Wilkinson complex, alkene 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... 

Most low-valence metal complexes are generally deactivated by air and sometimes also by water. Carbon monoxide, hydrogen cyanide, and PH3 frequently act as poisons for these catalysts. Poisoning by strongly co-ordinating molecules occurs by formation of catalytically inert complexes. An example is the poisoning of Wilkinson s catalyst for alkene hydrogenation ... 

The mechanism of alkene hydrogenation catalyzed by the neutral rhodium complex RhCl(PPh3)3 (Wilkinson s catalyst) has been characterized in detail by Halpern [36-38]. The hydrogen oxidative addition step involves initial dissociation of PPI13, which enhances the rate of hydrogen activation by a factor... 

One last remark concerning the two catalysts we have discussed in more detail, cationic rhodium catalysts and the neutral chloride catalyst of Wilkinson. The difference of the catalytic system discussed above from that of the Wilkinson catalyst lies in the sequence of the oxidative addition and the alkene complexation. The hydrogenation of the cinnamic acid derivative involves a cationic catalyst that first forms the alkene complex the intermediate alkene (enamide) complex can be observed spectroscopically. 

SCHEME 2. Hydrogenation of alkenes catalyzed by the Wilkinson complex. 

The difference of the catalytic system discussed above from that of the Wilkinson catalyst lies in the sequence of the oxidative addition and the alkene complexation. The hydrogenation of the cinnamic acid derivative involves a cationic catalyst of... 

Metal-phosphine complexes can be inunobihzed on phos-phinated polymers through simple ligand exchange reactions. Wilkinson s Catalyst, Rh(PPh3)3Cl, has been supported on phosphinated polystyrene as shown in equation (9). The supported complex can be used as an alkene hydrogenation catalyst. 

It is interesting to note that HRh(CO)(PR3)3 is an efficient alkene hydrogenation catalyst. It also resembles the very effective hydrogenation catalyst, ClRh(PPh3)3 (Wilkinson s catalyst, Section 9-4-2). Apparently, when the partial pressure of CO exceeds 10 bar, the capability of the complex to hydrogenate alkenes is suppressed. 

The metal complexes most often studied as polymer-bound catalysts have been Rh(I) complexes, such as analogues of Wilkinson s complex. The catalytic activity of a bound metal complex is nearly the same as that of the soluble analogue. Rhodium complexes are active for alkene hydrogenation, alkene hydroformylation, and, in the presence of CH3I cocatalyst, methanol carbonylation, etc. Polymer supports thus allow the chemistry of homogeneous catalysis to take place with the benefits of an insoluble, easily separated catalyst . ... 

Wilkinson s catalyst (26.4) has been widely studied, and in its presence alkene hydrogenation can be carried out at 298 K and 1 bar H2 pressure. The red, 16-electron Rh(I) complex 26.4 can be prepared from RhCl3 and PPh3, and is commonly used in benzene/ethanol solution, in which it dissociates to some extent (equilibrium 26.8) a solvent molecule (solv) fills the fourth site in RhCl(PPh3)2 to give RhCl(PPh3)2(solv). 

Rhodium is a good catalyst for alkene hydrogenation (Section 6.1), as are many of its complexes such as tris(triphenylphosphine)rhodium chloride (Wilkinson s catalyst). 

Diisocyano-Rh dimers photocatalytically decompose water [13] and diisocyano complexes catalyze hydrogenation and isomerisation of alkenes and alkynes (although they are far less active than the Wilkinsons catalyst [14]). Alkene hydrogenation is a probe reaction for such reaction centres, especially since the hydrogenation of alk-l-enes over Wilkinson s catalyst [hydrido-carbonyl tris(triphenylphosphine).Rh ] in benzene is quite selective (i.e. was 45 times faster than the cis-aIk-2-ene [15]). Surprisingly, the active centres in such catalysts are still not entirely understood, despite extensive analysis [16]. Rh complexed with 4,4 -diisocyanobiphenyl and 1,4-diisocyanobenzene is active in hydrogenation and isomerization of 1-hexene [14], while Rh complexed with aliphatic amines is active in catalysis of hydrogenation of alkenes and cycloalkenes [16]. 

Figure 3.2 The basic mechanism of alkene hydrogenation using Wilkinson s complex as...

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

The hydrogenation activity of the isolated hydrides 3 and 6 towards cyclooctene or 1-octene was much lower than the Wilkinson s complex, [RhCKPPhj) ], under the same conditions [2] furthermore, isomerisation of the terminal to internal alkenes competed with the hydrogenation reaction. The reduced activity may be related to the high stability of the Rh(III) hydrides, while displacement of a coordinated NHC by alkene may lead to decomposition and Rh metal formation. 

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