Hydrogenation rhodium catalysis

PEGs with average molecular weights above 1000 are waxy solids under ambient conditions, but they melt under C02 pressure to become liquids under typical conditions of scC02 catalysis [63], The approach was demonstrated for the rhodium catalysed hydrogenation of styrene as a test reaction using Wilkinson s complex [(PPhs RhCl] as the catalyst (Scheme 8.6) [61],... 

With cobalt catalysts, hydroformylation of ethyl cinnamate gave 91% of the hydrogenation product ethyl hydrocinnamate (15) and only 8% of the expected lactone, 16 (72). However, rhodium catalysis was effective in directing the reaction in favor of hydroformylation (70). The comparative results obtained with cobalt and rhodium are outlined in Table XXV. 

Optically active aldehydes are important precursors for biologically active compounds, and much effort has been applied to their asymmetric synthesis. Asymmetric hydroformylation has attracted much attention as a potential route to enantiomerically pure aldehyde because this method starts from inexpensive olefins and synthesis gas (CO/H2). Although rhodium-catalyzed hydrogenation has been one of the most important applications of homogeneous catalysis in industry, rhodium-mediated hydroformylation has also been extensively studied as a route to aldehydes. 

Chauvin, Y., Mussmann, L., and Olivier, H., A novel class of versatile solvents for two-phase catalysis hydrogenation, isomerization, and hydroformylation of alkenes catalyzed by rhodium complexes in liquid 1,3-dialkylimidazolium salts, Angew. Chem. Int. Ed., 34, 2698-2700,1996. 

Weis, M., Waloch, C., Seiche, W. and Breit, B. (2006) Self-assembly of bidentate ligands for combinatorial homogeneous catalysis Asymmetric rhodium-catalyzed hydrogenation. J. Am. Chem. Soc.. 128. 4188-4189. 

To extend the concept to combinatorial asymmetric catalysis a new library of chiral aminopyridine and isoquinolone systems equipped with phosphine and phospho-nite donors was prepared and applied to the asymmetric rhodium-catalyzed hydrogenation (Scheme 2.8) [21],... 

Elimination. Two kinds of elimination reaction are important in homogeneous catalysis. A catalytic cycle which has involved an oxidative addition generally ends with the reverse process of reductive elimination (equation 6). Rhodium-catalyzed hydrogenations end with this step. 

The first example of asymmetric rhodium-catalyzed hydrogenation of prochi-ral olefins in dendrimer catalysis was reported by Togni et al., who immobilized the chiral ferrocenyl diphosphine Josiphos at the end groups of dendrimers, thus obtaining systems of up to 24 chiral metal centres in the periphery (Fig. 2) [12-14]. The fact that the catalytic properties of the dendrimer catalysts were almost identical to those of the mononuclear catalysts was interpreted as a manifestation of the independence of the individual catalytic sites in the macromolecular systems. 

FIGURE 8 Molecular clips equipped with phosphines or phosphites used for host-guest catalysis in rhodium-catalyzed hydrogenation and hydroformylation. At the bottom are depicted the peculiar tandem cyclizations, found only for allyldihydr-oxybenzenes in the host molecule as the catalyst (37,32). (For a color version of this figure, the reader is referred to the Web version of this chapter.)... 

Ammonia N-donor Ligands Asymmetric Synthesis by Homogeneous Catalysis Decarbonylation Catalysis Hydride Complexes of the Transition Metals Hydroboration Catalysis Hydrogenation Isomerization of Alkenes Hydrosilation Catalysis P-donor Ligands Rhodium Organometallic Chemistry. 

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