Olefin Hydrogenation with Wilkinsons Catalyst

Olefin Hydrogenation with Wilkinson s Catalyst 2.2.1. Background and Rate Laws 

Spectroscopic studies under catalytic olefin hydrogenation conditions without added L show the presence of RhClLs (43), H2RhClL3 (44), (RhClL2)2 (45), and H2(RhClL2)2 (46). (In the case of ethylene (C2H4)RhClL2 (47) is also observed.) The amount of the hydrides increases with H2 pressure, while the dimeric species are increased by 

Both steps 5 and 3 were assumed to be rapid and reversible. 

The two terms arise from the two principal species present under catalytic cyclohexene hydrogenation conditions—RhClL3 and H2RhClL3, respectively. An additional constant term, like that shown in Rate Law No. 4 in Table 2.2, is not necessary unless a significant amount of another intermediate [H2RhClL2(S)] is also present in the system under conditions where the rate law applies, t 

The complexity of the Wilkinson hydrogenation system emphasizes the need to specify the ranges of variables over which a rate law applies, and it illustrates the power of combining spectroscopic studies with kinetic studies of the individual steps. 

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Example 8.9. Olefin hydrogenation with Wilkinson s catalyst. Wilkinson s catalyst is a dihydrido-chloro-phosphino complex of rhodium, H2RhClPh3, where Ph is an organic phosphine such as triphenyl phosphine [48-52]. The dominant mechanism of olefin hydrogenation with this catalyst, established chiefly by Halpem [53-55] in detailed studies that included measurements of equilibria in the absence of reactants and of reaction rates of isolated participants, backed by independent NMR studies [56] and ab initio molecular orbital calculations [57], is shown as 8.69 on the facing page (without minor parallel pathways and side reactions). 

Examples include acetal hydrolysis, base-catalyzed aldol condensation, olefin hydroformylation catalyzed by phosphine-substituted cobalt hydrocarbonyls, phosphate transfer in biological systems, enzymatic transamination, adiponitrile synthesis via hydrocyanation, olefin hydrogenation with Wilkinson s catalyst, and osmium tetroxide-catalyzed asymmetric dihydroxylation of olefins. 

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. 

The Hartree-Fock method was in any case the method of choice for the first quantitative calculations related to homogeneous catalysis. It was the method, for instance, on a study of the bonding between manganese and hydride in Mn-H, published in 1973 [28]. The first studies on single steps of catalytic cycles in the early 1980 s used the HF method [29]. And it was also the method applied in the first calculation of a full catalytic cycle, which was the hydrogenation of olefins with the Wilkinson catalyst in 1987 [30]. The limitations of the method were nevertheless soon noticed, and already in the late 1980 s, the importance of electron correlation was being recognized [31]. These approaches will be discussed in detail in the next section. 

When the catalyst was used for simple olefin systems, it was not as active as with the amino acid precursors. Table III shows the relative rates for a variety of substrates, special care being taken in each case to purge oxygen. The slow rate of a-phenylacrylic acid was unexpected, but, it may be the result of a stable olefin-rhodium complex similar to the one Wilkinson (15) experienced with ethylene. Such a contention is consistent with the increased speed of hydrogenation with increased pressure. 

Figure 2. Reactivity sequence of different types of olefins toward hydrogenation with the Wilkinson catalyst RhCKPPhs),.

The catalyst [CoH(CN)5] is soluble in water. It is selective for the reduction of olefinic double bonds in a,y9-unsaturated systems. Reduction of NO2 groups only occurs at elevated pressures. Hydrogenolysis of C-Hal bonds is observed [20]. Progress as far as water-soluble hydrogenation catalysts is concerned has also been made with Wilkinson-type catalysts by using phosphine ligands with sulfonic acid substituents [44]. 

Enantioselective hydrogenation of prochiral carbonyl compounds with Wilkinson-type catalysts is less successful than the hydrogenation of prochiral olefins. Both rates and enantioselectivities are greatly diminished in the hydrogenation of ketones, compared with olefins. Enantioselectivities only occasionally reach 80% ee, e. g., in the hydrogenation of acetophenone with the in-situ catalyst [Rh(nbd)Cl]2/DIOP, where nbd = norbomadiene [71]. The Ru-based BINAP catalysts improved this situation, by allowing the hydrogenation of a variety of functionalized ketones in enantioselectivities close to 100% ee [72]. 

Transition metal phosphine complexes provide another important class of hydrogenation catalysts. Wilkinson s complexes, RhClfPPhjlj and RuHClfPPhjlj, well known for their olefin hydrogenation activity, were shown by Fish [77, 97] to be also good precursors for the reduction of polyaromatic substrates under mild reaction conditions (85° C, ca. 20 atm H, ). following a general activity trend consistent with a combination of electronic and steric factors ... 

The development of catalysts for asymmetric hydrogenation began with the concept of replacing the triphenylphosphane ligand of the Wilkinson catalyst with a chiral ligand. With these new catalysts, it should be possible to hydrogenate pro-chiral olefins. 

Historically, since the Wilkinson catalyst [RhCKPPhjj] proved to be active for hydrosilylation of ketones with hydrosilanes in the early 1970s, the asymmetric version has been examined by using many optically active phosphorus ligands, which were mainly developed for the asymmetric hydrogenation of olefins. Many ketones were cataly tically reduced in high yields with moderate enantioselectiv-ities. As representative milestones, the first trial with (-l-)-DIOP (LI Fig. 1) and... 

While the homogeneous hydrogenation of olefins catalyzed by group VIII metal complexes is today a rather commonplace reaction and the CsMes-M complexes oflFer mainly rather higher rates by comparison with the Wilkinson catalyst, very few homogeneous catalysts so far reported are active for the hydrogenation of arenes. 

In order to eliminate the possibility for in situ carbene formation Raubenheimer et al. synthesized l-alkyl-2,3-dimethylimidazolium triflate ionic liquids and applied these as solvents in the rhodium catalyzed hydroformylation of l-hejEne and 1-dodecene [178]. Both, the classical Wilkinson type complex [RhCl(TPP)3] and the chiral, stereochemically pure complex (—)-(j7 -cycloocta-l,5-diene)-(2-menthyl-4,7-dimethylindenyl)rhodium(i) were applied. The Wilkinson catalyst showed low selectivity towards n-aldehydes whereas the chiral catalyst formed branched aldehydes predominantly. Hydrogenation was significant with up to 44% alkanes being formed and also a significant activity for olefin isomerization was observed. Additionally, hydroformylation was found to be slower in the ionic liquid than in toluene. Some of the findings were attributed by the authors to the lower gas solubility in the ionic liquid and the slower diffusion of the reactive gases H2 and CO into the ionic medium. 

The activity and selectivity of the bound Wilkinson catalyst parallels in many ways the homogeneous system. Terminal olefins are hydrogenated more rapidly than internal olefins, cw-olefins react faster than rran5-olefins, and olefins are reduced faster than acetylenes.There were some differences with the sterically hindered l(7)-p-menthene the polymeric catalyst, for example, produced appreciably more isomerized material. Furthermore, the attached complex was more selective in the hydrogenation of 1,3-cyclooctadiene. 

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. 

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