Hydrogenation Using Wilkinson’s catalyst

Figure 9.3 The mechanism of homogeneous hydrogenation using Wilkinson s catalyst.

The first two steps in this process give the catalytic species RhCl(H)2(PPh3)2, which has a vacant coordination site. A C=C double bond can coordinate to this site, gain the two hydrogens coordinated to Rh, and subsequently leave, if the double bond is not sterically hindered. This effect is illustrated in Table 14-3, which shows relative rates of hydrogenation using Wilkinson s catalyst. 

Equation 9.26 shows an example of hydrogenation using Wilkinson s catalyst. The use of D2 rather than H2 clearly indicates the stereochemistry of syn addition typical of Rh-catalyzed hydrogenation. 

Hydrogenation using Wilkinson s catalyst is still used for diastereoselective reduction as in the transformation of the functionalised enone 5 into the ketone 6 with high diastereoselectivity.3 Other (homogeneous or heterogeneous) catalysts were less effective. 

TABLE 14.5 Relative Rates of Hydrogenation Using Wilkinson s Catalyst at 25 °C... 

The mechanism of homogeneous catalytic hydrogenation involves reactions characteristic of transition metal organometallic compounds. A general scheme for hydrogenation using Wilkinson s catalyst is shown here. We have seen structural details of the mechanism in Section G.7. 

Because the insertion step 4 and the reductive elimination step 5 are stereospecific, the net result of the hydrogenation using Wilkinson s catalyst is a syn addition of hydrogen to the alkene. The following example (with D2 in place of H2) illustrates this aspect. 

Hydrogenation using Wilkinson s catalyst has been studied in great detail and some features are still not understood. A simplified scheme, based on Tolman s rules is given in Fig. 5.20. 

Scheme 11.79). The double cycloisomerization proceeds with double lactonization. The undesired double bonds, being the less hindered, were removed by hydrogenation using Wilkinson s catalyst to give the natural product 11.237. A synthesis of the related natural product, dehydrohomoancepsenolide, can be found in Scheme 8.131. 

Having seen steps such as oxidative addition, insertion, and reductive elimination in the context of transition metal-catalyzed hydrogenation using Wilkinson s catalyst, we can now see how these same types of mechanistic steps are involved in a mechanism proposed for the Heck-Mizokori reaction. Aspects of the Heck-Mizokori mechanism are similar to steps proposed for other cross-coupling reactions as well, although there are variations and certain steps that are specific to each, and not all of the steps below are involved or serve the same purpose in other cross-coupling reactions. 

FIGURE 23.27 Mechanism of hydrogenation using Wilkinson s catalyst. The sections in blue are not on the main catalytic pathway although they occur at measurable rates, these are at least 1000... 

Scheme 4.17 Simplified alkene hydrogenation mechanism using Wilkinson s catalyst...

FIGURE 22.8 The hydrogenation of alkenes using Wilkinson s catalyst. L is P(C6H5)3 or perhaps a solvent molecule. 

Some disagreement exists regarding the structure of the transition state (whether H or P3 is trans to the alkene) and whether or not solvent molecules occupy sites that are apparently vacant. In spite of some uncertainty regarding these details, the major issues regarding the catalyzed hydrogenation of alkenes using Wilkinson s catalyst are fairly well understood. 

For hydrogenation to take place, the substrate usually needs to bind to the metal complex, although exceptions are known to this rule [25]. Substrate inhibition can occur in a number of ways, for example if more than one molecule of substrate binds to the metal complex. At low concentration this may be a minor species, whereas at high substrate concentration this may be the only species. One example of this is the hydrogenation of allyl alcohol using Wilkinson s catalyst. Here, the rate dependence on the substrate concentration went through a maximum at 1.2 mmol IT1. The authors propose that this is caused by formation of a complex containing two molecules of allyl alcohol (Scheme 44.1) [26],... 

Eshova et al. studied the hydrogenation of C02 to formic acid in several solvents in the presence of an equivalent of Et3N using Wilkinson s catalyst. These authors conducted an extensive NMR study into the various decomposition pathways of the catalyst [56]. Apparently, DMSO is capable of rapidly displacing one equivalent of PPh3 on the catalyst a second equivalent is slowly displaced. 

Scheme 1. The mechanism of hydrogenation of simple alkenes using Wilkinson s catalyst RhCI(PPh3)3 (S = solvent, P = PPh3).[2]...

It is also worth noting that the solvent used can have a considerable influence on the outcome of a hydrogenation reaction. For example, the hydrogenation of the functional alkene shown in Scheme 8.3, using Wilkinson s catalyst, varies considerably according to the conventional organic solvent used [2],... 

The dispersions were obtained by emulsification via ultrasonication of a toluene solution of the unsaturated homopolymer in an aqueous surfactant solution. This was followed by exhaustive hydrogenation with Wilkinson s catalyst at 60°C and 80 bar H2 to produce a dispersion with an average particle size of 35 nm (dynamic light scattering and transmission electron microscopy analyses). The same a,co-diene was used as comonomer in the ADMET polymerization of a phosphorus-based monomer, also containing two 10-undecenoic acid moieties... 

The major problem with using Wilkinson s catalyst is that it also constitutes an excellent hydrogenation catalyst [56]. Thus, alkynes and terminal alkenes are not tolerated under the conditions of the coupled catalytic cycles. This implies that radical cyclizations terminated by a CHAT cannot be carried out under these conditions. 

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. 

Water-soluble rhodium complexes bearing sulfonated triphenylphos-phine ligands can catalyze the reduction of cyclohexene in a two-phase system. It is also possible to use Wilkinson s catalyst [(Ph3P)3RhCl] for the hydrogenation of water-soluble olefins in an aqueous-benzene solvent system (46). 

In the hydrogenation of an alkene using Wilkinson s catalyst, Rh(PPh3)2(RCH=CHi)-CI(H)(H) reacts to give Rh(PPh3)2CI(solvent) and an alkane. In this reaction rhodium is reduced from +3 to +1 and an alkene is reduced to an alkane. What is oxidized ... 

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... 

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