Ruthenium catalysts for olefin hydrogenation

Like Wilkinson s catalyst, the ruthenium-hydride complex RuH(Cl)(PPh3)3 selectively catalyzes the hydrogenation of terminal alkenes over internal alkenes. This catalyst reacts roughly 1000 times faster with terminal olefins than with internal olefins. The lack of hydrogenation of the more-substituted alkenes and the lack of a binding site for the docking of other fimctionalities has limited the use of this catalyst. 

Three of many examples of directed hydrogenation are shown in Equations 15.15-15.17. Equation 15.15 shows the reduction of a homoaUylic alcohol, which was one of the substrates first used to demonstrate this effect. Equation 15.16 shows a more complex substrate in which the diastereoselective reduction by Crabtree s catalyst is directed by the amide function as part of tlie synthesis of pulmitoxins. Equation 15.17 shows that the addition of hydrogen can be directed to a hindered face of a bicyclic system. In this case, the cationic rhodium system qf Brown, as well as Crabtree s catalyst, led to hi selectivity. Many other reactions occur with high selectivity in the presence of Brown s cationic rhodium system. Diastereoselective additions to acyclic systems, along with a rationalization for the selectivity in these types of substrates, can be found in the review by Evans.  

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