“On Water” Cationic Ruthenium(II) Catalysed Direct Aryl C(sp2)−H Amidation of Biorelevant Heterocyclic Scaffolds

Abstract

Ru(II)-Catalyzed “On Water” direct aryl C(sp²)−H amidation of 2-arylbenzo[d]-thiazole/oxazole with acyl azide is reported under silver-free condition. Deuterium scrambling experiments suggested reversible C−H activation catalyzed by active cationic ruthenium species. The organic solvents such as DCE, DMF, DMSO, MeCN, dioxane, and PhMe were not conducive for the C−H amidation except for PhCl in which case, however, inferior yield (31 %) was obtained. Water plays critical roles (i) during the formation of active cationic Ru-species, (ii) as proton scavenger during ligand-assisted C−H activation through hydrogen bond formation as evidenced by solvent kinetic isotope effect, and (iii) in the final protodissociation step. The mechanistic proposal resembles secondary coordination sphere hydrogen bond controlled transition metal catalysis showcasing the aryl C−H amidation through outer sphere nitrene insertion. The “on water” aryl C−H amidation protocol showed wide substrate scope with respect to the 2-arylbenzo[d]-thiazole/oxazole scaffold as well as the aryl moiety of the aroyl azide. While the applicability of the Ru(II)-catalysed “on water” C−H amidation protocol to 2-arylbenzo[d]-thiazole and 2-arylbenzo[d]-oxazole demonstrates its scope with respect to the directing group the effectiveness for sulfonamidation and phosphoramidation further broaden the synthetic scope.