Transfer hydrogenation

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Transfer hydrogenation is the reductive addition of hydrogen (H₂; dihydrogen in inorganic and organometallic chemistry) to a molecule using a source of hydrogen other than gaseous H₂. Transfer hydrogenation is applied in industry and in organic synthesis, in part because of the inconvenience and expense of using gaseous H₂. One large scale application of transfer hydrogenation is in coal liquifaction using "donor solvents" such as tetralin.

In the area of organic synthesis, a useful family of hydrogen transfer catalysts have been developed based on ruthenium and rhodium amine and phosphine complexes. These catalysts are mainly employed for the reduction of ketones and imines to alcohols and amines, respectively. The hydrogen-donor (transfer agent) is typically isopropanol, which gets oxidized to acetone upon donation of hydrogen.

Transfer hydrogenations can proceed with high enantioselectivities when the starting material is chiral:

RR'C=O + Me₂CHOH → RR'C*H-OH + Me₂C=O

where RR'C*H-OH is a chiral product. A typical catalyst is (arene)Ru(R,R-HNCHPhCHPhNTs), where Ts = SO₂C₆H₄Me and R,R refers to the absolute configuration of the two chiral carbon centers. This work was recognized with the 2003 Nobel Prize in Chemistry to Ryoji Noyori.

Another family of hydrogen transfer agents are those based on aluminium alkoxides, such as Aluminium isopropoxide.

A historically prominent transfer hydrogenation agent is diimide. Although diimide is an oxidized form of hydrazine, there is a thermodynamic imperative for it to be further oxidized to gaseous nitrogen.

More exotic transfer hydrogenations have been reported, including this intramolecular one:

References

T. Ikariya, K. Murata, R. Noyori "Bifunctional Transition Metal-Based Molecular Catalysts for Asymmetric Syntheses" Org. Biomol. Chem., 2006, volume 4, 393–406.
K. Muniz "Bifunctional Metal–Ligand Catalysis: Hydrogenations and New Reactions within the Metal–(Di)amine Scaffold" Angew Chemie, International Edition, 2005, volume 44, 6622 – 6627

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