enabling the use of bulk chemical feedstocks as coupling partners for the amination of both simple and complex alkynes (>100 examples), including those derived from pharmaceuticals, peptides and natural products (>30 examples). The isolation and full crystallographic characterization of gold hydride and hydride-bridged gold complexes has garnered insights into the catalyst differentiation process of fundamental organometallic gold hydride complexes.

IPr*F - Highly Hindered, Fluorinated N-Heterocyclic Carbenes

The introduction of fluorine atom has attracted considerable interest in molecular design owing to the high electronegativity and the resulting polarization of carbon-fluorine bonds. Simultaneously, sterically-hindered N-heterocyclic carbenes (NHCs) have received major interest due to high stabilization of the reactive metal centers, which has paved the way for the synthesis of stable and reactive organometallic compounds with broad applications in main group chemistry, inorganic synthesis and transition-metal-catalysis. Herein, we report the first class of sterically-hindered, fluorinated N-heterocyclic carbenes. These ligands feature variable fluorine substitution at the N-aromatic wingtip, permitting to rationally vary steric and electronic characteristics of the carbene center imparted by the fluorine atom. An efficient, one-pot synthesis of fluorinated IPr*F ligands is presented, enabling broad access of academic and industrial researchers to the fluorinated ligands. The evaluation of steric, electron-donating and π-accepting properties as well as coordination chemistry to Au(I), Rh(I) and Se is presented. Considering the unique properties of carbon-fluorine bonds, we anticipate that this novel class of fluorinated carbene ligands will find widespread application in stabilizing reactive metal centers.

Lewis Acid-Tethered (cAAC)-Copper Complexes: Reactivity for Hydride Transfer and Catalytic CO2 Hydrogenation

We present a series of borane-tethered cyclic (alkyl)(amino)carbene (cAAC)-copper complexes, including a borane-capped Cu(I) hydride. This hydride is unusually hydridic and reacts rapidly with both CO2 and 2,6-dimethylphenol at room temperature. Its reactivity is distinct from variants without a tethered borane, and the underlying principles governing the enhanced hydricity were evaluated experimentally and theoretically. These stoichiometric results were extended to catalytic CO2 hydrogenation, and the borane-tethered (intramolecular) system exhibits ~3-fold enhancement relative to an intermolecular system.

IPr# Complexes-Highly-Hindered, Sterically-Bulky Cu(I) and Ag(I) N-Heterocyclic Carbenes: Synthesis, Characterization, and Reactivity

Metal-N-heterocyclic carbene (M-NHC) complexes are well-known as an important class of organometallic compounds widely used in transition-metal catalysis. Taking into account that the steric hindrance around the metal center is one of the major effects in M-NHC catalysis, the development of new, sterically hindered M-NHC complexes is an ongoing interest in this field of research. Herein, we report the synthesis and characterization of exceedingly sterically hindered, well-defined, air- and moisture-stable Cu(I) and Ag(I) complexes, [Cu(NHC)Cl] and [Ag(NHC)Cl], in the recently discovered IPr# family of ligands that hinge upon modular peralkylation of anilines. The complexes in both the BIAN and IPr families of ligands are reported. X-ray crystallographic analyses and computational studies were conducted to determine steric effects, Frontier molecular orbitals, and bond orders. The complexes were evaluated in the model hydroboration of the alkynes. We identified [Cu(BIAN-IPr#)Cl] and [Ag(BIAN-IPr#)Cl] as highly reactive catalysts with the reactivity outperforming the classical IPr and IPr*. Considering the attractive features of well-defined Cu(I)-NHC and Ag(I)-NHC complexes, this class of sterically bulky yet wingtip-flexible complexes will be of interest for catalytic processes in various areas of organic synthesis and catalysis.

(CAAC)CuCl: A Competent Precatalyst for Carbonyl and Ester Hydrosilylation

Cu-catalyzed carbonyl hydrosilylation involves a ligated "[(L)CuH]" as the active catalyst, where the ligand L has a crucial role toward the stability, stereoselectivity, and enhancement of the hydridicity. Strongly σ-donating N-heterocyclic carbenes (NHCs), their ring-expanded form, and an abnormal NHC as ligands have yielded robust and efficient Cu catalysts. However, cyclic(alkyl)(amino)carbenes (CAACs), despite being stronger σ-donors than NHCs and already having a salient Cu(I) chemistry, are yet to be reported as a similar ligand platform for this purpose. We establish here the familiar [(Me2CAAC)CuCl] as a powerful precatalyst in this regard. Additionally, it also catalyzes the more challenging ester hydrosilylation, which is a rare feat for a Cu catalyst. Apart from the stronger σ-donating ability, the more steric "openness" of CAACs than bulky NHCs also seems to be advantageous. To corroborate, three new (CAAC)CuCl complexes [(ArCH2,MeCAAC)CuCl] (Ar = Ph, 1-naphthyl, and 1-prenyl) are devised, where the effective steric around the copper is practically unaltered from the case of [(Me2CAAC)CuCl]. All three are equally active in carbonyl and ester hydrosilylation as [(Me2CAAC)CuCl]. Computation suggests the carbonyl insertion into a "(CAAC)Cu-H" as the rate-limiting step. To elucidate the involvement of a "(CAAC)CuH", "(PhCH2,MeCAAC)CuH" is generated in situ and is trapped as its BH3 adduct (PhCH2,MeCAAC)CuBH4.

