N-Heterocyclic carbenes (NHCs) have been shown to be useful ligands for the Suzuki–Miyaura cross-coupling at low catalyst loadings. We now report that the commercially available and air-stable [Pd(IPr)(cin)Cl] pre-catalyst permits the formation of various functionalized biaryls from aryl chlorides and boronic acids (37 examples) under very mild conditions using a mixture of ethanol/water as solvent and an inorganic base.

A general methodology for the α-arylation of ketones using a nickel catalyst has been developed. The new well-defined [Ni(IPr*)(cin)Cl] (1 c) pre-catalyst showed great efficiency for this transformation, allowing the coupling of a wide range of ketones, including acetophenone derivatives, with various functionalised aryl chlorides. This cinnamyl-based Ni–N-heterocyclic carbene (NHC) complex has demonstrated a different behaviour to previously reported NHC-Ni catalysts. Preliminary mechanistic studies suggest a Ni⁰/Ni^II catalytic cycle to be at play.

A mechanistic study of the gold-catalysed protodecarboxylation is described. Each reaction step has been investigated experimentally and computationally. More specifically, the activation parameters for the decarboxylation step have been determined through kinetic studies. Further experimental studies on the hydrolysis of the arylgold intermediate have revealed that the protodeauration can become competitive with the decarboxylation process at high conversions. This switch in rate-limiting step has been shown to be pK_a-dependent. These studies have been supported by DFT calculations and permit a better understanding of which prevalent features of the reaction mechanism account for the decarboxylation process.

The facile insertion of CO₂ into iridium(I) hydroxide, alkoxide, and amide bonds was recently reported. In particular, [Ir(cod)(IiPr)(OH)] (IiPr=1,3-bis(isopropyl)imidazol-2-ylidene) reacted with CO₂ in solution and in the solid state in a matter of minutes to give the novel [{Ir(cod)(IiPr)}₂(μ-κ¹O:κ²O,O-CO₃)] complex. In the present study, this reaction is probed using kinetics and theoretical studies, which enabled us to analyse its facile nature and to fully elucidate the reaction mechanism with excellent correlation between the two methods.

We report the synthesis of nine new N-heterocyclic carbene gold bifluoride complexes starting from the corresponding N-heterocyclic carbene gold hydroxides. A new methodology to access N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene gold(I) fluoride starting from N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene gold(I) hydroxide and readily available potassium bifluoride is also reported. These gold bifluorides were shown to be efficient catalysts in the hydrofluorination of symmetrical and unsymmetrical alkynes, thus affording fluorinated stilbene analogues and fluorovinyl thioethers in good to excellent yields with high stereo- and regioselectivity. The method is exploited further to access a fluorinated combretastatin analogue selectively in two steps starting from commercially available reagents.

Two new [Ni(allyl)Cl(NHC)] complexes with the bulky yet flexible N-heterocyclic carbene (NHC) ligands IPr* {N,N′-bis[2,6-bis(diphenylmethyl)-4-methylphenyl]imidazole-2-ylidene} and IPr*^OMe {N,N′-bis[2,6-bis(diphenylmethyl)-4-methoxyphenyl]imidazol-2-ylidene} are reported. These complexes were employed in the amination and sulfination of aryl halide species and were shown to perform well in these reactions, which typically required less than half as much catalyst as previous state-of-the-art nickel complexes.

The decomposition of a series of benzylidene, methylidene, and 3-phenylindenylidene complexes has been probed in alcohol solution in the presence of base. Tricyclohexylphosphane-containing precatalysts are shown to yield [RuCl(H)(H₂)(PCy₃)₂] in isopropyl alcohol solutions, while 3-phenylindenylidene complexes lead to η⁵-(3-phenyl)indenyl products. The potential-energy surfaces for the formation of the latter species have been probed using density functional theory studies.

