Modular Synthesis of Azines Bearing a Guanidine Core from N-Heterocyclic Carbene (NHC)-Derived Selenoureas and Diazo Reagents.

N-Heterocyclic carbene (NHC)-derived selenoureas comprise a fundamentally important class of NHC derivatives, with key applications in coordination chemistry and the determination of NHC electronic properties. Considering the broad reactivity of chalcogen-containing compounds, it is surprising to note that the use of NHC-derived selenoureas as organic synthons remains essentially unexplored. The present contribution introduces a novel, straightforward transformation leading to azines bearing a guanidine moiety, through the reaction of a wide range of NHC-derived selenoureas with commercially available diazo compounds, in the presence of triphenylphosphine. This transformation offers a new approach to such products, having biological, materials chemistry, and organic synthesis applications. The guanidine-bearing azines are obtained in excellent yields, with all manipulations taking place in air. A reaction mechanism is proposed, based on both experimental mechanistic findings and density functional theory (DFT) calculations. A one-pot, multicomponent transesterification reaction between selenoureas, α-diazoesters, alcohols, and triphenylphosphine was also developed, providing highly functionalized azines.

Synthesis and reactivity of N-heterocyclic carbene (NHC) gold-fluoroalkoxide complexes.

We disclose a novel series of N-heterocyclic carbene (NHC) gold complexes with varied steric and electronic properties, bearing fluorinated alkoxide anions. Early reactivity studies involving these synthons, lead to the synthesis of various complexes of relevance to gold chemistry and catalysis.

Synthesis of N-heterocyclic carbene (NHC)-Au/Ag/Cu benzotriazolyl complexes and their catalytic activity in propargylamide cycloisomerization and carbonyl hydrosilylation reactions.

Carbene-metal-amide (CMA) complexes of gold, silver, and copper have been studied extensively for their photochemical/photocatalytic properties and as potential (pre-)catalysts in organic synthesis. Herein, the design, synthesis, and characterization of five bench-stable Au-, Ag-, and Cu-NHC complexes bearing the benzotriazolyl anion as an amide donor, are reported. All complexes are synthesized in a facile and straightforward manner, using mild conditions. The catalytic activity of the Ag and Cu complexes was studied in propargylamide cycloisomerization and carbonyl hydrosilylation reactions. Both CMA-catalyzed transformations proceed under mild conditions and are highly efficient for a range of propargylamides and carbonyl compounds, respectively, affording the desired corresponding products in good to excellent yields.

Dinuclear NHC-gold(I)-thiolato and -alkynyl complexes: synthesis, anticancer activity, and catalytic activity in lactonization reactions.

A series of novel dinuclear NHC-gold-thiolato and -alkynyl complexes bearing aromatic linkers were successfully synthesized by an efficient and simple synthetic route. The catalytic activity of these complexes was tested in a lactonization reaction. The reaction proceeds in high efficiency, in short reaction time and under mild conditions, and is complementary to existing methods. Furthermore, the digold(I)-thiolato derivatives exhibit remarkable cytotoxicity towards several cancer cell lines.

Correction: Simple synthetic access to [Au(IBiox)Cl] complexes.

Correction for 'Simple synthetic access to [Au(IBiox)Cl] complexes' by Ekaterina A. Martynova et al., Dalton Trans., 2023, 52, 7558-7563, https://doi.org/10.1039/D3DT01357J.

Synthesis of cyclobutane-fused chromanones via gold-mediated photocatalysis.

Energy transfer (EnT) photocatalysis has emerged as a valuable tool for constructing complex organic scaffolds via [2 + 2]-cycloaddition reactions. Herein, we present the use of [Au(SIPr)(Cbz)] as a sensitizer for the [2 + 2]-cycloaddition of coumarins and unactivated alkenes. Widely used in EnT catalysis, iridium and organic sensitizers proved less efficient under the examined catalytic conditions. The developed protocol permits the synthesis of cyclobutane-fused chromanones from readily available starting materials. A wide range of alkenes and substituted coumarins, including naturally occurring compounds, were reacted under mild conditions leading to structurally complex products with good functional group tolerance. Mechanistic studies reveal a previously overlooked reaction pathway for energy transfer catalysis involving coumarins.

Synthesis of cyclohepta[b]indoles via gold mediated energy transfer photocatalysis.

Photocatalysis involving energy transfer (EnT) has become a valuable technique for building intricate organic frameworks mostly through [2+2]-cycloaddition reactions. Herein, we report a synthetic method leading to functionalized cyclohepta[b]indoles, an important structural motif in natural products and pharmaceuticals, using gold-mediated energy transfer photocatalysis. The scope of this operationally simple and atom-economical strategy is presented. Density functional theory studies were employed in order to gain insights into the mechanism of formation of the cyclohepta[b]indole core.

