Rhodium complexes with olefins

The use of silver fluoroborate as a catalyst or reagent often depends on the precipitation of a silver haUde. Thus the silver ion abstracts a CU from a rhodium chloride complex, ((CgH )2As)2(CO)RhCl, yielding the cationic rhodium fluoroborate [30935-54-7] hydrogenation catalyst (99). The complexing tendency of olefins for AgBF has led to the development of chemisorption methods for ethylene separation (100,101). Copper(I) fluoroborate [14708-11-3] also forms complexes with olefins hydrocarbon separations are effected by similar means (102). 

Catalytic Asymmetric Hydroboration. The hydroboration of olefins with catecholborane (an achiral hydroborating agent) is cataly2ed by cationic rhodium complexes with enantiomericaHy pure phosphines, eg, [Rh(cod)2]BE4BINAP, where cod is 1,5-cyclooctadiene and BINAP is... 

Rhodium complexes with chelating bis(oxazoline) ligands have been described to a lesser extent for the cyclopropanation of olefins. For example, Bergman, Tilley et al. [32] have prepared a family of bis(oxazoline) complexes of coordinatively unsaturated monomeric rhodium(II) (see 20 in Scheme 13). Interestingly, the use of complex 20 in the cyclopropanation reaction of styrene afforded mainly the cis cyclopropane cis/trans = 63137), with 74% ee and not the thermodynamically favored trans isomer. No mechanistic suggestions are proposed by the authors to explain this unusual selectivity. 

Directed intramolecular transfer hydrogenations are catalyzed by rhodium complexes with the pendant alkene acting as an internal sacrificial olefin (Equation (39)). 

The discovery of Wilkinson complex, RhCl[P(C6H5)3]3, acting as an effective catalyst for hydrogenation of olefins opened the door for developing asymmetric reaction catalyzed by rhodium complexes with a chiral phosphine ligand. 

Neutral catalysts or catalyst precursors based on fluorinated ligand systems have been applied in compressed CO2 to a broad range of transformations such as Zn- and Cr-catalyzed copolymerization of epoxides and CO2 [53, 54], Mo-catalyzed olefin metathesis [9], Pd-catalyzed coupling reactions [43, 55, 56] and Pd-catalyzed hydrogen peroxide synthesis [57]. Rhodium complexes with perfluoroalkyl-substituted P ligands proved successful in hydroformylation of terminal alkenes [28, 42, 44, 58], enantioselective hydroformylation [18, 59, 60], hydrogenation [61], hydroboration [62], and polymerization of phenylacetylene... 

Diazo ester/rhodium(II) carboxylate combinations other than EDA/Rh2(OAc)4 have been tested It turned out that the solubility of the rhodium(II) carboxylate greatly influenced the efficiency of cyclopropanation. For the reaction of monoolefins with ethyl diazoacetate, markedly higher yields than with Rh(II) acetate were obtained with the better soluble rhodium(II) butanoate and rhodium(II) pivalate, the latter one being soluble even in pentane. However, only poor yields resulted from the use of rhodium(Il) trifluoroacetate, even though this compound is readily soluble, Rh CCFjCOO), in contrast to the other rhodium(II) carboxylates, is able to form 1 1 complexes with olefins particularly with electron-rich ones thus, competition of olefin and diazo compound for the only available coordination site at the metal atom could be responsible for the reduced catalytic action of Rh2(CF3COO)4 (as will be seen in Section 4.1, this complex is an excellent catalyst for cyclopropanation of aromatic substrates). The diazoester substituent also has some influence on the yields. Increasing yields were obtained in the series methyl ester, ethyl ester, n-butyl... 

In the DIPAMP diphosphine, the two phosphorus atoms themselves are optically active. Rhodium DIPAMP complexes, discovered by Knowles (recipient of the Nobel Prize in 2001) (120,121), have been used by Monsanto for the commercial synthesis of (S)-3-(3,4-dihydroxyphenyl)alanine,(S)-DOPA, extensively used for the treatment of Parkinson s disease. Cationic rhodium complexes with these chiral diphosphines are very effective for the asymmetric hydrogenation of prochiral olefins, such as the interesting a-acetamidoacrylates, which can bind... 

Dinuclear rhodium complex with 36 was active in the addition reactions of carbenes (Scheme 4.22). Moreover, Ihe yield decreased for the stericaUy hindered olefins compared with rhodium acetate (II) due to steric factors. Furthermore, this complex preferably catalyzed the inclusion of carbene to the CH bond of aromatic fragment [82],... 

The hydroformylation of propene in a biphasic system using rhodium complex with TPPTS ligand can be thought of as a perfect implementation of the ideal phase-separation technique. All new hydrophilic phosphine ligands are usually first tried in hydroformylation, with two primary goals (i) to improve selectivity with respect to the ratio of normal to branched products and (ii) to enhance productivity of the biphasic system. The latter goal depends on an intrinsic limitation of the biphasic system, in that the reaction takes place in the aqueous layer and the rate (turnovers per unit time) is limited by the sparse solubility of olefins in water and by mass transfer of olefin across the very small interfacial boundary between the organic and aqueous layers. 

The Wilkinson hydrogenation cycle shown in Figure 3 (16) was worked out in experiments that included isolation and identification of individual rhodium complexes, measurements of equiUbria of individual steps, deterrnination of rates of individual steps under conditions of stoichiometric reaction with certain reactants missing so that the catalytic cycle could not occur, and deterrnination of rates of the overall catalytic reaction. The cycle demonstrates some generally important points about catalysis the predominant species present in the reacting solution and the only ones that are easily observable by spectroscopic methods, eg, RhCl[P(CgH 2]3> 6 5)312 (olefin), and RhCl2[P(CgH )2]4, are outside the cycle, possibly in virtual equiUbrium with... 

Hydroformylation, or the 0X0 process, is the reaction of olefins with CO and H9 to make aldehydes, which may subsequently be converted to higher alcohols. The catalyst base is cobalt naph-thenate, which transforms to cobalt hydrocarbonyl in place. A rhodium complex that is more stable and mnctions at a lower temperature is also used. 

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