as a catalyst and a temperature of 100 °C is employed. More exotic transfer hydrogenations have been reported, including this intramolecular one: Many reactions exist with alcohol or amines as the proton donors, and alkali metals as electron donors. Of continuing value is the sodium metal-mediated Birch reduction of arenes (another name for aromatic hydrocarbons). Less important presently is the Bouveault–Blanc reduction of esters. The combination of magnesium and methanol is used in alkene reductions, e.g. the synthesis of asenapine: Organocatalytic transfer hydrogenation Organocatalytic transfer hydrogenation has been described by the group of List in 2004 in a system with a Hantzsch ester as hydride donor and an amine catalyst: In this particular reaction the substrate is an α,β-unsaturated carbonyl compound. The proton donor is oxidized to the pyridine form and resembles the biochemically relevant coenzyme NADH. In the catalytic cycle for this reaction the amine and the aldehyde first form an iminium ion, then proton transfer is followed by hydrolysis of the iminium bond regenerating the catalyst. By adopting a chiral imidazolidinone MacMillan organocatalyst an enantioselectivity of 81% ee was obtained: In a case of stereoconvergence, both the E-isomer and the Z-isomer in this reaction yield the (S)-enantiomer. Extending the scope of this reaction towards ketones or rather enones requires fine tuning of the catalyst (add a benzyl group and replace the t-butyl group by a furan) and of the Hantzsch ester (add more bulky t-butyl groups): With another organocatalyst altogether, hydrogenation can also be accomplished for imines. One cascade reaction is catalyzed by a chiral phosphoric acid: The reaction proceeds via a chiral iminium ion. Traditional metal-based catalysts, hydrogenation of aromatic or heteroaromatic substrates tend to fail. References See also Meerwein–Ponndorf–Verley reduction Oppenauer oxidation Dehydrogenation Hydrogenation Hydrogenolysis Borrowing hydrogen Chemical processes Industrial processes Organic redox reactions Hydrogenation