Nickel-Catalyzed Photoredox Allenylation of Alkyl Halides.

We report a dual Ni/photoredox-catalyzed cross-coupling method for propargyl carbonates and nonactivated alkyl bromides, facilitating the synthesis of a variety of substituted allenes under mild and practical conditions. Mechanistically, the reaction integrates Ni-catalyzed activation of the propargyl electrophile via S' oxidative addition at Ni(I) with silyl radical-induced activation of the alkyl halide through halogen-atom transfer. This methodology provides a gentle approach for introducing allenyl groups into complex halogenated aliphatic molecules, offering further opportunities for derivatization.

Photochemically Driven Palladium- Phosphinoacridine Catalysis: A Carboxylative Approach for Arylcarbamate Synthesis.

In this study, we demonstrate that phosphinoacridines are efficient bidentate ligands for palladium-catalyzed carboxylative C-N coupling reactions under blue LED irradiation. This method facilitates the direct synthesis of arylcarbamates using a range of non-activated aryl halides, such as iodides and bromides, with various amines under atmospheric pressure of CO2. The optimized conditions exhibit high tolerance to sensitive functional groups, resulting in very good to excellent yields of the desired products.

Photoinduced NiH Catalysis with Trialkylamines for the Stereodivergent Transfer Semi-Hydrogenation of Alkynes.

We report a selectivity-switchable nickel hydride-catalyzed methodology that enables the stereocontrolled semi-reduction of internal alkynes to E- or Z-alkenes under very mild conditions. The proposed transfer semi-hydrogenation process involves the use of a dual nickel/photoredox catalytic system and triethylamine, not only as a sacrificial reductant, but also as a source of hydrogen atoms. Mechanistic studies revealed a pathway involving photo-induced generation of nickel hydride, syn-hydronickelation of alkyne, and alkenylnickel isomerization as key steps.

Mechanism of Alkyne Hydroarylation Catalyzed by (P,C)-Cyclometalated Au(III) Complexes.

Over the last 5-10 years, gold(III) catalysis has developed rapidly. It often shows complementary if not unique features compared to gold(I) catalysis. While recent work has enabled major synthetic progress in terms of scope and efficiency, very little is yet known about the mechanism of Au(III)-catalyzed transformations and the relevant key intermediates have rarely been authenticated.

From Academia to the Market - Air-stable Ni()/Josiphos Catalysts.

The design, synthesis, commercialization and application of air-stable Ni(II)/Josiphos complexes has been realized in a collaboration between Solvias and ICBMS (University Lyon 1). The Ni-complexes are utilized as versatile precatalysts for diverse cross-coupling reactions. Apart from being active in established C-C and C-N couplings at low catalyst loadings, the novel Ni-precatalysts enabled the development of the challenging monoarylation of ammonia, ammonia surrogates and even alkylammonium chlorides with aryl carbamates.

Cu-Catalyzed P-C bond formation/cleavage: straightforward synthesis/ring-expansion of strained cyclic phosphoniums.

Upon reaction with copper(i), peri-halo naphthyl phosphines readily form peri-bridged naphthyl phosphonium salts. The reaction works with alkyl, aryl and amino substituents at phosphorus, with iodine, bromine and chlorine as a halogen. It proceeds under mild conditions and is quantitative, despite the strain associated with the resulting 4-membered ring structure and the naphthalene framework.

Nickel-Catalyzed Mono-Selective α-Arylation of Acetone with Aryl Chlorides and Phenol Derivatives.

The challenging nickel-catalyzed mono-α-arylation of acetone with aryl chlorides, pivalates, and carbamates has been achieved for the first time. A nickel/Josiphos-based catalytic system is shown to feature unique catalytic behavior, allowing the highly selective formation of the desired mono-α-arylated acetone. The developed methodology was applied to a variety of (hetero)aryl chlorides including biologically relevant derivatives.

Gold(I)/Gold(III) Catalysis that Merges Oxidative Addition and π-Alkene Activation.

Heteroarylation of alkenes with aryl iodides was efficiently achieved with a (MeDalphos)AuCl complex through Au /Au catalysis. The possibility to combine oxidative addition of aryl iodides and π-activation of alkenes at gold is demonstrated for the first time. The reaction is robust and general (>30 examples including internal alkenes, 5-, 6-, and 7-membered rings).

