Mickael.henrion

Pincer complexes are becoming increasingly important for organometallic chemistry and organic synthesis. Since numerous applications for such catalysts have been developed in recent decades, this Minireview covers progress in their use as catalysts for (de)hydrogenation and transfer (de)hydrogenation reactions during the last four years. Aside from noble-metal-based pincer complexes, the corresponding base metal complexes are also highlighted and their applications summarized.

Pincers of power: This minireview is focused on the recent progress of the application of various precious and non-precious metal pincer-type catalysts. Besides hydrogenation reactions, their utilization for dehydrogenation and transfer (de)hydrogenation reactions is also described. Especially, but not exclusively, the usage of ruthenium and iron pincer complexes is highlighted for CC, CN, and CO bond transformations.

The “green” reduction of carboxylic acids to alcohols is a challenging task in organic chemistry. Herein, we describe a general protocol for generation of alcohols by catalytic hydrogenation of carboxylic acids. Key to success is the use of a combination of Ru(acac)₃, triphos and Lewis acids. The novel method showed broad substrate tolerance and a variety of aliphatic carboxylic acids including biomass-derived compounds can be smoothly reduced.

Direct route to alcohols: The combination of a specific ruthenium complex and Lewis acids is found to be active for the direct hydrogenation of carboxylic acids to alcohols. A variety of carboxylic acids including biomass-based derivatives are converted into the desired alcohols in high yields.

A novel and simple hydrogen storage system was developed, based on the dehydrogenative coupling of inexpensive ethylenediamine with ethanol to form diacetylethylenediamine. The system is rechargeable and utilizes the same ruthenium pincer catalyst for both hydrogen loading and unloading procedures. It is efficient and uses a low catalyst loading. Repetitive reversal reactions without addition of new catalyst result in excellent conversions in both the dehydrogenation and hydrogenation procedures in three cycles.

In support of the hydrogen economy: An efficient and simple homogeneous hydrogen carrier system was developed based on the dehydrogenative coupling of ethylenediamine with ethanol to form diacetylethylenediamine. The same ruthenium pincer catalyst is used for both hydrogen loading and unloading reactions.

The last decades have seen a tremendous expanse in the application of CH activation of many different substrate classes, including the invaluable indole scaffold. Following the exciting emergence of CH activation as a multi-faceted platform for functionalization, a versatile tool box has been developed for the preparation of structurally diverse indoles. This review article discusses recent advances and strategies for transition metal-catalyzed CH activation of indoles.

A variety of chemical transformations benefit from the use of strong electron-donating ancillary ligands, such as alkylphosphines or N-heterocyclic carbenes when electron-rich metal centers are required. Herein, we describe a facile and highly modular access to monodentate and bidentate imidazolin-2-ylidenamino-substituted phosphines. Evaluation of the phosphine’s electronic properties substantiate that the formal replacement of alkyl or aryl groups by imidazolin-2-ylidenamino groups dramatically enhance their donor ability beyond that of alkylphosphines and even N-heterocyclic carbenes. The new phosphines have been coordinated onto palladium(II) centers, and the beneficial effect of the novel substitution patterns has been explored by using the corresponding complexes in the palladium-catalyzed Suzuki–Miyaura cross-coupling reaction of non-activated aryl chloride substrates.

Upgrading phosphines: A conceptually new approach to a family of extremely electron-rich phosphines is based on the use of imidazolin-2-ylidenamino groups directly attached to the phosphorus atom. The steric and electronic properties of the new ligands can be easily varied owing to the general and modular synthesis, which provides new prospects for phosphine ligands in catalysis.

Nickel nanoparticles, formed in situ and used in combination with micellar catalysis, catalyze Suzuki–Miyaura cross-couplings in water under very mild reaction conditions.

Under water: Nickel nanoparticles, formed in situ and used in combination with micellar catalysis, catalyze Suzuki–Miyaura cross-couplings in water under very mild reaction conditions. A wide range of substrates is tolerated and the reaction medium can be recycled.

Structurally diverse (hetero)aryl chloride, bromide, and tosylate electrophiles were employed in the Ni-catalyzed monoarylation of ammonia, including chemoselective transformations. The employed JosiPhos/[Ni(cod)₂] catalyst system enables the use of commercially available stock solutions of ammonia, or the use of ammonia gas in these reactions, thereby demonstrating the versatility and potential scalability of the reported protocol. Proof-of-principle experiments established that air-stable [(JosiPhos)NiCl₂] precatalysts can be employed successfully in such transformations.

Lighten Up: The substrate scope of the title reaction includes (hetero)aryl chloride, bromide, and tosylate electrophiles. The versatility and potential scalability of the reported protocol is demonstrated by the use of either commercially available stock solutions of ammonia or ammonia gas.

A highly efficient catalyst system based on ruthenium-pincer-type complexes has been discovered for N-formylation of various amines with CO₂ and H₂, thus affording the corresponding formamides with excellent productivity (turnover numbers of up to 1 940 000 in a single batch) and selectivity. Using a simple catalyst recycling protocol, the catalyst was reused for 12 runs in N,N-dimethylformamide production without significant loss of activity, thus demonstrating the potential for practical utilization of this cost-effective process. A one-pot two-step procedure for hydrogenation of CO₂ to methanol via the intermediacy of formamide formation has also been developed.

Just a pinch: CO₂ is efficiently hydrogenated for N-formylation of various amines using ruthenium-pincer catalysts, thus affording the corresponding formamides with extremely high turnover numbers (TONs). The catalyst was readily reused for 12 runs without significant loss of activity in N,N-dimethylformamide production, thus demonstrating potential for practical utilization of this cost-effective process.