Computational Insight into a Gold(I) N-Heterocyclic Carbene Mediated Alkyne Hydroamination Reaction

Hydroamination of alkynes catalyzed by NHC-Gold(I) complexes: the non-monotonic effect of substituted arylamines on the catalyst activity

Imines are valuable key compounds for synthesizing several nitrogen-containing molecules used in biological and industrial fields. They have been obtained, as highly regioselective Markovnikov products, by reacting several alkynes with arylamines in the presence of three new N-Heterocyclic carbene gold(I) complexes (3b, 4b, and 6b) together with the known 1-2b and 7b gold complexes as well as silver complexes 1-2a. Gold(I) complexes were investigated by means of NMR, mass spectroscopy, elemental analysis, and X-ray crystallographic studies. Accurate screening of co-catalysts and solvents led to identifying the best reaction conditions and the most active catalyst (2b) in the model hydroamination of phenylacetylene with aniline. Complex 2b was then tested in the hydroamination of alkynes with a wide variety of arylamines yielding a lower percentage of product when arylamines with both electron-withdrawing and electron-donating substituents were involved. Computational studies on the rate-determining step of hydroamination were conducted to shed light on the significantly different yields observed when reacting arylamines with different substituents.

Accessing Bioactive Hydrazones by the Hydrohydrazination of Terminal Alkynes Catalyzed by Gold(I) Acyclic Aminooxy Carbene Complexes and Their Gold(I) Arylthiolato and Gold(III) Tribromo Derivatives: A Combined Experimental and Computational Study

Hydrohydrazination of terminal alkynes with hydrazides yielding hydrazones 5-14 were successfully catalyzed by a series of gold(I) acyclic aminooxy carbene complexes of the type [{(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)}methylidene]AuCl, where R² = H, R¹ = Me (1b); R² = H, R¹ = Cy (2b); R² = t-Bu, R¹ = Me (3b); R² = t-Bu, R¹ = Cy (4b). The mass spectrometric evidence corroborated the existence of the catalytically active solvent-coordinated [(AAOC)Au(CH₃CN)]SbF₆ (1-4)A species and the acetylene-bound [(AAOC)Au(HC≡CPhMe)]SbF₆ (3B) species of the proposed catalysis cycle. The hydrohydrazination reaction was successfully employed in synthesizing several bioactive hydrazone compounds (15-18) with anticonvulsant properties using a representative precatalyst (2b). The DFT studies favored the 4-ethynyltoluene (HC≡CPhMe) coordination pathway over the p-toluenesulfonyl hydrazide (NH₂NHSO₂C₆H₄CH₃) coordination pathway, and that proceeded by a crucial intermolecular hydrazide-assisted proton transfer step. The gold(I) complexes (1-4)b were synthesized from the {[(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)]CH}⁺OTf^- (1-4)a by treatment with (Me₂S)AuCl in the presence of NaH as a base. The reactivity studies of (1-4)b yielded the gold(III) [{(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)}methylidene]AuBr₃ (1-4)c complexes upon reaction with molecular bromine and the gold(I) perfluorophenylthiolato derivatives, [{(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)}methylidene]AuSC₆F₅ (1-4)d, upon treatment with C₆F₅SH.

N-heterocyclic Carbene Gold Complexes Active in Hydroamination and Hydration of Alkynes

Until the year 2000, gold compounds were considered catalytically inert. Subsequently, it was found that they are able to promote the nucleophilic attack on unsaturated substrates by forming an Au–π-system. The main limitation in the use of these catalytic systems is the ease with which they decompose, which is avoided by stabilization with an ancillary ligand. N-heterocyclic carbenes (NHCs), having interesting s-donor capacities, are able to stabilize the gold complexes (Au (I/III) NHC), favoring the exploration of their catalytic activity. This review reports the state of the art (years 2007–2022) in the nucleophilic addition of amines (hydroamination) and water (hydration) to the terminal and internal alkynes catalyzed by N-heterocyclic carbene gold (I/III) complexes. These reactions are particularly interesting both because they are environmentally sustainable and because they lead to the production of important intermediates in the chemical and pharmaceutical industry. In fact, they have an atom economy of 100%, and lead to the formation of imines and enamines, as well as the formation of ketones and enols, all important scaffolds in the synthesis of bioactive molecules, drugs, heterocycles, polymers, and bulk and fine chemicals.

Gold-Catalyzed Hydroamination of Propargylic Alcohols: Controlling Divergent Catalytic Reaction Pathways To Access 1,3-Amino Alcohols, 3-Hydroxyketones, or 3-Aminoketones

A versatile approach to the valorization of propargylic alcohols is reported, enabling controlled access to three different products from the same starting materials. First, a general method for the hydroamination of propargylic alcohols with anilines is described using gold catalysis to give 3-hydroxyimines with complete regioselectivity. These 3-hydroxyimines can be reduced to give 1,3-amino alcohols with high syn selectivity. Alternatively, by using a catalytic quantity of aniline, 3-hydroxyketones can be obtained in high yield directly from propargylic alcohols. Further manipulation of the reaction conditions enables the selective formation of 3-aminoketones via a rearrangement/hydroamination pathway. The utility of the new chemistry was exemplified by the one-pot synthesis of a selection of N-arylpyrrolidines and N-arylpiperidines. A mechanism for the hydroamination has been proposed on the basis of experimental studies and density functional theory calculations.