Silyl Radical Mediated Cross-Electrophile Coupling of -Acyl-imides with Alkyl Bromides under Photoredox/Nickel Dual Catalysis.

A photoredox Ni-catalyzed cross-coupling of -acyl-imides with unactivated alkyl bromides has been developed that enables efficient access to a variety of functionalized alkyl ketones, including unsymmetrical dialkyl ketones, under very mild and operationally practical conditions. The reaction that operates without the need for any preformed carbon nucleophile proceeds via the combination of two different bond activation processes, i.e.

Catalytic Au(i)/Au(iii) arylation with the hemilabile MeDalphos ligand: unusual selectivity for electron-rich iodoarenes and efficient application to indoles.

The ability of the hemilabile (P,N) MeDalphos ligand to trigger oxidative addition of iodoarenes to gold has been thoroughly studied. Competition experiments and Hammett correlations substantiate a clear preference of gold for electron-enriched substrates both in stoichiometric oxidative addition reactions and in catalytic C-C cross-coupling with 1,3,5-trimethoxybenzene. This feature markedly contrasts with the higher reactivity of electron-deprived substrates typically encountered with palladium.

π Complexes of P^P and P^N chelated gold(i).

Tricoordinate gold(i) π-complexes containing P-based chelating ligands (P^P and P^N) were prepared. The structure of the gold(i) styrene complexes has been analysed in detail based on NMR and XRD data. The P^N complex is a competent catalyst for indole alkylation.

Evidence for genuine hydrogen bonding in gold(I) complexes.

The ability of gold to act as proton acceptor and participate in hydrogen bonding remains an open question. Here, we report the synthesis and characterization of cationic gold(I) complexes featuring ditopic phosphine-ammonium (P,NH) ligands. In addition to the presence of short Au∙∙∙H contacts in the solid state, the presence of Au∙∙∙H-N hydrogen bonds was inferred by NMR and IR spectroscopies.

(P,C) Cyclometalated Gold(III) Complexes: Highly Active Catalysts for the Hydroarylation of Alkynes.

The first catalytic application of well-defined (P,C) cyclometalated gold(III) complexes is reported. The bench-stable bis(trifluoroacetyl) complexes 2 a,b perform very well in the intermolecular hydroarylation of alkynes. The reaction is broad in scope, it proceeds within few hours at 25 °C at catalytic loadings of 0.

Gold(iii) π complexes.

π Hydrocarbons are widespread ligands and substrates in transition metal chemistry. Although gold(iii) has been known for some time to efficiently activate alkenes and alkynes towards nucleophilic addition, and to readily promote the C-H auration of arenes, the corresponding π-complexes have remained elusive until recently. This frontier article highlights recent achievements in the preparation and characterization of gold(iii) π complexes.

Formation of a peri-Bridged Phosphonio-Naphthalene by Cu-Mediated Phosphine-Aryl Coupling.

The peri-iodo naphthyl phosphine 1 reacts with CuI to give the peri-bridged phosphonio-naphthalene 2, which has been fully characterized (multi-nuclear NMR, MS, XRD). The outcome of the reaction differs markedly from that observed with gold. A two-step pathway involving P-assisted C-I oxidative addition to copper, followed by P-C reductive elimination is shown to be energetically feasible by DFT calculations.

Cyclometalated gold(iii) complexes: noticeable differences between (N,C) and (P,C) ligands in migratory insertion.

Gold(iii) complexes are garnering increasing interest for opto-electronic, therapeutic and catalytic applications. But so far, very little is known about the factors controlling their reactivity and the very influence of the ancillary ligand. This article reports the first comprehensive study on this topic.

Isolation of a Reactive Tricoordinate α-Oxo Gold Carbene Complex.

The [(P,P)Au=C(Ph)CO Et] complex 3 [where (P,P) is an o-carboranyl diphosphine ligand] was prepared by diazo decomposition at -40 °C. It is the first α-oxo gold carbene complex to be characterized. Its crystallographic structure was determined and DFT calculations have been performed, unraveling the key influence of the chelating (P,P) ligand.

