Hydrogen transfer reduction bonds

Solutions of Moiseev s giant Pd colloids [49,161-166] were shown to catalyze a number of reactions in the quasi homogeneous phase, namely oxidative ace-toxylation reactions [162], the oxidative carbonylation of phenol to diphenyl carbonate [166], the hydrogen-transfer reduction of multiple bonds by formic acid [387], the... 

Moiseev, I.I., Tsirkov, G.A., Gekhman, A.E., and Vargaftik, M.N., Facile hydrogen- transfer reduction of multiple bonds by formic acid catalysed with a Pd-561 giant cluster, Mendeleev Commun., 7,1-3,1997. 

Catalytic hydrogenation transfers the elements of molecular hydrogen through a series of complexes and intermediates. Diimide, HN=NH, an unstable hydrogen donor that can be generated in situ, finds specialized application in the reduction of carbon-carbon double bonds. Simple alkenes are reduced efficiently by diimide, but other easily reduced functional groups, such as nitro and cyano are unaffected. The mechanism of the reaction is pictured as a concerted transfer of hydrogen via a nonpolar cyclic TS. 

A wide variety of iridium-based hydrogenation catalysts are currently under development, notably for organic syntheses including enantioselective synthesis. Hydrogenation by hydrogen transfer is well known [15], and the reduction of C=0 and C=N double bonds is also possible [16, 17]. 

The scope of hydrogen transfer reactions is not limited to ketones. Imines, carbon-carbon double and triple bonds have also been reduced in this way, although homogeneous and heterogeneous catalyzed reductions using molecular hydrogen are generally preferred for the latter compounds. 

Hydrogen transfer reactions are highly selective and usually no side products are formed. However, a major problem is that such reactions are in redox equilibrium and high TOFs can often only be reached when the equilibria involved are shifted towards the product side. As stated above, this can be achieved by adding an excess of the hydrogen donor. (For a comparison, see Table 20.2, entry 8 and Table 20.7, entry 3, in which a 10-fold increase in TOF, from 6 to 60, can be observed for the reaction catalyzed by neodymium isopropoxide upon changing the amount of hydrogen donor from an equimolar amount to a solvent. Removal of the oxidation product by distillation also increases the reaction rate. When formic acid (49) is employed, the reduction is a truly irreversible reaction [82]. This acid is mainly used for the reduction of C-C double bonds. As the proton and the hydride are removed from the acid, carbon dioxide is formed, which leaves the reaction mixture. Typically, the reaction is performed in an azeotropic mixture of formic acid and triethylamine in the molar ratio 5 2 [83],... 

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