Wilkinson olefin hydrogenation

Figure 2 Catalytic cycle of the Wilkinson olefin hydrogenation, as proposed by Halpem et al. ...

Wilkinson Hyd.rogena.tion, One of the best understood catalytic cycles is that for olefin hydrogenation in the presence of phosphine complexes of rhodium, the Wilkinson hydrogenation (14,15). The reactions of a number of olefins, eg, cyclohexene and styrene, are rapid, taking place even at room temperature and atmospheric pressure but the reaction of ethylene is extremely slow. Complexes of a number of transition metals in addition to rhodium are active for the reaction. 

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. 

Potential Energy Surface of Olefin Hydrogenation by Wilkinson Catalyst... 

In this work, we have compared the potential energy profiles of the model catalytic cycle of olefin hydrogenation by the Wilkinson catalyst between the Halpern and the Brown mechanisms. The former is a well-accepted mechanism in which all the intermediates have trans phosphines, while in the latter, proposed very recently, phosphines are located cis to each other to reduce the steric repulsion between bulky olefin and phosphines. Our ab initio calculations on a sterically unhindered model catalytic cycle have shown that the profile for the Halpern mechanism is smooth without too stable intermediates and too high activation barrier. On the other hand, the key cis dihydride intermediate in the cis mechanism is electronically unstable and normally the sequence of elementary reactions would be broken. Possible sequences of reactions can be proposed from our calculation, if one assumes that steric effects of bulky olefin substituents prohibits some intermediates or reactions to be realized. 

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 ... 

Olefin hydrogenation has been known since 1966, when Wilkinson and cowork-ers reported the homogeneous hydrogenation of olefins by rhodium catalysts (see... 

Another example, to which we will return later, is square-planar [Rh(PH3)3 Cl] (Fig. 2). This molecule is used as a model for Wilkinson s catalyst, [Rh(PPh3)3Cl], in an extensive investigation of the catalytic cycle for olefin hydrogenation (Sect. 4.4) [85]. However, Bertran has demonstrated [86]... 

Fig. 5. Reaction scheme for olefin hydrogenation by the Wilkinson s catalyst model complex Rh(PH3)2Cl studied by the Hartree-Fock method in Ref. 85...

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

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. 

All of the olefins discussed so far contain a functional group, other than the C=C bond, that binds to the metal to create a defined structure. The asymmetric hydrogenation of olefins that lack this second functional group has been a major challenge. Few complexes of any type catalyze the hydrogenation of tri-substituted and tetra-substituted olefins, let alone catalyze asymmetric hydrogenation of these olefins. Recall from Section 15.3 on achiral catalysts for olefin hydrogenation that Wilkinson s catalyst and ruthenium-hydride complexes display little reactivity for the reduction of tri-substituted alkenes, and no reactivity for... 

Kinetics of the olefin hydrogenation reaction were investigated most accurately in the case of cyclohexene and styrene with the application of Wilkinson s catalyst [RhCl(PPh3)3]. 2"-"" ... 

Halpern s proposed mechanism (13.28) is similar in nature. Although his main steps are analogous to the those in the mechanism proposed by Wilkinson, Petit, and de Croon, the kinetic equations for the olefin hydrogenation reaction are considerably different for the latter authors ... 

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." ... 

In 1965, Wilkinson (together with his former Ph.D. student John Osborn), and Coffey independently discovered the first homogeneous olefin hydrogenation catalyst, Wilkinson s catalyst, [RhCl(PPh3)3]. In 1970, Henri Kagan reported in a patent... 

Figure 4 Potential energy profile of the entire catalytic cycle of the Halpem mechanism of olefin hydrogenation by the Wilkinson catalyst, in kcal mol at the RHF level relative to 13 + C2H4 + H2. Numbers in parenthesis are MP2 energies for the RHF structures, relative to 15...

Thus, the mechanisms of olefin hydrogenation are different for the catalyst systems. The Wilkinson complex accepts H2 prior to olefin, whereas the cationic Rh(I) catalyst forms the olefin complex first, which reacts with H2 subsequently. In either case the dihydride complex is the key intermediate, and two hydride ligands add successively to the olefin molecule situated at the adjacent coordination site. 

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