Rational development of catalytic Au(I)/Au(III) arylation involving mild oxidative addition of aryl halides.

The reluctance of gold to achieve oxidative addition reaction is considered as an intrinsic limitation for the development of gold-catalyzed cross-coupling reactions with simple and ubiquitous aryl halide electrophiles. Here, we report the rational construction of a Au(I)/Au(III) catalytic cycle involving a sequence of Csp-X oxidative addition, Csp-H auration and reductive elimination, allowing a gold-catalyzed direct arylation of arenes with aryl halides. Key to this discovery is the use of Me-Dalphos, a simple ancillary (P,N) ligand, that allows the bottleneck oxidative addition of aryl iodides and bromides to readily proceed under mild conditions.

A Nucleophilic Gold(III) Carbene Complex.

The first Au carbene complex was prepared by reacting a geminal dianion with a (P,C) cyclometalated Au precursor. Its structure and bonding situation have been thoroughly investigated by experimental and computational means. The presence of a high-energy highest occupied molecular orbital (HOMO) centered at the carbene center suggests nucleophilic character for the Au carbene complex.

Gold(iii)-arene complexes by insertion of olefins into gold-aryl bonds.

The synthesis and characterization of the first gold(iii)-arene complexes are described. Well-defined (P,C)-cyclometalated gold(iii)-aryl complexes were prepared and characterized by NMR spectroscopy. These complexes swiftly and cleanly reacted with norbornene and ethylene to provide cationic gold(iii)-alkyl complexes, in which the remote phenyl ring was η-coordinated to gold.

β-Hydride Elimination at Low-Coordinate Gold(III) Centers.

This Article reports the first comprehensive study of β-hydride elimination at gold(III). The stability/fate of gold(III) alkyl species have been investigated experimentally and computationally. A series of well-defined cationic cyclometalated gold(III) alkyl complexes [(P,C)gold(III)(R)][NTf2] [(P,C) = 8-diisopropylphosphino-naphthyl; R = Me, nPr, nBu] have been synthesized and spectroscopically characterized.

Experimental and Theoretical Evidence for an Agostic Interaction in a Gold(III) Complex.

The first agostic interaction in a gold complex is described. The presence of a bonding C-H⋅⋅⋅Au interaction in a cationic "tricoordinate" gold(III) complex was suggested by DFT calculations and was subsequently confirmed by NMR spectroscopy at low temperature. The agostic interaction was analyzed computationally using NBO and QTAIM analyses (NBO=natural bond orbital; QTAIM=quantum theory of atoms in molecules).

Reactivity of Gold Complexes towards Elementary Organometallic Reactions.

For a while, the reactivity of gold complexes was largely dominated by their Lewis acid behavior. In contrast to the other transition metals, the elementary steps of organometallic chemistry-oxidative addition, reductive elimination, transmetallation, migratory insertion-have scarcely been studied in the case of gold or even remained unprecedented until recently. However, within the last few years, the ability of gold complexes to undergo these fundamental reactions has been unambiguously demonstrated, and the reactivity of gold complexes was shown to extend well beyond π-activation.

Oxidative addition of carbon-carbon bonds to gold.

The oxidative addition of strained CC bonds (biphenylene, benzocyclobutenone) to DPCb (diphosphino-carborane) gold(I) complexes is reported. The resulting cationic organogold(III) complexes have been isolated and fully characterized. Experimental conditions can be adjusted to obtain selectively acyl gold(III) complexes resulting from oxidative addition of either the C(aryl)C(O) or C(alkyl)C(O) bond of benzocyclobutenone.

Enhanced π-backdonation from gold(I): isolation of original carbonyl and carbene complexes.

The specific electronic properties of bent o-carborane diphosphine gold(I) fragments were exploited to obtain the first classical carbonyl complex of gold [(DPCb)AuCO](+) (ν(CO)=2143 cm(-1) ) and the diphenylcarbene complex [(DPCb)Au(CPh2 )](+) , which is stabilized by the gold fragment rather than the carbene substituents. These two complexes were characterized by spectroscopic and crystallographic means. The [(DPCb)Au](+) fragment plays a major role in their stability, as substantiated by DFT calculations.