Rhodium complexes olefin

Rhodium-olefin complexes have been identified as intermediate species in rhodium-catalyzed olefin-to-olefin addition reactions (5a, 150a, 151) and olefin hydrogenation reactions (450). Although the ethylene-Rh(I) complexes are not in themselves catalysts for dimerization of ethylene, both [(C2H4)2RhCl]2 and (C2H4)2Rh( Cac) react with... 

A phosphine-amine pincer ligand reacts with a rhodium olefin complex more easily than diphosphine pincer ligands to give a C-C bond activated complex in minutes at room temperature. In this case, a C-H activated complex was not observed upon monitoring the reaction even at -50°C [55]. 

Typical square-planar rhodium-olefin complexes such as acetylacetonates (48) have a stoichiometry of two coordinated olefins per metal-atom. Since chelating olefins are bidentate in their cationic rhodium biphosphine complexes, it would be surprising if bis-olefin complexes were never found under hydrogenation conditions. It seems clear, in fact, that they can be the major coordinated species under certain conditions. Thus examples of 2 1 rhodium enamide complexes with biz-diphenyl-phosphinopropane have been observed (49), although the majority of cases involve a8-unsaturated acids co-complexed with DIOP. 

Todd et al. (24) measured Jwrh-c of 10 to 16 Hz and substantial upheld shifts (50 to 115 ppm) of the olefinic carbons in a series of rhodium-olefin complexes. A value of 15% character in the rhodium-olefinic carbon bond was calculated from the Rh-C coupling constants. The estimate of 15%. s character implied approximately 60% contribution of the bonding form in which there is a ct bond between an sp rehybridized olefinic carbon and a dsp rhodium orbital. 

The NMR parameters for iron and rhodium olefinic complexes are given in Table XLIV. 

Some kinetic parameters for restricted rotation in rhodium-olefin complexes were reported some time ago. These values have recently been revised, and parameters for related rhodium compounds determined. Activation parameters have also been determined for some platinum-olefin complexes. Here intramolecularity of mechanism is proved by the persistence of Pt- H coupling and the lack of change in the n.m.r. spectra of the non-olefinic ligands with varying temperature. The roles of p and d orbitals both in fixing the preferred orientation of the olefin perpendicular to the co-ordination square around the platinum and in the rotational process, are discussed. Wide-line n.m.r. spectra indicate some... 

In the rhodium-olefin complexes [Rh(C5H702XC2H4)J (60a) and [Rh(C5H 02)(C2H4)(C2F4)] (60b), the acetylacetonato oxygen atoms and the midpoints of two C=C bonds define a square plane around the metal atom. The molecule (60a) possesses ciystallographic C, symmetry the... 

The formation of a stable 3-rhoda-l,2-dioxolane by dioxygenation of a rhodium-olefin complex supported the involvement of such complexes as intermediates in rhodium-catalyzed oxygenation of olefins to ketones. This 3-rhoda-l,2-dioxolane complexes rearranged to rhodium formylmethyl hydroxycomplexcs upon exposure to light or protons (Scheme 25). ... 

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). 

Finally, selective hydrogenation of the olefinic bond in mesityl oxide is conducted over a fixed-bed catalyst in either the Hquid or vapor phase. In the hquid phase the reaction takes place at 150°C and 0.69 MPa, in the vapor phase the reaction can be conducted at atmospheric pressure and temperatures of 150—170°C. The reaction is highly exothermic and yields 8.37 kJ/mol (65). To prevent temperature mnaways and obtain high selectivity, the conversion per pass is limited in the Hquid phase, and in the vapor phase inert gases often are used to dilute the reactants. The catalysts employed in both vapor- and Hquid-phase processes include nickel (66—76), palladium (77—79), copper (80,81), and rhodium hydride complexes (82). Complete conversion of mesityl oxide can be obtained at selectivities of 95—98%. 

