Iridium‐Catalyzed Acid‐Assisted Hydrogenation of Oximes to Hydroxylamines

We found that cyclometalated cyclopentadienyl iridium(III) complexes are uniquely efficient catalysts in homogeneous hydrogenation of oximes to hydroxylamine products. A stable iridium C,N‐chelation is crucial, with alkoxy‐substituted aryl ketimine ligands providing the best catalytic performance. Several Ir‐complexes were mapped by X‐ray crystal analysis in order to collect steric parameters that might guide a rational design of even more active catalysts. A broad range of oximes and oxime ethers were activated with stoichiometric amounts of methanesulfonic acid and reduced at room temperature, remarkably without cleavage of the fragile N−O bond. The exquisite functional group compatibility of our hydrogenation system was further demonstrated by additive tests. Experimental mechanistic investigations support an ionic hydrogenation platform, and suggest a role for the Brønsted acid beyond a proton source. Our studies provide deep understanding of this novel acidic hydrogenation and may facilitate its improvement and application to other challenging substrates. Half‐sandwich iridium(III) complexes are found to be efficient catalysts for homogeneous oxime hydrogenation to hydroxylamine products, only if equipped with an aryl‐imine ligand and operating under strong acidic conditions. Remarkably they do not hydrogenolyse the fragile N−O bond. Experimental evidence suggests an unusual acid‐assisted ionic hydrogenation mechanism which may be applicable to the reduction of other challenging substrates.