A synthetic protocol making use of a well-defined cationic ruthenium complex 2 enabling the racemization of enantiomerically pure secondary alcohols in the presence of a weak base (K₂CO₃) is described. The compatibility of 2 with Candida Antarctica lipase B (Novozym 435) allows the development of an efficient dynamic kinetic resolution of sec-alcohols in the absence of an additional strong base. This procedure involves the first example of a dynamic kinetic resolution of alcohols in the presence of a cationic ruthenium catalyst. In addition, we describe the conversion of ketones to the enantioenriched acetates in a one-pot reaction, probing the versatility of complex 2.

A new synthetic protocol that combines the advantages offered by eco-friendly solvent-free reactions and sequential transformations is reported. This strategy offers straightforward access to benzo[c]chromenes and benzo[b]furans from commercially available starting materials. This two-step, one-pot strategy consists of an Au-catalyzed hydrophenoxylation process followed by Pd-catalyzed CH activation or Mizoroki–Heck reactions. The selectivity of the process towards CH activation or Mizoroki–Heck reaction can be easily tuned.

A general catalytic protocol for the α-arylation of aryl ketones has been developed. It involves the use of a preformed, bench-stable Pd–N-heterocyclic carbene pre-catalyst bearing IHept as an ancillary ligand, and allows the coupling of various functionalized coupling partners at very low catalyst loading. Careful choice of the solvent/base system was crucial to obtain optimum catalyst performance. The pre-catalyst was also successfully tested in the synthesis of an industrially relevant compound.

The decomposition of a series of benzylidene, methylidene, and 3-phenylindenylidene complexes has been probed in alcohol solution in the presence of base. Tricyclohexylphosphane-containing precatalysts are shown to yield [RuCl(H)(H₂)(PCy₃)₂] in isopropyl alcohol solutions, while 3-phenylindenylidene complexes lead to η⁵-(3-phenyl)indenyl products. The potential-energy surfaces for the formation of the latter species have been probed using density functional theory studies.

A straightforward and scalable eight-step synthesis of new N-heterocyclic carbenes (NHCs) has been developed from inexpensive and readily available 2-nitro-m-xylene. This process allows for the preparation of a novel class of NHCs coined ITent (“Tent” for “tentacular”) of which the well-known IMes (N,N′-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), IPr (N,N′-bis(2,6-di(2-propyl)phenyl)imidazol-2-ylidene) and IPent (N,N′-bis(2,6-di(3-pentyl)phenyl)imidazol-2-ylidene) NHCs are the simplest and already known congeners. The synthetic route was successfully used for the preparation of three members of the ITent family: IPent (N,N′-bis(2,6-di(3-pentyl)phenyl)imidazol-2-ylidene), IHept (N,N′-bis(2,6-di(4-heptyl)phenyl)imidazol-2-ylidene) and INon (N,N′-bis(2,6-di(5-nonyl)phenyl)imidazol-2-ylidene). The electronic and steric properties of each NHC were studied through the preparation of both nickel and palladium complexes. Finally the effect of these new ITent ligands in Pd-catalyzed Suzuki–Miyaura and Buchwald–Hartwig cross-couplings was investigated.

A family of iridium(I) hydroxides of the form [Ir(cod)(NHC)(OH)] (cod=1,5-cyclooctadiene, NHC=N-heterocyclic carbene) is reported. Single-crystal X-ray analyses and computational methods were used to explore the structural characteristics and steric properties of these new complexes. The model complex [Ir(cod)(IiPr)(OH)] (IiPr=1,3-(diisopropyl)imidazol-2-ylidene) undergoes reaction with a wide variety of substrates including boronic acids and silicon compounds. In addition, OH, NH and CH bond activation was achieved with alcohols, carboxylic acids, amines and various sp-, sp²- and sp³-hybridised carbon centres, giving access to a wide range of new Ir^I complexes. These studies have allowed us to explore the exciting reactivity of this motif, revealing a versatile and useful synthon capable of activating important chemical bonds under mild (typically room temperature) conditions. No additives were required and, in the case of XH bond activation, water was the only waste product, rendering this an atom efficient procedure for bond activation. This system has great potential for the construction of new catalytic cycles for organic synthesis and small-molecule activation.

