Hydrogenation rhodium

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). 

The aldehyde formed in the first cycle reacts with an amine (E) yielding an enamine. This coordinates to the rhodium dihydrido species (G) of the second catalytic cycle. The insertion of the enamine in the rhodium hydrogen bond (H) is the next step before the hydrogenated molecule is cleaved (J) and the active rhodium complex is restored by adding a hydrogen molecule (F). 

Abstract The applications of hybrid DFT/molecular mechanics (DFT/MM) methods to the study of reactions catalyzed by transition metal complexes are reviewed. Special attention is given to the processes that have been studied in more detail, such as olefin polymerization, rhodium hydrogenation of alkenes, osmium dihydroxylation of alkenes and hydroformylation by rhodium catalysts. DFT/MM methods are shown, by comparison with experiment and with full quantum mechanics calculations, to allow a reasonably accurate computational study of experimentally relevant problems which otherwise would be out of reach for theoretical chemistry. 

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). 

Platinum/rhodium Hydrogen cyanide synthesis Gauzes... 

Selective reduction of both double bonds in a,jS-unsaturated carbonyl compounds that also contain an isolated olefin is possible with appropriate catalyst and reaction conditions . Chlorotris(biphenylphosphine)rhodium hydrogenates the isolated double bond. Thus, carvone, 13, is reduced to dihydrocarvone, 14, with careful measure of the uptake of Hj ... 

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. 

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