Simply accessible platinum(II) complexes enabling alkene hydrosilylation at ppm catalyst loadings.

An efficient olefin hydrosilylation protocol utilising Pt(II)-thioether-based pre-catalysts is reported. These simple and readily available complexes exhibit excellent catalytic performance and offer significant advantages over existing alternatives, enabling rapid and high conversions at ppm-level catalyst loadings.

Site selective gold(i)-catalysed benzylic C-H amination via an intermolecular hydride transfer to triazolinediones.

Triazolinediones are known as highly reactive dienophiles that can also act as electrophilic amination reagents towards enolisable C-H bonds (ionic pathway) or weak C-H bonds (free radical pathway). Here, we report that this C-H amination reactivity can be significantly extended and enhanced via gold(i)-catalysis. Under mild conditions, several alkyl-substituted aryls successfully undergo benzylic C-H aminations at room temperature. The remarkable site selectivity that is observed points towards strong electronic activation and deactivation effects, that go beyond a simple weakening of the C-H bond. The observed catalytic C-H aminations do not follow the expected trends for a free radical-type C-H amination and show complementarity to existing methods. Density functional theory (DFT) calculations and distinct experimental trends provide a clear mechanistic rationale for observed selectivity patterns, postulating a novel pathway for triazolinedione-induced aminations via a carbon-to-nitrogen hydride transfer.

Suzuki-Miyaura Cross-Coupling of Amides by N-C Cleavage Mediated by Air-Stable, Well-Defined [Pd(NHC)(sulfide)Cl2] Catalysts: Reaction Development, Scope, and Mechanism.

The Suzuki-Miyaura cross-coupling of amides by selective N-C acyl bond cleavage represents a powerful tool for constructing biaryl ketones from historically inert amide bonds. These amide bond activation reactions hinge upon efficient oxidative addition of the N-C acyl bond to Pd(0). However, in contrast to the well-researched activation of aryl halides by C(sp2)-X oxidative addition, very few studies on the mechanism of C(acyl)-N bond oxidative addition and catalyst effect have been reported. Herein, we report a study on [Pd(NHC)(sulfide)Cl2] catalysts in amide N-C bond activation. These readily prepared, well-defined, air- and moisture-stable Pd(II)-NHC catalysts feature SMe2 (DMS = dimethylsulfide) or S(CH2CH2)2 (THT = tetrahydrothiophene) as ancillary ligands. The reaction development, kinetic studies, and reaction scope are presented. Extensive DFT studies were conducted to gain insight into the mechanism of C(acyl)-N bond oxidative addition and catalyst activation. We expect that [Pd(NHC)(sulfide)Cl2] precatalysts featuring sulfides as well-defined, readily accessible ancillary ligands will find application in C(acyl)-X bond activation in organic synthesis and catalysis.

Gold complexes with remote-substituted amino N-heterocyclic carbenes.

The 4-RN-1,3-Ar2-imidazolium salt, R = iPr, tBu, Ar = Mes, Dipp, Mes = mesityl, Dipp = 2,6-bis-diisopropyl-phenyl was metalated by AuI at the C2-, C5- and 4-RN positions depending on the reactants and conditions employed; a rare direct rearrangement of a AuI aminide to an abnormal imidazol-5-ylidene AuI complex was also observed and based on a DFT study it may involve TfO- facilitated H+ transfer.

Exploiting the inductive effect of the trifluoromethyl group: regioselective gold-catalyzed hydration of 2,2,2-trifluoroethyl-substituted alkynes.

The gold-catalyzed hydration of 2,2,2-trifluoroethyl-substituted alkynes is highly regioselective, producing ß-trifluoromethylketones as major products. This transformation illustrates the strong directing effect of the trifluoromethyl group, through its inductive effect, in gold-catalyzed addition to alkynes.

An air-stable [Cu(NHC)(OR)] (R = C(H)(CF3)2) complex for C-H, N-H and S-H bond activation.

The synthesis, isolation and full characterisation of a [Cu(IPr)(OC(H)(CF3)2)] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) complex are reported. This new Cu(I) complex is a versatile synthon and can activate numerous X-H bonds including C-H, N-H and S-H bonds. [Cu(IPr)(OC(H)(CF3)2)] was investigated as a pre-catalyst in several catalytic reactions.

Gold(I)-N-Heterocyclic Carbene Synthons in Organometallic Synthesis.

