Transfer Hydrogenation Including the Meerwein-Ponndorf-Verley Reduction

More recently, Lnm alkoxides were shown to have much higher catalytic activity in this reaction, which allowed their use in only catalytic amounts [6, 7]. Later, however, much higher reactivities for Alln-catalyzed Meerwein-Ponndorf-Verley and Oppenauer (MPVO) reactions have also been achieved with dinuc-lear Alnl complexes [8,9] and with Alln alkoxides generated in situ [10]. Several reviews on the MPVO reactions have been published [11-14], 

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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. 

The advantages of hydrogen transfer over other methods of hydrogenation comprise the use of readily available hydrogen donors such as 2-propanol, the very mild reaction conditions, and the high selectivity. High concentrations of the reductant can be applied and the hydrogen donor is often used as the solvent, which means that mass transfer limitations cannot occur in these reactions. The uncatalyzed reduction of ketones requires temperatures of 300 °C [29]. 

In the direct transfer mechanism, the metal ion coordinates both reactants enabling an intramolecular reaction, and activates them via polarization. Consequently, strong Lewis acids including Alln and the Lnln ions are the most suitable catalysts in this type of reactions. In the hydride mechanism, a hydride is transferred from a donor molecule to the metal of the catalyst, hence forming a metal hydride. Subsequently, the hydride is transferred from the metal to the acceptor molecule. Metals that have a high affinity for hydrides, such as Ru, Rh and Ir, are therefore the catalysts of choice. The Lewis acidity of these metals is too weak to catalyze a direct hydride transfer and, vice versa, the affinity of Alm and Lnm to hydride-ions is too low to catalyze the indirect hydrogen transfer. Two distinct pathways are possible for the hydride mechanism one in which the catalyst takes up two hydrides from the donor molecule and another in which the catalyst facilitates the transfer of a single hydride. 

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Transfer Hydrogenation Including the Meerwein-Ponndorf-Verley Reduction... 

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