Wilkinson hydrogenation

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]...

C2.7.6.1 WILKINSON HYDROGENATION OF ALKENES CATALYSED BY A RHODIUM COMPLEX... 

There is more to tire Wilkinson hydrogenation mechanism tlian tire cycle itself a number of species in tire cycle are drained away by reaction to fomi species outside tire cycle. Thus, for example, PPh (Ph is phenyl) drains rhodium from tire cycle and tlius it inliibits tire catalytic reaction (slows it down). However, PPh plays anotlier, essential role—it is part of tire catalytically active species and, as an electron-donor ligand, it affects tire reactivities of tire intemiediates in tire cycle in such a way tliat tliey react rapidly and lead to catalysis. Thus, tliere is a tradeoff tliat implies an optimum ratio of PPh to Rli. 

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... 

The strategy of the catalyst development was to use a rhodium complex similar to those of the Wilkinson hydrogenation but containing bulky chiral ligands in an attempt to direct the stereochemistry of the catalytic reaction to favor the desired L isomer of the product (17). Active and stereoselective catalysts have been found and used in commercial practice, although there is now a more economical route to L-dopa than through hydrogenation of the prochiral precursor. 

Wilkinson hydrogenation, 5 210 Williams-Landel-Ferry (WLF) equation, 21 710... 

Figure 1 Mechanism of action of the Wilkinson hydrogenation catalyst the dashed line encloses the predominant part of the cycle.

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. 

These activities are understood to reflect the catalytic actions of metal phosphines, e.g., Wilkinson hydrogenation catalyst, (PPh3)3RhCl, metal carbonyls, e.g., hydrogenation catalysts, Ni(CO)5), and cyclopetadienyl metal compounds, e.g., olefln polymerization catalysts, Cp2ZrCl2. 

The steric and electronic properties of phosphines also affect the overall geometry of a complex. For example, bulky phosphines tend to bind trans to one another. Furthermore, the presence of several bulky phosphine ligands in the same coordination sphere can cause deviations from the idealized coordination geometry. For example, the Wilkinson hydrogenation catalyst, RhCl(PPh3)3, contains a nonplanar arrangement of the four donor atoms, as determined by X-ray diffraction, instead of the square planar arrangement expected for a d , Rh(I) complex. 

Mechanistic Study of Wilkinson Hydrogenation Tolman et al were able to deduce the initial events in the mechanism of Wilkinson hydrogenation (Eq. 10.1 and Section 9.2) from the P NMR data shown in Fig. 10.4. Spectrum A shows the proton-decoupled P NMR of RhCl(PPh3)3 itself. Two typ s of phosphorus are seen in a 2 1 ratio, P and Pb in 10.7, each showing coupling to Rh (/ = i, 100% abundance). P, also shows a cis coupling to Pb... 

The catalytic cycle The mechanism of Wilkinson hydrogenation is briefly outlined here, mainly to demonstrate a procedure for formulating appropriate catalytic cycles and formulating rate models. The complete cycle for Wilkinson hydrogenation is given in Figure 15.4, based on which the following conclusions are important ... 

Wilkinson and Coffey independently discover the Wilkinson hydrogenation complex (PPh3)3RhCl [106]. 

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