Cyanosilylation of Aromatic Aldehydes by Cationic Ruthenium(II) Complexes of Benzimidazole-Derived O-Functionalized N-Heterocyclic Carbenes at Ambient Temperature under Solvent-Free Conditions

A series of ruthenium complexes, namely, [{1-(N-R₁-2-acetamido)-3-(R₂)-benzimidazol-2-ylidine}Ru(p-cymene)Cl]Cl, where {R₁ = 2,6-(i-Pr)₂C₆H₃, R₂ = i-Pr (1c); R₁ = 2,6-(i-Pr)₂C₆H₃, R₂ = Et (2c); R₁ = 2,4,6-(CH₃)₃C₆H₂, R₂ = Et (3c)}, of benzimidazole-derived N/O-functionalized N-heterocyclic carbene ligands successfully carried out the cyanosilylation reaction of aromatic aldehydes and heteroaryl aldehydes with trimethylsilyl cyanide, providing good to excellent yields (ca. 60-95%) at room temperature under solvent-free condition. The ruthenium (1-3)c complexes were synthesized from the silver (1-3)b analogues in ca. 67-80% yields. The silver (1-3)b complexes exhibited an argentophilic d ¹⁰···d ¹⁰ interaction in its dinuclear macrometallacyclic motif, as observed by a short Ag···Ag contact of 3.1894(3) Å in single-crystal X-ray diffraction studies for a representative silver complex 2b and also in photoluminescence studies that showed characteristic emission band(s) at ca. 534-536 nm in the CHCl₃ solution and at ca. 482-487 and 530-533 nm in the solid state.

Computational Insight Into the Hydroamination of an Activated Olefin, As Catalyzed by a 1,2,4-Triazole-Derived Nickel(II) N-Heterocyclic Carbene Complex

A density functional theory (DFT) investigation performed at the B3LYP/TZVP//B3LYP/6-31G(d)-LANL2DZ level of theory on the hydroamination of dimethylamine (Me₂NH) on an activated olefin (namely, acrylonitrile (CH₂═CHCN)), as catalyzed by a 1,2,4-triazol based nickel(II) N-heterocyclic carbene complex (namely, [1,4-dimethyl-1,2,4-triazole-5-ylidene]₂ nickel dichloride) revealed that the olefin coordination pathway is favorable over the amine coordination pathway, although the initial olefin coordination step is higher in energy than the initial amine coordination step. Significantly enough, the reaction involved a crucial 1,3-proton transfer step between the resonance intermediates, i.e., the C-bound [(NHC)₂Ni(CH(CN)CH₂NHMe₂)]⁺ (D) species or N-bound [(NHC)₂Ni(NCCHCH₂NHMe₂)]⁺ (E) species and the intermediate [(NHC)₂Ni(NCCH₂CH₂NMe₂)]⁺ (F), depicting the cleavage of a N-H bond and the formation of a C-H bond facilitated by a water-assisted/amine-assisted proton shuttle. Overall, among the various pathways explored, the lowest energy pathway involved alkene coordination, followed by an amine-assisted 1,3-proton transfer step.

A D3h-symmetry hexaazatriphenylene-tris-N-heterocyclic carbene ligand and its coordination to iridium and gold: preliminary catalytic studies

A planar star-shaped tris-NHC ligand with a hexaazatriphenylene core was coordinated to gold and iridium. The tris–Au(i) complex shows enhanced catalytic activities due to the electron-deficient character of the HAT core.

Theoretical investigation on the structures and bonding properties of Pd(II), Pt(II) and Ni(II) complexes with tridentate CNC-pincer N-heterocyclic carbene ligands

The density functional theory (DFT) has been applied for the analysis of the bond between group 10 metals and N-heterocyclic carbene (NHC) in complexes (MCl(L-X): M [Formula: see text] Pd(II), Pt(II), and Ni(II), L-X[Formula: see text][2-(3-methylimidazolin-4,5-bisX-2-yliden-1-yl)-4-phenyl] amido, X [Formula: see text]H, Cl and CN). Full geometry optimizations have been performed for all the ligands (L-X[Formula: see text] anions), MCl[Formula: see text] cations, and the complexes. In the ligands, the energy levels of the carbon [Formula: see text] lone-pair orbitals suggest the trend L-H[Formula: see text] L-Cl[Formula: see text] L-CN[Formula: see text] for the donor strength. The role of the M–NHC interaction in complexes was investigated by natural bond orbital (NBO) analysis. The results show that the NHC–M bond consists of the components originating from the L[Formula: see text]M donation and the M[Formula: see text]Carbene C back-donation and the metal[Formula: see text]the ring of NHC back-donation. The transition-metal strongly affects the donation and back-donation. The interaction between the metal and the NHC ligand can be influenced by the central metal and the substituent on the ring of NHC.

An ab Initio Benchmark and DFT Validation Study on Gold(I)-Catalyzed Hydroamination of Alkynes

High level ab initio calculations have been carried out on an archetypal gold(I)-catalyzed reaction: hydroamination of ethyne. We studied up to 12 structures of possible gold(I)-coordinated species modeling different intermediates potentially present in a catalytic cycle for the addition of a protic nucleophile to an alkyne. The benchmark is used to evaluate the performances of some popular density functionals for describing geometries and relative energies of stationary points along the reaction profile. Most functionals (including hybrid or meta-hybrid) give accurate structures but large nonsystematic errors (4-12 kcal/mol) along the reaction energy profile. The double hybrid functional B2PLYP outperforms all considered functionals and compares very nicely with our reference ab initio benchmark energies. Moreover, we present an assessment of the accuracy of commonly used approaches to include relativistic effects, such as relativistic effective potentials and a scalar ZORA Hamiltonian, by a comparison with the results obtained using a relativistic all-electron four-component Dirac-Kohn-Sham method. The contribution of nonscalar relativistic effects in gold(I)-catalyzed reactions, as we investigated here, is expected to be on the order of 1 kcal/mol.