Rhodium-catalyzed hydrogenation Wilkinson complex

The potential of microemulsions for organometaUic-catalyzed hydrogenations in water/scC02 biphasic systems has been assessed using the rhodium-catalyzed hydrogenation of styrene as a common test reaction [Eq. (7)] [31]. The water-soluble Wilkinson complex [RhCl(TPPTS)3] was applied as catalyst precursor together with anionic perfluoropolyether carboxylates, cationic Lodyne A, or nonionic poly-(butene oxide)-b-poly(ethylene oxide) surfactants. The interfacial tension is small in the presence of the supercritical fluid and small amounts of surfactant (0.1-2.0 wt.%) suffice to form stable microemulsions. The droplet diameter of the microemulsions varied between 0.5 and 15 pm and a surface area of up to 10 m was obtained. 

The discovery of Wilkinson complex, RhCl[P(C6H5)3]3, acting as an effective catalyst for hydrogenation of olefins opened the door for developing asymmetric reaction catalyzed by rhodium complexes with a chiral phosphine ligand. 

In order to eliminate the possibility for in situ carbene formation Raubenheimer et al. synthesized l-alkyl-2,3-dimethylimidazolium triflate ionic liquids and applied these as solvents in the rhodium catalyzed hydroformylation of l-hejEne and 1-dodecene [178]. Both, the classical Wilkinson type complex [RhCl(TPP)3] and the chiral, stereochemically pure complex (—)-(j7 -cycloocta-l,5-diene)-(2-menthyl-4,7-dimethylindenyl)rhodium(i) were applied. The Wilkinson catalyst showed low selectivity towards n-aldehydes whereas the chiral catalyst formed branched aldehydes predominantly. Hydrogenation was significant with up to 44% alkanes being formed and also a significant activity for olefin isomerization was observed. Additionally, hydroformylation was found to be slower in the ionic liquid than in toluene. Some of the findings were attributed by the authors to the lower gas solubility in the ionic liquid and the slower diffusion of the reactive gases H2 and CO into the ionic medium. 

All of these reactions involve transition metals such as palladium, copper, and ruthenium, usually in complex with certain types of ligands. After we see the practical applications of these reactions for carbon—carbon bond formation, we shall consider some general aspects of transition metal complex structure and representative steps in the mechanisms of transition metal—catalyzed reactions. We shall consider as specific examples the mechanism for a transition metal—catalyzed hydrogenation using a rhodium complex called Wilkinsons catalyst, and the mechanism for the Heck—Mizoroki reaction. 

Catalyst or Catalyst Precursor In Situ Catalyst or Preformed Catalyst. A major consideration in homogeneous processes is whether to use a catalyst precursor or to use a compound actually involved in the catalytic cycle. A catalyst precursor is a compound that is not itself the catalyst but can react to form a catalyst. For example, rhodium trichloride does not itself catalyze hydrogenation but can be reacted with triphenylphosphine to form Wilkinson s catalyst (1). Catalyst precursors are generally cheaper and are easier to handle than the catalysts themselves. However, a catalyst formed in the complex chemical mixture of a reaction vessel will normally be less pure than if it were synthesized separately. In some cases, catalyst purity is critical to the reaction. For example, in enantioselective reactions an optically pure catalyst may be needed. Small differences in catalyst structure can have a large effect on enantioselectivity (2). 

A proposed mechanism [9] for the hydrosilylation of olefins catalyzed by platinum(II) complexes (chloroplatinic acid is thought to be reduced to a plati-num(II) species in the early stages of the catalytic reaction) is similar to that for the rhodium(I) complex-catalyzed hydrogenation of olefins, which was advanced mostly by Wilkinson and his co-workers [10]. Besides the Speier s catalyst, it has been shown that tertiary phosphine complexes of nickel [11], palladium [12], platinum [13], and rhodium [14] are also effective as catalysts, and homogeneous catalysis by these Group VIII transition metal complexes is our present concern. In addition, as we will see later, hydrosilanes with chlorine, alkyl or aryl substituents on silicon show their characteristic reactivities in the metal complex-catalyzed hydrosilylation. Therefore, it seems appropriate to summarize here briefly recent advances in elucidation of the catalysis by metal complexes, including activation of silicon-hydrogen bonds. 

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... 

Water-soluble rhodium complexes bearing sulfonated triphenylphos-phine ligands can catalyze the reduction of cyclohexene in a two-phase system. It is also possible to use Wilkinson s catalyst [(Ph3P)3RhCl] for the hydrogenation of water-soluble olefins in an aqueous-benzene solvent system (46). 

One of the most carefully studied hydrogenations is the one catalyzed by the Rh(I) complex RhCl(PPh3)3, usually known as the Wilkinson catalyst. It was discovered in 1965 and is easily prepared by the reduction of rhodium trichloride hydrate in the presence of triphenylphosphine. 

Asymmetric hydrogenation. Morrison et al. have reported on asymmetric hydrogenations catalyzed by rhodium(I) complexes of the Wilkinson type containing chiral ligands. This type of asymmetric synthesis had been carried out previously with relatively inaccessible phosphine ligands that are asymmetric at phosphorus. Phosphines that are asymmetric at carbon are more readily available and appear to be more efficient. Thus reduction of (E)- 3-methylcinnamic acid with prereduced tris(neomenthyldiphenylphosphine)chlororhodium in the presence of triethylamine leads to 3-phenylbutanoic acid, +34.5°, which contains 61% enantiomeric excess of the S-isomer. Hydrogenations of olefins exhibit a lower degree of asymmetric bias. 

Hydroborations. Addition of Catecholborane to alkenes is accelerated by Wilkinson s catalyst, and other sources of rhodium-(I) complexes. Unfortunately, the reaction of Wilkinson s catalyst with catecholborane is complex hence if the conditions for these reactions are not carefully controlled, competing processes result. In the hydroboration of styrene, for instance, the secondary alcohol is formed almost exclusively (after oxidation of the intermediate boronate ester, eq 37) however, the primary alcohol also is formed if the catalyst is partially oxidized and this can be the major product in extreme cases. Conversely, hydroboration of the allylic ether (12) catalyzed by pure Wilkinson s catalyst gives the expected alcohol (13), hydrogenation product (14), and aldehyde (15), but alcohol (13) is the exclusive (>95%) product if the RhCl(PPh3)3 is briefly exposed to air before use. The 5yn-alcohol is generally the favored diastereomer in these and related reactions (eq 38), and the catalyzed reaction is therefore stereocomplementary to uncatalyzed hydroborations of allylic ether derivatives. ... 

In contrast to a number of studies on the homogeneous hydrogenation of carbon-carbon multiple bonds [25], there had been few papers about hydrogenation of simple ketones before Schrock and Osborn [26] reported in 1970 a catalytic activity of cationic rhodium complexes with relatively basic phosphines as ligands. In fact, the Wilkinson s rhodium(I) complex usually lacks activity towards hydrogenation of carbonyl groups, and rather catalyzes decarbonylation of aldehydes. The catalytic cycle of the hydrogenation of ketones proposed by Schrock and Osborn is depicted in Scheme 3. 

---