Rhodium catalysts hydrogenation

To date, reports have involved palladium catalysts for Suzuki and Sono-gashira coupling reactions [63-66], rhodium catalysts for silylations of alcohols by trialkylsilanes [67,68], and tin-, hafnium-, and scandium-based Lewis acid catalysts for Baeyer-Villiger and Diels-Alder reactions [69]. Regardless of exact mechanism, this recovery strategy represents an important direction for future research and applications development. Finally, a particularly elegant protocol where CO2 pressure is used instead of temperature to desorb a fluorous rhodium hydrogenation catalyst from fluorous silica gel deserves emphasis [28]. 

Although carbonyl groups generally tend to deactivate rhodium hydrogenation catalysts, a number of complexes have been studied. [RhH(CO)(PPh3)3] (34), which is better known as a... 

Table 3 Some Other Rhodium Hydrogenation Catalysts...

In the hydroboration/oxidation reaction of 2-cyclohexen-l-ol derivatives the regio- as well as the stereoselectivities can be steered in either of two directions by use of different reagents and reaction conditions. Dialkylboranes, e.g., 9-BBN, give mainly tn rs-l,2-diols, whereas with 1,3,2-benzodioxaborole ( catecholborane") in the presence of the rhodium hydrogenation catalyst Rh(PPh3)jCl the trans-1,3-diols become predominant. The cis- 1,2-diols are usually only formed in trace amounts, but the cis-l,3-diols are always produced as byproducts in 10 — 20% yield (D.A. Evans, 1988). 

A ligand with a more direct interaction between chiral centers and the substrate binding pocket is the bis(phospholane) (22-XII) 11 rhodium hydrogenation catalysts... 

FIGURE 34 Hydrogen-bonding motifs of Frechet dendrimer serving as host for phosphite-containing rhodium hydrogenation catalyst, shown for the second-generation dendrimer G2 (724). 

A water-soluble rhodium hydrogenation catalyst has been found by Patin and co-workers [38]. Hydrogenations are carried out in very mild conditions, with Rh/ TPPTS in HzO/EtOH (1 1) and the catalyst is not air-sensitive and is easily prepared. However, the hydrogenation of linoleic acid is not selective, but yields only stearic acid. 

Figure 7.6. Synthetic sequence for the preparation of a protein bound rhodium hydrogenation catalyst.

The resulting polymer was then converted to a rhodium hydrogenation catalyst ... 

There have been several examples of multi-phase systems in asymmetric syntheses, notably a polymeric chiral rhodium hydrogenation catalyst for selectively producing i -amino-acids, and the use of phase-transfer catalysis in asymmetric carbon-carbon bond formation. The incorporation of optically active cinchona alkaloids, e.g. quinine (29), into a polymer by virtue of their vinyl group has been described, with preliminary results on application to asymmetric syntheses. At the other extreme of structural complexity, enzymes themselves have been incorporated into polymers to some advantage. ... 

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