Phosphine versus Carbene Metal Interactions: Bond Energies

A variety of different ground-state structures of carbene and phosphine groups 1 and 2 cationic, group 11 cationic, and group 10 neutral complexes were studied using density functional theory (DFT) and correlated molecular orbital theory (CCSD(T)) methods. Geometries of complexes with phosphines were studied and compared to available experimental data. Among the three analyzed phosphine ligands, PH3, PMe3, and PPh3, PH3 was found to have noticeably smaller ligand binding energies (LBEs, ΔH298 K). PPh3 has the greatest LBEs with group 2 dications. The difference in LBEs for PMe3 and PPh3 in complexes with group 1 monocations and transition-metal (TM) complexes was significantly less pronounced. The stability and reactivity of phosphine complexes were analyzed and compared with those of previously studied N-heterocyclic carbenes (NHC). PH3 has smaller LBEs compared to NHC carbenes. The lower LBEs correlate with the hardness for M(11)+ complexes and correlate with both the hardness and ionic radii for the M(1)+ and M(2)2+ complexes. The presence of additional PH3 substituents on the metal center makes the LBE smaller compared to their unsubstituted or less substituted analogs. The presence of NH3 in a structure causes a smaller effect on binding, and, except for carbene-PtNH3, an increase in LBE was observed. Composite-correlated molecular orbital theory (G3MP2) was used to predict the LBE of various Lewis acidic ligands with PH3 and NHCs to contrast their binding behavior. Binding either phosphine or carbene to metal diamine complexes caused ligand exchange and transfer of NH3 to an outer coordination sphere.

Asymmetric Synthesis of SCF3-Substituted Alkylboronates by Copper-Catalyzed Hydroboration of 1-Trifluoromethylthioalkenes

A synthetic method for preparation of optically active trifluoromethylthio (SCF3) compounds by a copper-catalyzed regio- and enantioselective hydroboration of 1-trifluoromethylthioalkenes with H-Bpin has been developed. The enantioselective hydrocupration of an in situ generated CuH species and subsequent boration reaction generate a chiral SCF3-containing alkylboronate, of which Bpin moiety can be further transformed to deliver various optically active SCF3 molecules. Computational studies suggest that the SCF3 group successfully controls the regioselectivity in the reaction.

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

A new class of photoswitchable NHC ligands, named azImBA, has been developed by integrating azobenzene into a previously unreported imidazobenzoxazol-1-ylidene framework. These rigid photochromic carbenes enable precise control over confinement around a metal's coordination sphere. As a model system, gold(I) complexes of these NHCs exhibit efficient bidirectional E-Z isomerization under visible light, offering a versatile platform for reversibly photomodulating the reactivity of organogold species. Comprehensive kinetic studies of the protodeauration reaction reveal rate differences of up to 2 orders of magnitude between the E and Z isomers of the NHCs, resulting in a quasi-complete visible-light-gated ON/OFF switchable system. Such a high level of photomodulation efficiency is unprecedented for gold complexes, challenging the current state-of-the-art in photoswitchable organometallics. Thorough investigations into the ligand properties paired with structure-reactivity correlations underscored the unique ligand's steric features as a key factor for reactivity. This effective photocontrol strategy was further validated in gold(I) catalysis, enabling in situ photoswitching of catalytic activity in the intramolecular hydroalkoxylation and -amination of alkynes. Given the significance of these findings and its potential as a widely applicable, easily customizable photoswitchable ancillary ligand platform, azImBA is poised to stimulate the development of adaptive, multifunctional metal complexes.

Remote Steric and Electronic Effects of N-Heterocyclic Carbene Ligands on Alkene Reactivity and Regioselectivity toward Hydrocupration Reactions: The Role of Expanded-Ring N-Heterocyclic Carbenes

