A Boomerang Catalyst Combining High Catalytic Activity and Reusability for Aqueous Transfer Hydrogenation of Ketones

We report a boomerang-type iridium catalyst that combines the high activity of homogeneous catalysis with spontaneous and quantitative catalyst recovery without external operation. The catalyst consists of a Cp*Ir complex immobilized on a cross-linked poly(acrylic acid) gel (PAA). Upon addition of formic acid (FA), Ir species are released into solution, enabling the aqueous transfer hydrogenation of a broad range of ketones under air without additional additives. In situ analyses confirmed the presence of soluble Cp*Ir reaction intermediates during the reaction. Upon complete consumption of formic acid (FA), the dissolved Ir species were spontaneously and quantitatively recaptured by the support without requiring temperature modulation or solvent exchange. The catalyst retains over 90% of its initial activity over five consecutive runs, with no detectable Ir remaining in the reaction solution after recovery. This autonomous boomerang mechanism provides a promising strategy for combining the high activity of homogeneous catalysts with the reusability of heterogeneous catalysts.