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.