The prominent role of gold-N-heterocyclic carbene (NHC) complexes in numerous research areas such as homogeneous (photo)catalysis, medicinal chemistry and materials science has prompted organometallic chemists to design gold-based synthons that permit access to target complexes through simple synthetic steps under mild conditions. In this review, the main gold-NHC synthons employed in organometallic synthesis are discussed. Mechanistic aspects involved in their synthesis and reactivity as well as applications of gold-NHC synthons as efficient pre-catalysts, antitumor agents and/or photo-emissive materials are presented.

PtII -N-Heterocyclic Carbene Complexes in Solvent-Free Alkene Hydrosilylation.

Herein, we report the catalytic activity of a series of platinum(II) pre-catalysts, bearing N-heterocyclic carbene (NHC) ligands, in the alkene hydrosilylation reaction. Their structural and electronic properties are fully investigated using X-ray diffraction analysis and nuclear magnetic resonance spectroscopy (NMR). Next, our study presents a structure-activity relationship within this group of pre-catalysts and gives mechanistic insights into the catalyst activation step. An exceptional catalytic performance of one of the complexes is observed, reaching a turnover number (TON) of 970 000 and a turnover frequency (TOF) of 40 417 h-1 at 1 ppm catalyst loading. Finally, an attractive solvent-free and open-to-air alkene hydrosilylation protocol, featuring efficient platinum removal (reduction of residual Pt from 582 ppm to 5.8 ppm), is disclosed.

Simple synthetic access to [Au(IBiox)Cl] complexes.

Green and sustainable access to chiral and achiral gold-IBiox complexes is reported. The gold complexes were synthesized using a simple, air-tolerant, weak base protocol carried out in a green solvent. Their catalytic activity was examined in the hydroamination of alkynes. The steric protection afforded the gold center by these ligands was quantified using the %Vbur model and compared with the most commonly encountered NHCs.

Correlation of Experimental and Calculated Reaction Enthalpies with Ligand Donor Strengths.

Ylide-functionalized phosphines (YPhos) have recently proven to be strongly donating ligands that enable high catalyst activities in gold(I)-mediated transformations. We now report on a calorimetric study dealing with the [Au(YPhos)Cl] system and assess YPhos-Au bond dissociation enthalpies (BDE). Comparison with other commonly used phosphines confirmed the high binding strengths of the YPhos ligands. Furthermore, the values of the reaction enthalpies were shown to correlate with the electronic properties of the ligands measured via the Tolman electronic parameter or the calculated molecular electrostatic potential at phosphorus. Notably, the reaction enthalpies can conveniently be derived by computational methods, thus making these easy-to-obtain descriptors for ligand donor property quantification.

Liquid Dynamics Determine Transition Metal-N-Heterocyclic Carbene Complex Formation.

Invited for the cover of this issue are Oldamur Hollóczki and co-workers at the Universities of Bonn, Ghent and Debrecen. The image depicts the search of an ionic base for the acidic proton of an imidazolium cation in order to form a carbene complex. Read the full text of the article at 10.1002/chem.202203636.

Synthesis, characterization, and reactivity of [Pd(phosphine)(py)Cl2] (PEPPSI) and [Pd(phosphine)Cl2]2 complexes.

The synthesis of novel phosphine palladium PEPPSI and dimer complexes bearing RuPhos, SPhos and XPhos phosphines is reported. The crystal structures of XPhos Pd PEPPSI with pyridine, SPhos Pd PEPPSI with 3-chloropyridine as throw-away ligands and the RuPhos palladium dimer were obtained and compared with previously reported congeners. The catalytic activity of these novel complexes was examined via a C-N coupling reaction involving 4-chloroanisole and morpholine. RuPhos complex 2b proved most active, leading to 97% yield with a low (0.2 mol%) catalyst loading, while phosphine palladium dimers showed significantly lower catalytic activity. However, the addition of 3-pentanone as an activator/stabilizer significantly improved the yields of phosphine dimers and PEPPSI complexes when the reactions were performed in THF.

Access to Azetidines via Gold Mediated Energy Transfer Photocatalysis.

The area of energy transfer photocatalysis to generate four-membered rings is experiencing an unprecedented level of activity. Here, we report an operationally simple method toward azetidines from 2-isoxasoline-3-carboxylates and alkenes, using [Au(cbz)(NHC)] complexes as photocatalysts. The procedure enables the reaction for a wide range of substrates. Mechanistic studies confirm the energy transfer pathway. This contribution adds to the earlier reported use of these gold catalysts as a potentially versatile tool in energy transfer chemistry and catalysis.