Transfer Hydrogenation from Glycerol: Activity and Recyclability of Iridium and Ruthenium Sulfonate-Functionalized N-Heterocyclic Carbene Catalysts

Three ruthenium(II) and two iridium(III) N-heterocyclic carbene (NHC) complexes functionalized with sulfonates are compared with respect to their activity and selectivity for the transfer hydrogenation of imines, aldehydes, ketones, and olefins using neat glycerol as hydrogen donor and solvent. Four of the five catalysts likely proceed through a monohydride mechanism and are more active for transfer hydrogenation of imines than aldehydes, ketones, and olefins. The fifth catalyst likely proceeds through a dihydride mechanism and is found to be more active for carbonyls than imines and olefins. Lactic acid is observed as the only detectable byproduct from glycerol. Quantitative poisoning experiments with 1,10-phenanthroline suggest that the predominant catalytically active species is a ligated homogeneous complex with weak binding to the poison. The potential for catalyst recycling is explored: the ruthenium NHC catalysts with chelating ligands are found to be more robust and recyclable relative to the iridium catalysts and the ruthenium mono-NHC catalyst. The reason is traced to the relative rate of degradation of the catalyst in the presence of glycerol and KOH but no substrate. This degradation is only observed when glycerol is used, as replacing glycerol with isopropanol allows the ruthenium catalyst to be fully recyclable.