Three-legged piano stool Cp*Ir<sup>III</sup>Cl complexes (Cp* = pentamethylcyclopentadienyl) bearing pyridinesulfonamide ligands with varying electronic parameters were synthesized and characterized by NMR spectroscopy and single crystal X-ray diffraction. These complexes are highly active precatalysts for the base-free transfer hydrogenation of substituted aryl and linear ketones and aldehydes, cycloaliphatic and diaryl ketones α,β-unsaturated ketones, diones, β-ketoesters, and biomass derived 5-hydroxymethylfurfural and levulinic acid substrates with 2-propanol at 85 °C, using 1 mol% precatalyst. Catalysis was also efficient using a 0.1 mol% loading. Selective initial reduction of the activated alkene of α,β-unsaturated ketones is observed and upon longer reaction times carbonyl reduction is observed. All transfer hydrogenation catalysis was conducted in air without dried and degassed substrates and basic additives are not required for high catalytic activity. Control experiments and a mercury poisoning experiment support a homogeneous catalyzed pathway. Overall, the fastest reactions are observed using electron poor substrates and precatalysts bearing electron rich ligands. Progress towards the application of these Cp*Ir(pyridinesulfonamide)Cl precatalysts in tandem with acid catalysis to convert biomass-derived linear aldehydes to fuel precursors and acteophenones to styrenes and ethyl benzenes is also described.

Base-free transfer hydrogenation of aldehydes and ketones using Cp*Ir(pyridinesulfonamide)Cl precatalysts

251st National Meeting (2016)

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