Highly pH-Dependent Chemoselective Transfer Hydrogenation of α,β-Unsaturated Aldehydes in Water

The pH-dependent selective Ir-catalyzed hydrogenation of α,β-unsaturated aldehydes was realized in water. Using HCOOH as the hydride donor at low pH, the unsaturated alcohol products were obtained exclusively, while the saturated alcohol products were formed preferentially by employing HCOONa as the hydride donor at high pH. A wide range of functional groups including electron-rich as well as electron-poor substituents on the aryl group of α,β-unsaturated aldehydes can be tolerated, affording the corresponding products in excellent yields with high TOF values. High selectivity and yields were also observed for α,β-unsaturated aldehydes with aliphatic substituents. Our mechanistic investigations indicate that the pH value is critical to the chemoselectivity.