Ionic Hydrogenation of Ketones Using Metal Hydrides and Added Acid

Marko and coworkers found that hydrogenation of ketones (acetone, acetophenone, cyclohexanone, etc.) or aldehydes (benzaldehyde, n-butyraldehyde) were catalytically hydrogenated by Cr, Mo or W carbonyl complexes in methanol under 100bar H2 [34]. Their experiments used M(CO)6 as the catalyst precursor, with NaOCHs added under these conditions (COlsMH and [( x-H)M2(CO)ior are formed, so these reactions are similar to those discussed above. Most of their experiments were carried out at 100-160 °C, but with the Mo complex hydrogenations could be carried out at temperatures as low as 70 °C. 

Ionic Hydrogenation of Ketones Using Metai Hydrides and Added Acid 

The use of stoichiometric ionic hydrogenations in organic synthetic applications was estabUshed prior to the use of transition metal hydride complexes for ketone hydrogenations [36, 37]. In traditional stoichiometric ionic hydrogenations, CF3CO2H is used as the acid and HSiEts serves as the hydride donor, though other acids and other hydride sources have been used as well. The C=0 bonds of ketones can be hydrogenated by this method, as well as certain types of C=C bonds. 

Using transition metal hydrides, rather than the H-Si bond of silanes, as hydride donors offers several significant advantages. The steric and electronic properties 

Stoichiometric ionic hydrogenations using acids as the H+ source and transition metal hydrides as the H donor were studied prior to the development of catalytic hydrogenations. Hydrogenation of C=C bonds through the reaction with triflic acid (HOTf) and metal hydrides can be accomplished within minutes at low temperature (3.12). 

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