Abstract
Hydrogen transfer reactions have exceptional importance, due to their applicability in numerous synthetic pathways, with academic as well as industrial relevance. The most important transformations are, e.g., reduction, ring-closing, stereoselective reactions, and the synthesis of heterocycles. The present review provides insights into the hydrogen transfer reactions in the condensed phase in the presence of noble metals (Rh, Ru, Pd) as catalysts. Since the H-donor molecules (such as alcohols/ethers and amines (1°, 2°, 3°)) and the acceptor molecules (alkenes (C=C), alkynes (C≡C), and carbonyl (C=O) compounds) play a crucial role from mechanistic viewpoints, the present summary points out the key mechanistic differences with the interpretation of current contributions and the corresponding historical achievements as well.
Highlights
Transfer hydrogenation (TH) is a reaction in which H is transferred to an acceptor molecule from another molecular species acting as a donor
Based on the large spectrum of hydrogen transfer reactions, the present review will focus on the results obtained with noble metals (Rh, Ru, Pd), in combination with alcohols/ethers and amines as H-donor species, whereas carbonyl compounds, alkenes, and alkynes act as acceptor molecules
The results showed that the cationic Pd hydride generated from the corresponding Pd(μ-OH) complex and ethanol can act as a chiral reducing agent, driving the reaction to the saturated carbonyl species (Scheme 22) [92]
Summary
Transfer hydrogenation (TH) is a reaction in which H is transferred to an acceptor molecule from another molecular species (other than molecular hydrogen) acting as a donor. In the 1960s, Mitchell published the saturation reaction of cyclohexanone to cyclohexanol in the presence of chloroiridic acid (H2 IrCl6 ) with trimethyl phosphite in aqueous 2-propanol (IPA), with water acting as the H-source [15] Transition metal catalysts such as the Wilkinson complex (Rh(PPh3 ) Cl) showed remarkable activity in hydrogen transfer reactions as well [16]; the system is best known for hydrogenation reactions with molecular hydrogen. Based on the large spectrum of hydrogen transfer reactions, the present review will focus on the results obtained with noble metals (Rh, Ru, Pd), in combination with alcohols/ethers and amines as H-donor species, whereas carbonyl compounds, alkenes, and alkynes act as acceptor molecules. The dominance of the reduction/oxidation pathways are highly dependent on the concentration of the donor and acceptor molecules and the thermodynamic stability of the species involved in the redox equilibrium [1]
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