Base-Free Transfer Hydrogenation Catalyzed by Ruthenium Hydride Complexes of Coumarin-Amide Ligands

Rutheniumcomplexes of coumarin-amide ligands functionedas base-free catalysts for the transfer hydrogenation of carbonylcompounds including a few biologically relevant substrates. Two sets of Ru-(II) complexes, [Ru-Cl] (1-3) and [Ru-H] (4-6), supported with coumarin-amide-basedligands have been synthesized and characterized. In these complexes,the coumarin-amide-based ligands show different coordination modes:N-N, N-O, and N-N-O as confirmed by thecrystallographic studies. While [Ru-Cl] complexes (1-3) required the presence of a base for promoting the transfer hydrogenation(TH), the corresponding [Ru-H] complexes (4-6) acted as excellent catalysts for the base-free TH of assorted carbonylcompounds. The [Ru-H] complexes (4-6) provideda nearly quantitative yield of the corresponding alcohols by usingisopropanol both as a solvent and a hydrogen source. TH of a few biologicallyrelevant substrates such as furfural, nicotinaldehyde, vanillin, isovanillin,chalcone, and 3,6-dihydroxyflavone illustrated the noteworthy catalyticperformance of the [Ru-H] complexes. The entire catalytic procedureincluding reagents, solvents, and conditions was evaluated with thehelp of the CHEM21 green metrics toolkit to assess the 'greencredentials' of the catalysis.

Automated summary

Ruthenium complexes of coumarin-amide ligands were synthesized and characterized. The Ru-H complexes (4-6) exhibited excellent catalytic performance in the base-free transfer hydrogenation of carbonyl compounds. The green credentials of the catalysis were evaluated.