In gold chemistry, the hydroxide function on the metal center was observed to act as a potent transmetalation group facilitator when reacted with boronic acids leading to the exceedingly rapid formation of gold-aryl bonds.

An efficient and green protocol for the transfer hydrogenation of carbonyl and imine compounds is presented. The transformations are catalysed by the inexpensive and easily synthesised complex [RuCl(PPh₃)(3-phenylindenyl)]. Its catalytic activity was compared to that of the most commonly encountered ruthenium complexes in transfer hydrogenation reactions involving several protypical substrates.

The well-defined [Pd(IPr*)(cinnamyl)Cl] complex is reported as one of the best N-heterocyclic carbene (NHC)-based pre-catalysts for the Buchwald–Hartwig amination reaction. This catalytic system displays high efficiency for the coupling of numerous (hetero)aryl chlorides, with a wide range of amines, at room temperature or at extremely low catalyst loading (as low as 0.025 mol%).

The formation of 4-alkoxy-2(5H)-furanones was achieved via tandem alkoxylation/lactonization of γ-hydroxy-α,β-acetylenic esters catalyzed by 2 mol% of [2,6-bis(diisopropylphenyl)imidazol-2-ylidine]gold bis(trifluoromethanesulfonyl)imidate [Au(IPr)(NTf₂)]. The economic and simple procedure was applied to a series of various secondary propargylic alcohols allowing for yields of desired product of up to 95%. In addition, tertiary propargylic alcohols bearing mostly cyclic substituents were converted into the corresponding spiro derivatives. Both primary and secondary alcohols reacted with propargylic alcohols at moderate temperatures (65–80 °C) in either neat reactions or using 1,2-dichloroethane as a reaction medium allowing for yields of 23–95%. In contrast to [Au(IPr)(NTf₂)], reactions with cationic complexes such as [2,6-bis(diisopropylphenyl)imidazol-2-ylidine](acetonitrile)gold tetrafluoroborate [Au(IPr)(CH₃CN)][BF₄] or (μ-hydroxy)bis{[2,6-bis(diisopropylphenyl)imidazol-2-ylidine]gold} tetrafluoroborate or bis(trifluoromethanesulfonyl)imidate – [{Au(IPr)}₂(μ-OH)][X] (X=BF₄, NTf₂) – mostly stop after the alkoxylation. Analysis of the intermediate proved the exclusive formation of the E-isomer which allows for the subsequent lactonization.

The synthesis of a series of dinuclear gold hydroxide complexes has been achieved. These complexes of type [{Au(IPr)}₂(μ-OH)]X (X=BF₄, NTf₂, OTf, FABA, SbF₆; IPr=2,6-bis(disopropylphenyl)imidazol-2-ylidene; NTf₂=bis(trifluoromethanesulfonyl)imidate; OTf=trifluoromethanesulfonate; FABA=tetrakis(pentafluorophenyl)borate) are easily formed in the presence of water and prove highly efficient in the catalytic hydration of nitriles. Their facile formation in aqueous media suggests they are of relevance in gold-catalyzed reactions involving water. Additionally, a series of [Au(IPr)(NCR)][BF₄] (R=alkyl, aryl) complexes was synthesized as they possibly occur as intermediates in the catalytic reaction mechanism. ¹H and ¹³C NMR data as well as key bond lengths obtained by X-ray diffraction studies are compared and reveal an interesting structure–activity relationship. The collected data indicate a negligible effect of the nature of the nitrile on the reactivity of [Au(L)(NCR)][X] complexes in catalysis.

