Hydrogenation by Wilkinson’s catalyst

In the following section, we discuss the basic mechanism of homogeneous hydrogenation by Wilkinson s catalyst, RhCl(PPh3)3. Many other complexes of rhodium as well as complexes of other metals such as ruthenium, platinum, lutetium, etc. have also been used as homogeneous, laboratory-scale, hydrogenation catalysts. The mechanisms in all these cases may differ substantially. 

Figure 7.3 Hydrogenation by Wilkinson s catalyst. For butene, oxidative addition of H2 precedes alkene coordination but not necessarily for all other alkenes.

In order to appreciate the mechanism of rhodium asymmetric hydrogenation, it may be useful first to examine the simpler achiral case of alkene hydrogenation by Wilkinson s catalyst. In early studies [13] assumptions were made about the dihydrogen activation step which proved to be incorrect because of the direct... 

Figure 2.5. The mechanism of cyclohexene (S) hydrogenation by Wilkinson s catalyst. L is PPha or P(p-tolyl)3.

Relative reactivities of alkenes for hydrogenation by Wilkinson s catalyst. 

The overall mechanism for olefin hydrogenation by Wilkinson s catalyst can be divided into three parts the addition of hydrogen to RhClLj, the reaction of RhHjClLj with the olefin by migratory insertion, and the reductive elimination of the reduced product. Because the hydrogen adds before the olefin, the mechanism shown in Schemes 15.1 and 15.6 is referred to as the hydride or hydrogen-first path. Mechanistic information on the first tv m parts of the catalytic cycle that control the rates of these reactions are discussed in ttie following two sections. 

Magnetisation transfer experiments in and P nmr suggest that in the catalytic cycle of hydrogenation by Wilkinson s catalyst, [RhCl(PPhg)3], a cis-(PPha) Rh conformation occurs at key stages . Wilkinson s catalyst hydrogenates allyl alcohol with an activation energy of 19.7 kcal mol but under certain... 

The equilibrium and transition state structures involved in the catalytic cycles of olefin hydrogenation by Wilkinson s catalyst and the mechanism of the reaction was examined by the ONIOM method. The energy profiles for both trans- and cis-forms were optimized and determined." ... 

Some use of NMR has also been made in elucidating the stereochemistry of the hydrogenation by Wilkinson s catalyst of the deuterium-labeled N-acetylisodehydrovaline (XXIX) with a hydrogen tritide—hydrogen mixture (61). The two diastereoiso-... 

In Section 5.9, we saw that alkenes can be converted to alkanes by catalytic hydrogenation by a variety of catalysts, such as palladium and platinum. These are heterogeneous catalysts. We also noted that homogeneous catalytic hydrogenation can be carried out by Wilkinsons catalyst, Ru[(PPh3)3Cl. We now return to that subject to discuss the reaction mechanism. We will find that hydrogenation by Wilkinson s catalyst occurs in a catalytic cycle that is strikingly similar to the catalytic cycles of the reactions we have discussed thus far in this chapter. The transition metal in the Wilkinson catalyst, however, is ruthenium, not palladium. 

Under a wide range of conditions, homogeneous hydrogenation by Wilkinson s catalyst is consistent with the approximate rate law ... 

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