The Achiral Case

the dihydrogen bond is already broken and the unsaturated bond is weakened. Next, the unsaturated bond inserts into one of the Rh-H bonds and thus the first hydrogen is transferred to the product molecule. Transfer of the second hydrogen leads to reductive elimination of the product molecule and the catalyst is ready for the next cycle. 

The general catalytic cycle thus typically consists of the four steps addition, association, insertion, and elimination. 

4 you can see a selection of the more prominent and powerful examples of chiral phosphine ligands which, coordinated to a rhodium center, form homogeneous hydrogenation catalysts. In contrast to WUldnsoris catalyst, all of these are cationic rather than neutral. Typically, weaMy-coordination anions such as tetrafluorobo-rate, hexafluorophosphate or triflate are used here as the counter-ions. 

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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]. 

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