Neutral Wilkinson type catalysts

In principle, the mechanism of homogeneous hydrogenation, in the chiral as well as in the achiral case, can follow two pathways (Figure 9.5). These involve either dihydrogen addition, followed by olefin association ( hydride route , as described in detail for Wilkinson s catalyst, vide supra) or initial association of the olefin to the rhodium center, which is then followed by dihydrogen addition ( unsaturate route ). As a rule of thumb, the hydride route is typical for neutral, Wilkinson-type catalysts whereas the catalytic mechanism for cationic complexes containing diphosphine chelate ligands seems to be dominated by the unsaturate route [1]. 

Neutral Wilkinson type catalysts with chiral ligands are quite effective for the hydrogenation of C=0 bonds in prochiral 2-oxocarbo Q lic acids or their esters and are applied to the as5mimetric S5mthesis of 2-hydroxyesters with rather high ee s of up to 80-95%. The reduction of alpha- iQ o esters, such as alkyl-pymvates into alkyl-lactates (Scheme 7.18.) on chiral complexes is one of the most important methods of the synthesis of chiral synthones for practical use. 

The first rapid and practical system for the homogeneous reduction of alkenes, alkynes, and other unsaturated substrates at room temperature and 1 bar pressure used the complex RhCl(PPh3)3, also known as Wilkinson s catalyst. Since then many other tertiary phosphine complexes have been studied as hydrogenation catalysts, including neutral compounds like RuHCl(PPh3)3 and cationic complexes such as [Rh(diene)(PPh3)2]+ and [RuH(PMe2Ph)5]+. There are three main types of complexes ... 

Catalytic hydrogenation of a-keto-esters can be achieved in the presence of homogeneous neutral Rh complexes of the Wilkinson type. Asymmetric reduction occurs when chiral bis-phosphines are employed as ligands, and one of the best optical yields known for homogeneous a-keto-ester hydrogenation (76%) is observed with (20a) as a ligand and propyl pyruvate as substrate. Use of the ligand (20b) increases the lipophilicity of such rhodium catalysts, and hence their solubility in non-polar solvents. ... 

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