Olefin hydrogenation monohydride catalysts

One of the classic catalysts that reacts through a pathway involving the insertion of olefins into monohydride intermediates is [Rh(H)(PPh3)3(CO)]. The reactions of terminal olefins occur imder mild conditions (Equation 15.24). The pronounced substrate selectivity of this catalyst is illustrated by its lack of reactivity with cyclohexene. This complex catalyzes the isomerization of internal olefins, but it does not catalyze their hydrogenation. Other unsaturated fimctionalities, such as aldehydes, nitriles, and esters, are stable to this catalyst under these mild conditions. 

Ruthenium catalysts are now widely used for olefin hydrogenation, and many examples of enantioselective ruthenium-catalyzed hydrogenation are discussed in Section 15.7. Here, before addressing the issues of stereoselectivity, the elementary steps of ruthenium-catalyzed hydrogenation are discussed. These catalysts react through monohydride species containing a second anionic ligand. 

Monohydrides play an important role in the following rhodium-complex-catalyzed hydrogenations in aqueous media. The catalyst precursor is [RhCl(PTA)3], which gives the catalytically active species (HRh(PTA)3] formed by dehydrochlorination of the primary product of H2 oxidative addition (88). The complex is an active catalyst for several reactants, e.g., olefinic and oxo adds, allyl alcohol, and sulfostyrene. 

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