In chemistry, transfer hydrogenation is a chemical reaction involving the addition of hydrogen to a compound from a source other than molecular . It is applied in laboratory and industrial organic synthesis to saturate organic compounds and reduce ketones to alcohols, and imines to amines. It avoids the need for high-pressure molecular used in conventional hydrogenation. Transfer hydrogenation usually occurs at mild temperature and pressure conditions using organic or organometallic catalysts, many of which are chiral, allowing efficient asymmetric synthesis. It uses hydrogen donor compounds such as formic acid, isopropanol or dihydroanthracene, dehydrogenating them to , acetone, or anthracene respectively. Often, the donor molecules also function as solvents for the reaction. A large scale application of transfer hydrogenation is coal liquefaction using "donor solvents" such as tetralin. Organometallic catalysts In the area of organic synthesis, a useful family of hydrogen-transfer catalysts have been developed based on ruthenium and rhodium complexes, often with diamine and phosphine ligands. A representative catalyst precursor is derived from (cymene)ruthenium dichloride dimer and the tosylated diphenylethylenediamine. 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 converts to acetone upon donation of hydrogen. Transfer hydrogenations can proceed with high enantioselectivities when the starting material is prochiral: RR'C=O{} + Me2CHOH -> RR'C^{\star}H-OH{} + Me2C=O where is a chiral product. A typical catalyst is , where Ts refers to a tosyl group () 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 Ryōji Noyori. Another family of hydrogen-transfer agents are those based on aluminium alkoxides, such as aluminium isopropoxide in the MPV reduction; however their activities are relatively low by comparison with the transition metal-based systems. Metal-free routes Prior to the development of catalytic hydrogenation, many methods were developed for the hydrogenation of unsaturated substrates. Many of these methods are only of historical and pedagogical interest. One prominent transfer hydrogenation agent is diimide or (NH)2, also called diazene. This becomes oxidized to the very stable N2: The diimide is generated from hydrazine. Two hydrocarbons that can serve as hydrogen donors are cyclohexene or cyclohexadiene. In this case, an alkane is formed, along with a benzene. The gain of aromatic stabilization energy when the benzene is formed is the driving force of the reaction. Pd can be used