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2.7.1.1.2 Transfer Hydrogenation

DOI: 10.1055/sos-SD-227-00139

Puylaert, P.; Savini, A.; Hinze, S.Science of Synthesis: Catalytic Reduction in Organic Synthesis, (20182338.

Transfer hydrogenations have been known for decades.[‌59‌‌62‌] Prominent catalytically active metals are ruthenium, iridium, rhodium, osmium, and iron. In metal-catalyzed transfer hydrogenations, formic acid, hydrazine, and alcohols are among the most common hydrogen sources. Despite the need for (super)stoichiometric amounts of hydrogen sources, transfer hydrogenations offer an alternative way to reduce nitroaromatic compounds if high-pressure equipment is not available or the use of hydrogen gas is undesirable. As early as 1972, Landesberg and co-workers observed the transfer hydrogenation of nitroaromatics to the corresponding amines in the presence of dodecacarbonyltriiron(0) [Fe3(CO)12] and methanol.[‌63‌] Although the reaction requires stoichiometric dodecacarbonyltriiron(0), a large number of functional groups are tolerated, including halides, double bonds, alcohols, ketones, amides, and esters. Since then, a plethora of catalytic methods has been developed.[‌2‌,‌3‌,‌41‌] Both homogeneous and heterogeneous catalysts have been reported to successfully catalyze the transfer hydrogenation of nitroarenes using alcohols, for the major part propan-2-ol (isopropanol), as the hydrogen source. However, a stoichiometric amount of base (with respect to the substrate) is required in the majority of the reported examples.[‌64‌‌69‌]

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