Sustainable Amidation Reactions – Recent Advances

Synthesis of Ruthenium complexes and their catalytic applications: A review

Photocatalytic aldehydes/alcohols/toluenes oxidative amidation over bifunctional Pd/MOFs: Effect of Fe-O clusters and Lewis acid sites

A practical and sustainable protocol for direct amidation of unactivated esters under transition-metal-free and solvent-free conditions

The direct amide synthesis from alcohols and amines applying various transition metal catalysts has been demonstrated as an attractive and promising process. Among various catalytic systems, N-heterocyclic carbene (NHC)-based ruthenium (Ru) ones have been testified to be active for this atom-economic transformation. Although a variety of imidazole-based NHC/Ru catalytic systems were reported to be active for this reaction, the benzimidazole-based analogs exhibited higher catalytic performance in most cases. However, these catalytic systems, which comprise a monodentate NHC ligand and a Cl or phosphine ligand as the key components, require relatively high catalyst loadings. In order to obtain more active and robust catalytic systems, we aim to bridge two monodentate ligands with one bidentate NHC ligand. Therefore, a number of CNHCC bidentate NHC precursors were designed and synthesized. Through screening of the NHC precursors and other reaction conditions, potent and phosphine-free bidentate NHC/Ru catalytic systems were discovered for the efficient amide synthesis. Interestingly, from the in situ generated catalytic system, two NHC/Ru intermediates were isolated and structurally confirmed by X-ray crystallography. Notably, these two complexes are active for the amide synthesis even at a low catalyst loading of 0.5 mol%, which could verify that they should be key intermediates during the catalysis. Probably, the current catalytic systems, featuring high efficiency and ready accessibility, could be valuable for more interesting applications.

Efficient and phosphine-free bidentate N-heterocyclic carbene/ruthenium catalytic systems for the dehydrogenative amidation of alcohols and amines

Well‐defined N‐heterocyclic carbene/ruthenium complexes for the alcohol amidation with amines: The dual role of cesium carbonate and improved activities applying an added ligand

The atom‐economic direct amidation of alcohols with amines has been recently highlighted as an attractive and promising transformation. Among the versatile reported catalytic systems, in situ generated N‐heterocyclic carbene (NHC)/ruthenium (Ru) catalytic systems have demonstrated their advantages such as easy operation and use of commercial Ru compounds. However, the existing catalyst loadings are relatively high, and additional insights for the in situ catalyst generation are still not well‐documented. In this work, a variety of benzimidazole‐based NHC precursors were initially synthesized. Through the screening of various NHC precursors and other reaction conditions, active in situ catalytic systems were discovered for the efficient amide synthesis. Notably, the catalyst loading is as low as 0.5 mol %. Furthermore, additional experiments were performed to validate the rationale for the superiority of the current catalytic systems over our previous system. It was observed that the ligand structure is one of the reasons for the higher activity. In addition, the higher ratio of the NHC precursor/[Ru] is another important factor for the improvement. Further HR‐MS analysis identified the formation of two mono‐NHC‐Ru species as major species and two Ru species bearing multiple NHC ligands as minor species. Hopefully, the efficient and readily‐accessible catalytic systems reported herein could demonstrate great potential for further practical applications.

Efficient N‐Heterocyclic Carbene/Ruthenium Catalytic Systems for the Alcohol Amidation with Amines: Involvement of Poly‐Carbene Complexes?

Transition-metal-catalyzed amide-bond formation from alcohols and amines is an atom-economic and eco-friendly route Herein, we identified a highly active in situ N-heterocyclic carbene (NHC)/ruthenium (Ru) catalytic system for this amide synthesis Various substrates, including sterically hindered ones, could be directly transformed into the corresponding amides with the catalyst loading as low as 025 mol% In this system, we replaced the p-cymene ligand of the Ru source with a relatively labile cyclooctadiene (cod) ligand so as to more efficiently obtain the corresponding poly-carbene Ru species Expectedly, the weaker cod ligand could be more easily substituted with multiple mono-NHC ligands Further high-resolution mass spectrometry (HRMS) analyses revealed that two tetra-carbene complexes were probably generated from the in situ catalytic system

Ruthenium-Based Catalytic Systems Incorporating a Labile Cyclooctadiene Ligand with N -Heterocyclic Carbene Precursors for the Atom-Economic Alcohol Amidation Using Amines.

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Well‐defined N‐heterocyclic carbene/ruthenium complexes for the alcohol amidation with amines: The dual role of cesium carbonate and improved activities applying an added ligand

Efficient N‐Heterocyclic Carbene/Ruthenium Catalytic Systems for the Alcohol Amidation with Amines: Involvement of Poly‐Carbene Complexes?

Ruthenium-Based Catalytic Systems Incorporating a Labile Cyclooctadiene Ligand with N -Heterocyclic Carbene Precursors for the Atom-Economic Alcohol Amidation Using Amines.