The synthesis and characterization of two new ruthenium indenylidene complexes [RuCl₂(SIPr)(Py)(Ind)] 6 and [RuCl₂(SIPr)(3-BrPy)(Ind)] 7 featuring the sterically demanding N-heterocyclic carbene 1,3-bis(2,6-di isopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr) are reported. Remarkable activity was observed with these complexes in ring closing, enyne, and cross metathesis of olefins at low catalyst loadings. The performance of SIPr-bearing complexes 6 and 7 as well as [RuCl₂(SIPr)(PCy₃)(Ind)] 5 in ring opening metathesis polymerization is also disclosed. This work highlights the enormous influence of the neutral “spectator” ligands on catalyst activity and stability.

Several ruthenium-indenylidene complexes bearing N-heterocyclic carbenes (NHCs) and phosphanes have been investigated for the ring rearrangement of cyclic compounds by alkene metathesis. These catalysts were found to promote efficiently such domino reactions, especially sterically hindered NHC-containing complexes. Moreover, indenylidene-type catalysts were compared to benzylidene- and Hoveyda-type analogues. The scope of the reaction highlights the parameters governing the RRM.

The synthesis of a series of [(IPr)Pd(R-acac)Cl] precatalysts (acac=acetylacetonato; IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), where the acac ligand on palladium has been systematically modified through terminal substitution, is reported. The following substituted acac ligands are employed in this study: dibenzoylmethanato (dbm), benzoylacetonato (bac), tetramethylheptanedionato (tmhd), and hexafluoroacetylacetonato (hfac). Full spectroscopic characterization of the new complexes is provided along with X-ray studies for three of these. Investigation of their catalytic activity in cross-coupling is also presented through a comparative study in an aryl amination reaction. The catalytic results show a strong correlation between the increased steric bulk of the acac substituents and an increased activation rate of the precatalyst, going from the acac to the tmhd ligand. This observation, along with the inertness of the hfac compound, seems to support our previous proposal for the activation mechanism of these complexes under cross-coupling conditions.

The aim of the present study is to develop readily available and stable pre-catalysts that could be easily prepared on large scale from simple starting materials. Based on the hypothesis that substitution of classical PCy₃ with phosphanes of varying electron-donating properties could be a straightforward manner to improve catalytic activity, a methodical study dealing with the effect of phosphane fine-tuning in ruthenium–indenylidene catalysts was performed. Challenged to establish how the electronic properties of para-substituted phosphane ligands translate into catalyst activity, the versatile behaviour of these new ruthenium–indenylidene complexes was investigated for a number of metathesis reactions.

The use of a versatile N-heterocyclic carbene (NHC) gold(I) hydroxide precatalyst, [Au(OH)(IPr)], (IPr=N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) permits the in situ generation of the [Au(IPr)]⁺ ion by simple addition of a Brønsted acid. This cationic entity is believed to be the active species in numerous catalytic reactions. ¹H NMR studies in several solvent media of the in situ generation of this [Au(IPr)]⁺ ion also reveal the formation of a dinuclear gold hydroxide intermediate [{Au(IPr)}₂(μ-OH)], which is fully characterized and was tested in gold(I) catalysis.

A study on the enyne metathesis reaction leading to the formation cyclic compounds using ruthenium–indenylidene complexes is presented. Several 1,11-dien-6-ynes have been subjected to ruthenium metathesis cyclization by using ruthenium–indenylidene complexes bearing various phosphine and N-heterocyclic carbene (NHC) ligands. Interestingly, for some substrates chemodivergent metathesis occurs and is a function of the catalyst employed. This led us to investigate the competing “ene-then-yne” or “yne-then-ene” reaction pathways apparently at play in these systems using both experimental observations and DFT calculations. Experimental and computational studies were found in good agreement and permit to conclude that for phosphine-containing catalysts, the “ene-then-yne” pathway is exclusively adopted. On the other hand, for catalysts bearing NHC ligands, both pathways are possible.