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

Abstract

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 0.25 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.

Highly Efficient N-Heterocyclic Carbene/Ruthenium Catalytic Systems for the Acceptorless Dehydrogenation of Alcohols to Carboxylic Acids: Effects of Ancillary and Additional Ligands

“…It appeared that metallic mercury had almost no impact on the activity of our optimized catalytic system (entries 2-4 vs. entry 1), demonstrating the homogeneous nature of the real catalyst. Since the generation of Ru hydride species was reported to be crucial for the NHC/Ru-catalyzed dehydrogenative coupling of alcohols with amines or hydroxides [33,[51][52][53][54][55][56], Ru hydride formation was monitored by two NMR reactions utilizing [Ru]-5 with and without L5, respectively ( Figures S1 and S2, Supplementary Materials). These results implied that Ru-hydride species were involved in the reactions and probably paramount in the catalytic cycle.…”