Sylvain Gaillard

A N-heterocyclic carbene gold hydroxide complex: a golden synthon

The synthesis and complete characterisation of a well-defined N-heterocyclic carbene bearing gold hydroxide complex, [Au(OH)(IPr)], is reported. Its reaction chemistry appears rich and diverse.

Knölker’s Iron Complex: An Efficient In Situ Generated Catalyst for Reductive Amination of Alkyl Aldehydes and Amines

An aminated series: a well-defined iron-catalyzed reductive amination reaction of aldehydes and ketones with aliphatic amines using molecular hydrogen is presented. Under mild conditions, good yields for a broad range of alkyl ketones as well as aldehydes were achieved.

[{Au(IPr)}2(μ‐OH)]X Complexes: Synthetic, Structural and Catalytic Studies

The synthesis of a series of dinuclear gold hydroxide complexes has been achieved. These complexes of type [{Au(IPr)}(2)(μ-OH)]X (X=BF(4), NTf(2), OTf, FABA, SbF(6); IPr=2,6-bis(disopropylphenyl)imidazol-2-ylidene; NTf(2)=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)][BF(4)] (R=alkyl, aryl) complexes was synthesized as they possibly occur as intermediates in the catalytic reaction mechanism. (1)H and (13)C 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.

Bifunctional (cyclopentadienone)iron-tricarbonyl complexes: Synthesis, computational studies and application in reductive amination

Reductive amination under hydrogen pressure is a valuable process in organic chemistry to access amine derivatives from aldehydes or ketones. Knölkers complex has been shown to be an efficient iron catalyst in this reaction. To determine the influence of the substituents on the cyclopentadienone ancillary ligand, a series of modified Knölkers complexes was synthesised and fully characterised. These complexes were also transformed into their analogous acetonitrile iron-dicarbonyl complexes. Catalytic activities of these complexes were evaluated and compared in a model reaction. The scope of this reaction is also reported. For mechanistic insights, deuterium-labelling experiments and DFT calculations were undertaken and are also presented.

Gold(I)-catalyzed synthesis of furans and pyrroles via alkyne hydration

Furans and pyrroles were prepared via the gold(I)-catalyzed alkyne hydration of diynes. The use of [Au(IPr)OH] as precatalyst in a silver-free protocol permits low catalyst loadings and in situ generation of the active cationic gold species. A detailed computational study confirmed the experimental results and supports the proposed mechanism.

Ligand influence in the selective gold-mediated synthesis of allenes

The use of [(IPr)AuOH] permits the generation of gold(i)-amine complexes by a silver-free protocol. These in situ or well-defined complexes are used in a straightforward synthetic route to substituted allenes from propargylic acetates. The catalytic activity can be modulated as a function of the nature of the ligand bound to the gold-NHC moiety.

NHC Copper(I) Complexes Bearing Dipyridylamine Ligands: Synthesis, Structural, and Photoluminescent Studies

We describe the synthesis of new cationic tricoordinated copper complexes bearing bidentate pyridine-type ligands and N-heterocyclic carbene as ancillary ligands. These cationic copper complexes were fully characterized by NMR, electrochemistry, X-ray analysis, and photophysical studies in different environments. Density functional theory calculations were also undertaken to rationalize the assignment of the electronic structure and the photophysical properties. These tricoordinated cationic copper complexes possess a stabilizing CH-π interaction leading to high stability in both solid and liquid states. In addition, these copper complexes, bearing dipyridylamine ligands having a central nitrogen atom as potential anchoring point, exhibit very interesting luminescent properties that render them potential candidates for organic light-emitting diode applications.

Bifunctional Iron Complexes: Efficient Catalysts for CO and CN Reduction in Water

The application of bifunctional iron complexes for the hydrogenation of polarized CX bonds is reported. Two modified Knölker complexes bearing ionic fragments were synthesized and fully characterized. With these well‐defined complexes, the reaction proceeded under mild conditions in pure water, and various alcohols and amines were isolated in good yields. Compared with hydrogenation in organic solvents, better reactivities were observed.

The role of silver additives in gold-mediated C-H functionalisation.

