Hydrogenation using transition metal catalysts

Some general reviews on hydrogenation using transition metal complexes that have appeared within the last five years are listed (4-7), as well as general reviews on asymmetric hydrogenation (8-10) and some dealing specifically with chiral rhodium-phosphine catalysts (11-13). The topic of catalysis by supported transition metal complexes has also been well reviewed (6, 14-29), and reviews on molecular metal cluster systems, that include aspects of catalytic hydrogenations, have appeared (30-34). 

In hydrogenation, early transition-metal catalysts are mainly based on metallocene complexes, and particularly the Group IV metallocenes. Nonetheless, Group III, lanthanide and even actinide complexes as well as later metals (Groups V-VII) have also been used. The active species can be stabilized by other bulky ligands such as those derived from 2,6-disubstituted phenols (aryl-oxy) or silica (siloxy) (vide infra). Moreover, the catalytic activity of these systems is not limited to the hydrogenation of alkenes, but can be used for the hydrogenation of aromatics, alkynes and imines. These systems have also been developed very successfully into their enantioselective versions. 

Most alkenes react quantitatively with molecular hydrogen, Hg in the presence of a transition metal catalyst to give alkanes. Commonly used transition metal catalysts include platinum, palladium, ruthenium, and nickel. Idelds are usually quantitative or nearly so. Because the conversion of an alkene to an alkane involves reduction by hydrogen in the presence of a catalyst, the process is called catalytic reduction or, alternatively, catalytic hydrogenation. 

Several methanol dehydrogenation processes have recently been developed using transition-metal catalysts to afford formaldehyde, hydrogen, and carbon dioxide [131-137]. 

The reaction occurs with stepwise addition of hydrogen to first give an alkene. The second step occurs even faster than the first. Therefore, catalytic hydrogenation with transition metal catalysts cannot be used to partially hydrogenate alkynes and stop at an alkene. The hydrogenation goes all the way to the alkane. 

Use of transition metal catalysts as 19 selective hydrogenation catalysts (121)... 

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