The remote groups in N-heterocyclic carbene (NHC) ligands have a significant influence on metal-catalyzed reactions. We examine how remote bulkiness, electronic groups, and expanded-ring NHCs (ER-NHCs) influence alkene reactivity and regioselectivity toward hydrocupration using density functional theory calculations. The impact of remote steric bulkiness on the Cu-H insertion rate is analyzed, revealing a strong correlation between the steric substituent constant and rate ratio, where a bulky group increases the rate due to reduced steric effects in the transition state (TS). The steric properties of the examined catalysts (with a remote group R2 = CPh3, CHPh2, CH2Ph, CH3, and H) and their corresponding TSs are found to be modulated greatly by the remote steric substitution group and the ring size of the NHC ligand. Enhanced bulkiness enhances the nucleophilic Cu-H moiety. The remote electronic groups have a smaller impact on insertion barrier compared to that of steric hindrance. Furthermore, ER-NHC exploration indicates that NHCs with over five-membered rings have a significantly negative influence on the reaction rate. Finally, with a highly bulky group (R2 = CPh3), anti-Markovnikov insertion preference is attributed to high interaction energy and improved steric properties. Overall, our findings here provide valuable insights for the development of a more effective catalyst in metal-catalyzed reactions.

Stereoselective Synthesis of Trisubstituted Alkenes via Copper Hydride-Catalyzed Alkyne Hydroalkylation

Alkenes are ubiquitous in organic chemistry, yet many classes of alkenes remain challenging to access by current synthetic methodology. Herein, we report a copper hydride-catalyzed approach for the synthesis of Z-configured trisubstituted alkenes with high stereo- and regioselectivity via alkyne hydroalkylation. A DTBM-dppf-supported Cu catalyst was found to be optimal, providing a substantial increase in product yield compared to reactions conducted with dppf as the ligand. DFT calculations show that the DTBM substitution leads to the acceleration of alkyne hydrocupration through combined ground and transition state effects related to preventing catalyst dimerization and enhancing catalyst-substrate dispersion interactions, respectively. Alkyne hydroalkylation was successfully demonstrated with methyl and larger alkyl tosylate electrophiles to produce a variety of (hetero)aryl-substituted alkenes in moderate to high yields with complete selectivity for the Z stereochemically configured products. In the formation of the key C-C bond, computational studies revealed a direct SN2 pathway for alkylation of the vinylcopper intermediate with in situ-formed alkyl iodides.

Synthesis, Structural Characterization, and CO2 Reactivity of a Constitutionally Analogous Series of Tricopper Mono-, Di-, and Trihydrides

The formation of hydrides at heterogeneous copper surfaces results in dramatic structural and reactivity changes, yet the morphologies of these materials and their respective roles in catalysis are not well understood. Of particular interest is the reactivity of heterogeneous copper hydrides with carbon dioxide (CO₂), an early mechanistic branching point in the CO₂ reduction reaction. Herein, we report the synthesis, characterization, and reactivity of tricopper compounds supported by a facially biased, chelating tris(carbene) ligand scaffold. This sterically bulky environment affords access to an isolable series of tricopper hydrides: [LCu₃H]²⁺ (4), [LCu₃H₂]⁺ (3), and LCu₃H₃ (6). Single-crystal X-ray diffraction and solution NMR spectroscopy studies reveal both geometric flexibility within the Cu₃ core and fluxionality of hydride ligands across the Cu₃ cluster, providing both atomically precise experimental analogues of static surface species and emulating dynamic ligand behavior proposed for surfaces. Electronic structure calculations serve as a predictor of hydricity, which was likewise benchmarked experimentally via both protonolysis and hydride abstraction reactions. Increased hydride number (and commensurately lower cluster charge) results in more hydridic complexes, with a thermodynamic hydricity range spanning >30 kcal/mol. These thermochemical studies serve as an accurate predictor of CO₂ reactivity. Together, this Cu₃H_x series exhibits the structure/reactivity relationships proposed for catalytically active copper surfaces, validating the application of carefully designed molecular clusters toward elucidating mechanisms in surface science.

CAAC-IPr*: easily accessible, highly sterically-hindered cyclic (alkyl)(amino)carbenes

IPr* (IPr* = 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene) has emerged as a powerful highly hindered and sterically-flexible ligand platform for transition-metal catalysis. CAACs (CAAC = cyclic (al-kyl)(amino)carbenes) have gained major attention as strongly electron-rich carbon analogues of NHCs (NHC = N-heterocyclic carbene) with broad applications in both industry and academia. Herein, we report a merger of CAAC ligands with highly-hindered IPr*. The efficient synthesis, electronic characterization and application in model Cu-catalyzed hydroboration of alkynes is described. The ligands are strongly electron-rich, bulky and flexible around the N-Ar wingtip. The availability of various IPr* and CAAC templates offers a significant potential to expand the existing arsenal of NHC ligands to electron-rich bulky architectures with critical applications in metal stabilization and catalysis.

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

We disclose a 1,4,7-triazacyclononane (TACN) ligand featuring an appended boron Lewis acid. Metalation with Cu(I) affords a series of tetrahedral complexes including a boron-capped cuprous hydride. We demonstrate distinct reactivity modes as a function of chemical oxidation: hydride transfer to CO2 in the copper(I) state and oxidant-induced H2 evolution as well as alkyne reduction.