Transfer hydrogenation

Transfer hydrogenation is the addition of hydrogen (H2; dihydrogen in inorganic and organometallic chemistry) to a molecule from a source 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 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 diamine 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 coverts 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 2001 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, which becomes oxidized to N2.



Half of the hydrogen in the hydrazine is transferred to the substrate. The reaction produces the very stable molecule N2 molecule.

Another similar example of the use of transfer hydrogenation where the product is an alkane is when the hydrogen supplier is cyclohexane. In this case an alkane is formed along with the formation of benzene. The driving force of the reaction being the gain of aromatic stabilization energy when benzene is formed. Pd can be used as a catalyst and a temperature of 100 °C is employed.

One limitation of using transfer hydrogenation for the production of alkane is that it cannot be used to prepare methane as no unsaturated hydrocarbon contain only one carbon.

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