Wilkinson catalyst, catalytic cycle hydrogenation

A well-understood catalytic cycle is tliat of the Wilkinson alkene hydrogenation (figure C2.7.2) [2]. Like most catalytic cycles, tliat shown in figure C2.7.2 is complex, involving intennediate species in tire cycle (inside tire dashed line) and otlier species outside tire cycle and in dead-end patlis. Knowledge of all but a small number of catalytic cycles is only fragmentary because of tire complexity and because, if tire catalyst is active, tire cycle turns over rapidly and tire concentrations of tire intennediates are minute thus, tliese intennediates are often not even... 

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. 

Perhaps the best known homogeneous hydrogenation catalyst is Wilkinson s catalyst, Rh(PPh3)3Cl, named after the Nobel Laureate who discovered this extremely important compound. The mechanism by which Rh(PPh3)3Cl catalyses the hydrogenation reaction has been intensively studied and involves a series of steps which are illustrated in the catalytic cycle in Scheme 8.2. 

The widely known Wilkinson catalyst is proposed to operate through this reaction mechanism. Computational evaluation of the full catalytic cycle showed that the rate-determining step implies the insertion and the subsequent isomerization process (27). Moreover, this catalyst has the particularity that the reaction mechanism depends on the hydrogen source since a monohydridic route has been proposed when 2-propanol is the hydrogen source (28). 

This sequence of events may be illustrated by the homogeneous hydrogenation of ethylene in (say) benzene solution by Wilkinson s catalyst, RhCl(PPh3)3 (Ph = phenyl, CeH5 omitted for clarity in cycle 18.10). In that square-planar complex, the central rhodium atom is stabilized in the oxidation state I by acceptance of excess electron density into the 3d orbitals of the triphenylphosphane ligands but is readily oxidized to rhodium (III), which is preferentially six coordinate. Thus, we have a typical candidate for a catalytic cycle of oxidative addition and subsequent reductive elimination ... 

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. 

It has been our goal to design a catalytic system theoretically. To the end of this goal, we have so far analyzed the organometallic reactions by using the ab initio MO calculations. Recently, we have completed the theoretical study of the catalytic cycle of hydrogenation by the Wilkinson catalyst (2), of which mechanism has been proposed by Halpern (3). This catalytic cycle shown in Scheme 1 consists of oxidative addition of H, coordination of olefin, olefin insertion, isomerization, and reductive elimina-... 

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. 

Every late-metal hydrogenation catalyst, whether homogeneous or heterogeneous, probably uses exactly the same catalytic cycle, although some catalysts, particularly sterically encumbered ones such as Wilkinson s catalyst, require that ligand dissociation or substitution occur before the catalytic cycle gets underway. In the case of metals supported on solids such as activated C, silica, and alumina, the support may participate in the reaction in ways that need not concern you here. 

The discovery of Wilkinson s catalyst led to the development of a new class of complexes capable of promoting hydrogenation these have the general formula I. M+ (M = Rh or Ir). The Rh series was first reported by Schrock and Osborn 81 equation 9.30 demonstrates how such a complex may be prepared. The cationic Rh(I) complex (60), interacts with a solvent such as THF or acetone to give a 12-electron, unsaturated diphosphine intermediate (61), which is considered the active catalyst. The catalytic cycle begins this time with alkene binding, followed by the oxidative addition of H2. 

Fig.3. The catalytic cycle for Wilkinson s catalyst in hydrogenation of simple alkenes...

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. 26.5 Catalytic cycle for the hydrogenation of RCH=CH2 using Wilkinson s catalyst, RhCl(PPh3)3.

Figure 8.6 Catalytic cycle for hydrogenation of olefins using a Wilkinson catalyst (redrawn from Halpern et al., 1976). L = PPh3, = vacant coordination site, rds = ratedetermining step...

Halpern studied the rate behavior of the overall catalytic hydrogenation of cyclohexene by Wilkinson s catalyst, as well as the rate behavior of the individual steps of the proposed catalytic cycle. The core of the catalytic cycle was shown in Scheme 15.1 to illustrate the hydrogen-first pathway, but a more complete view of the pathway deduced from Halpern s studies is shown in Scheme 15.6. This overall scheme was deduced by isolating and determining the rate behavior of each individual stage of the catalytic cycle, demonstrating that these steps can account for the kinetic behavior of the overall catalytic reaction, and demonstrating that several previously isolated compounds were not kinetically competent to be intermediates in the catalytic process. 

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