Wilkinson catalytic cycle

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... 

Figure C2.7.2. Catalytic cycle (witliin dashed lines) for tire Wilkinson hydrogenation of alkene [2]. Values of rate and equilibrium constants are given in [2]...

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. 

The Wilkinson hydrogenation cycle shown in Figure 3 (16) was worked out in experiments that included isolation and identification of individual rhodium complexes, measurements of equiUbria of individual steps, deterrnination of rates of individual steps under conditions of stoichiometric reaction with certain reactants missing so that the catalytic cycle could not occur, and deterrnination of rates of the overall catalytic reaction. The cycle demonstrates some generally important points about catalysis the predominant species present in the reacting solution and the only ones that are easily observable by spectroscopic methods, eg, RhCl[P(CgH 2]3> 6 5)312 (olefin), and RhCl2[P(CgH )2]4, are outside the cycle, possibly in virtual equiUbrium with... 

A reaction mechanism may involve one of two types of sequence, open or closed (Wilkinson, 1980, pp. 40,176). In an open sequence, each reactive intermediate is produced in only one step and disappears in another. In a closed sequence, in addition to steps in which a reactive intermediate is initially produced and ultimately consumed, there are steps in which it is consumed and reproduced in a cyclic sequence which gives rise to a chain reaction. We give examples to illustrate these in the next sections. Catalytic reactions are a special type of closed mechanism in which the catalyst species forms reaction intermediates. The catalyst is regenerated after product formation to participate in repeated (catalytic) cycles. Catalysts can be involved in both homogeneous and heterogeneous systems (Chapter 8). 

A variety of six-membered carbocycles and heterocycles were synthesized by Shibata et al.81 using Wilkinson s catalyst (Equation (79)). The proposed catalytic cycle (Scheme 24) rationalizes the exclusive formation of the (Z)-isomer. Additionally, the mechanism is supported by the results of a isotope-labeling study reported by Brummond... 

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 first step in the catalytic cycle is the dissociation of a phosphine ligand from Wilkinson s catalyst which produces a highly reactive trigonal planar rhodium centre, compound B. Oxidative addition of H2 to B affords C which then undergoes association of the C=C compound to afford D. One of the hydride ligands undergoes transfer to the C=C bond affording a coordinated alkyl as... 

Historically, high-pressure IR spectroscopy has been one of the most important methods to measure intermediates or resting-state species in catalytic cycles. In 1%8, Wilkinson observed HRh(PPh3)2(CO)2 in the Rh/PPh3 catalyst system by IR spectroscopy where an IR cell was connected via a tube to the autoclave. A related study was performed more recently by Moser et who applied their cylindrical internal reflectance IR cell. They determined the rate-... 

This rate is usually referred to as the turnover frequency and it is the number of molecules reacting per active site per unit time at the conditions of the experiment (Boudart, 1985 McNaught and Wilkinson, 1997 Fogler, 1999). Boudart (1995) used the term turnover frequency to define the number of revolutions of the catalytic cycle per unit time and active site. In each revolution, one mole of reactant is consumed. For example, the revolution of a catalytic cycle for S02 oxidation is shown in Figure 3.1. 

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 ... 

According to more recent studies,599 the catalytic cycle shown here for the Wilkinson complex and a boronic ester includes the most probable 48 Rh(m) intermediate formed by the addition of the B—H bond (Scheme 6.9). 

Figure 31-3 Catalytic cycle for the hydroformylation of alkenes as developed by G. Wilkinson. The stereochemical configurations of the participants in the cycle are uncertain.

The widely accepted mechanism for olefin hydroformylation using a HRh(PR3)2(CO) catalyst system was proposed over 30 years ago by Wilkinson et al [104]. The catalytic cycle comprises many of the fundamental reac-... 

Figures 7 and 8 show the intermediates generated by TAMREAC. It should be noted that we have not discarded 20-electron intermediates (intermediates 2 and 8, Fig. 7) in order to find the associative pathway proposed by Wilkinson and co-workers (124a,124b). Figure 9 shows the most probable catalytic cycle, which proceeds via a dissociative pathway (124a), and Figs. 10 and 11 the novel cycles found by the program involving an intramolecular carbonylation.

The coupling of independent catalytic cycles for both radical generation and reduction has been realized by the combination of the titanocene catalyzed reductive epoxide opening [36—4-0] via electron transfer and the catalytic reduction of radicals after H2 activation by Wilkinson s complex [Rh(PPh3)3Cl] as shown in Scheme 16 [41—43],... 

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. 

The basic mechanism of hydrogenation is shown by the catalytic cycle in Fig. 7.3. This cycle is simplified, and some reactions are not shown. Intermediate 7.9 is a 14-electron complex (see Section 2.1). Phosphine dissociation of Wilkinson s complex leads to its formation. Conversion of 7.9 to 7.10 is a simple oxidative addition of H2 to the former. Coordination by the alkene, for example, 1-butene, generates 7.11. Subsequent insertion of the alkene into the metal-hydrogen bond gives the metal alkyl species 7.12. The latter undergoes reductive elimination of butane and regenerates 7.9. 

The most important catalyst in this class is Wilkinson s Catalyst, chlorotris(triphenylphosphine)rhodium(I). The catalytic cycle of this complex is shown in Scheme 7. The basic catalytic cycle is very simple, but parasitic side reactions make its study more difficult. 

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. 

The hydroformylation mechanism for phosphine-modified rhodium catalysts follows with minor modifications the Heck-Breslow cycle. HRh(CO)(TPP)3 [11] is believed to be the precursor of the active hydroformylation species. First synthesized by Vaska in 1963 [98] and structurally characterized in the same year [99], Wilkinson introduced this phosphine-stabilized rhodium catalyst to hydroformylation five years later [100]. As one of life s ironies, Vaska even compared HRh(CO)(TPP)3 in detail with HCo(CO)4 as an example of structurally related hy-drido complexes [98]. Unfortunately he did not draw the conclusion that the rhodium complex should be used in the oxo reaction. According to Wilkinson, two possible pathways are imaginable the associative and the dissociative mechanisms. Preceding the catalytic cycle are several equilibria which generate the key intermediate HRh(CO)2(TPP)2 (Scheme 4 L = ligand). 

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