Summary The role of silver additives is examined in the context of gold-mediated functionalisation of aromatic C–H bonds. Doubt is cast on the commonly cited route of halide abstraction from gold and evidence of substrate activation is given.

Iron‐Catalyzed Hydrogenation of Esters to Alcohols

The reduction of esters to alcohols is an important chemical transformation frequently used in industry and in academic research laboratories as well. The two classical protocols for reduction are based on the use of a stoichiometric amount of a hydride reagent and, alternatively, catalytic heterogeneous hydrogenation. The former method suffers from severe drawbacks such as the toxicity of the reagents and the generated waste. From an ecological point of view, the latter method is more promising as the reducing agent is molecular hydrogen. One excellent example is the large-scale reduction of fatty esters to give the corresponding alcohols, precursors for plasticizers and surfactants, in the presence of a copper chromite catalyst. However, heterogeneous hydrogenation using metal oxide based catalysts is poorly chemoselective; high pressures and temperatures are often required. These harsh conditions are not compatible with other unsaturated functional groups (alkenes, alkynes, nitro functions). Homogeneous catalysis is an alternative as the reduction can be performed under milder conditions and the functional group tolerance is higher. Since the pioneering work by Grey and Pez, tremendous improvements, concerning reaction conditions, selectivities, and catalytic efficiencies, have been described for the homogeneous hydrogenation of esters. Most of the catalytic systems reported up to now are based on ruthenium. However, owing to environmental concerns, these noble metals should be replaced by Earth-abundant ones, such as iron. In this context, the groups lead by Milstein, Guan and Fairweather, and Beller reported in 2014 the first examples of homogenous iron-based complexes as catalysts for the hydrogenation of esters under relatively mild conditions. The three groups reported the use of structurally related iron complexes with [Fe(H)2(CO)] or [Fe(H)(HBH3)(CO)] fragments coordinated to a phosphorous/nitrogen-based tridentate pincer-type ligand (Figure 1), and it appeared that the nature of the PNP ligand has an important influence on the substrate scope. Milstein and co-workers reported the hydrogenation of highly activated trifluoroacetate derivatives using PNP–iron complex 1 (PNP = 2,6-(di-tert-butylphosphinomethyl)pyridine) in the presence of a strong base (KH or potassium or sodium alkoxide). Reactions were performed under moderate hydrogen pressure (usually 5 to 25 bars) at mild temperatures (40 8C) in the presence of 1 mol% of 1 and 5 mol% NaOMe in 1,4-dioxane. In related studies with ruthenium complexes a cooperative effect between the PNP ligand and the metal was evoked. Again, ligand involvement was also proposed to activate molecular hydrogen. The proposed ligand dearomatization/aromatization process intrinsically requires only one equivalent of base relative to metal, but in this reduction a higher amount of base (up to 10 mol%) increased the catalytic activity and was thought to facilitate both the formation of the dearomatized intermediate and the removal of the acetal intermediate. Complex 1 displays some nice selectivity towards fluorinated aryl groups and ethers but also more interestingly towards nonpolarized and poorly substituted C=C bonds introduced on the ester moiety. Interestingly, the authors reported that further heating led to unexpected lower conversions, probably due to catalyst deactivation/degradation. Moreover, complex 1 appeared to be quite sensitive to steric hindrance of the ester moiety. Conversions dramatically decreased with an increase of the steric demand on the ester (isopropyl or methylcyclohexyl derivatives). Nevertheless, the interest in such a system is understandably quite limited due to the lack of conversion of slightly less activated difluoroacetate and less electron-demanding esters, such as acetate and benzoate derivatives. Independently, Guan and Fairweather, and Beller reported the use of an iron pincer complex bearing a bis(diisopropylphosphinoethyl)amine) ligand, which was previously described by Gusev for the homogeneous osmium-catalyzed hydrogenation of esters. With this ligand, Guan and Fairweather reported that the corresponding [(PNP)Fe(H)(Br)(CO)] precursor was slightly active in the presence of KOtBu. But more importantly, both research groups noticed that the Figure 1. Iron complexes for the hydrogenation of esters.