Inhibition of diazoester decomposition by a large excess of olefin speaks in favor of intermediarily liberated W(CO)5 as direct metal precursor of425. Stereoselectivities in the cyclopropanation reaction are very similar to those observed in the Rh2(OAc)4 catalyzed version, which underlines once more the close relationship of tungsten and rhodium carbene complexes. 

Chalk and Elarrod (11a) compared the above ethylene Pt(II) complex with chloroplatinic acid for hydrosilation, and found that each gave essentially the same results in terms of rate, yields, and products. Plati-num(II) complexes and rhodium(I) complexes were very much alike in their behavior. No system was found in which a palladium olefin complex brought about hydrosilation. In most systems the palladium complex was very rapidly reduced to the metal. 

Recently, various rhodium carbene complexes were investigated as catalysts for hydrosilation of olefins, acetylenes, and dienes to see whether carbene ligands modify catalytic activity. All reactions were... 

The 0/7/fo-alkylation of aromatic ketones with olefins can also be achieved by using the rhodium bis-olefin complex [C5Me5Rh(C2H3SiMe3)2] 2, as shown in Equation (9).7 This reaction is applied to a series of olefins (allyltrimethyl-silane, 1-pentene, norbornene, 2,2 -dimethyl-3-butene, cyclopentene, and vinyl ethyl ether) and aromatic ketones (benzophenone, 4,4 -dimethoxybenzophenone, 3,3 -bis(trifluoromethyl)benzophenone, dibenzosuberone, acetophenone, />-chloroacetophenone, and />-(trifluoromethyl)acetophenone). 

In summary, the asymmetric hydrogenation of olefins or functionalized ketones catalysed by chiral transition metal complexes is one of the most practical methods for preparing optically active organic compounds. Ruthenium and rhodium-diphosphine complexes, using molecular hydrogen or hydrogen transfer, are the most common catalysts in this area. The hydrogenation of simple ketones has proved to be difficult with metallic catalysts. However,... 

In 2004 Caporali investigated the hydroformylation of 1-hexene and cyclohexene using HRh(CO)(PPh3)3 [61]. The collected data indicated that the rate-determining step in the hydroformylation cycle depends upon the structure of the olefin. With an alpha-olefin like 1-hexene, the slowest step seems to be the hydrogenolysis of the acyl rhodium complex. In the presence of cyclohexene as a model for an internal olefin, the rate-determining step is the reaction of the olefin with the rhodium hydride complex (intermediate II in Fig. 6). 

P-Cyclodextrin was modified by attaching 2-(diphenylphosphinoethyl)-thio- (127) and 2-bis(diphenylphosphinoethyl)amino- (126) moieties at the C-6 position [8-11]. The resulting macroligands were reacted with [ RhCl(NBD) 2] to provide the corresponding cationic rhodium-bisphosphine complexes. These catalysts showed pronounced selectivity due to complexation of the substrate by the CD unit adjacent to the catalyticaUy active metal center. For example, in competitive hydrogenation of similarly substituted terminal olefins (Scheme 10.4), 4-phenyl-but-l-ene was... 

Historically, NHC complexes were investigated for the first time as catalysts and discussed as catalytic intermediates in the dismutation of electron rich tetraamino-ethylenes [Eq, (49)] Mixtures of two differently substituted olefins were reacted in the presence of rhodium(I) complexes and the products obtained showed mixed substitution patterns. Starting from Wilkinson s catalyst [(Ph3P)3RhCl], NHC complexes are formed as intermediates which could be isolated and used as even more active catalysts. In this first example, however, the NHC actively participates in... 

Rhodium(I) and ruthenium(II) complexes containing NHCs have been applied in hydrosilylation reactions with alkenes, alkynes, and ketones. Rhodium(I) complexes with imidazolidin-2-ylidene ligands such as [RhCl( j -cod)(NHC)], [RhCl(PPh3)2(NHC)], and [RhCl(CO)(PPh3)(NHC)] have been reported to lead to highly selective anti-Markovnikov addition of silanes to terminal olefins [Eq. 

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