Catalytic hydrogenation involving heterolytic cleavage

Fig. 3. Mechanism for catalytic hydrogenation involving heterolytic cleavage ofri2-H2.

The mechanism of action, and organization of the catalytic sites, in hydrogenases are different from a solid catalyst such as platinum. For a start, the reaction of H2 with hydrogenase involves heterolytic cleavage into a hydron and a hydride. This contrasts with the reaction of H2 at the surface of a metal such as platinum, which is usually considered to involve the homolytic cleavage into two hydrogen atoms. Moreover in the enzyme, the catalyst is a cluster of metal ions (with oxidation states +2 or -h3) rather than the metal (oxidation state 0). 

The side-on (r)2) bonding in M r 2-H2 and other a-complexes has been termed non-classical, on analogy to the 3-center, 2-electron bonding in non-classical carbocations and boranes (Fig. 2). One of the first questions raised when H2 complexes were discovered is whether they would be important in catalytic reactions. As will be shown below the answer is an emphatic yes, as exemplified by the elegant asymmetric catalytic hydrogenation systems of Nobel-laureate Ryoji Noyori. Also, the mechanism of catalytic silane alcoholysis directly involves two different a complexes M(r 2-Si-H) and M(r 2-H2). In both of these systems, the crucial step is heterolytic cleavage of the H H and/or Si-H bond, the primary subject of this review. 

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