Ionic rhodium -compounds

The reaction of alcohols with CO can also be catalysed by palladium iodides, and various ligands or solvents. Acetic acid is prepared by the reaction of MeOH with CO in the presence of a catalyst system comprising a palladium compound, an ionic iodide compound, a sulfone solvent at conditions similar to those of the rhodium system (180 °C, 60 bar), and, in some cases, traces of a nickel-bipyridine compound were added. Sulfones or phosphine oxides play a stabilising role in preventing metal precipitation [26], Palladium(II) salts catalyse the carbonylation of methyl iodide in methanol to methyl acetate in the presence of an excess of iodide, even without amine or phosphine co-ligands platinum(II) salts are less effective [27],... 

A truly hemilabile amino functionalised NHC ligand was introduced by Jimdnez et al. who synthesised a series of rhodium(I) compounds using anunonium functionalised imidazolium salts as starting materials [150] (see Figure 3.52). Interestingly, initially an ionic rhodium(I) compound was obtained that did not contain a carbene-rhodium bond. The rhodium carbene complex could be obtained after further deprotonation and coordination of the amine sidearm to the metal occurred only after chloride abstraction with AgBF. ... 

The best rhodium-catalyst proved to be bis(ethene)acetylacetonate-rhodium which was activated by basic trialkyl phosphines such as tri-ethylphosphine. Also ionic rhodium(I)-compounds and monomeric as well as dimeric rhodium(III)-complexes yielded the lactone selectively. 

Methanol Carbonylation. Some researchers have described the possibility of supporting a rhodium compound on an ionic resin such as a copolymer of styrene and 4-vinylpyridine alkylated with methyl iodide forming a methylpyridinium-functionalized polymer (16). They have concluded that their ionic polymer-supported rhodium catalyst for methanol carbonylation in liquid phase is approximately equal in catalytic activity to the dissolved complex and that leaching of the complex could be minimized by suitable choice of solvent and by selecting high resin-to-rhodium ratios. However, experiments carried out only at low temperature (120°C) and low pressure were reported. 

Rhodium- and cobalt-catalyzed hydrogenation of butadiene and 1-hexene [47, 48] and the Ru-catalyzed hydrogenation of aromatic compounds [49] and acrylonitrile-butadiene copolymers [50] have also been reported to be successful in ionic liquids. 

Because coordination compounds are usually considered to be covalently bonded, io a first approximation (see Chapter 11). extended complex structures in the solid can readily be related io them. Consider, for example, rhodium penlafluonde Obviously, it could be considered as an ionic structure, Rh5+5F , and indeed the crystal structure7 consists, in part, of hep-arranged fhjonde ions with rhodium in octahedral holes. However, closer inspection of the structure reveals that it consists of tetrnmeric units, Rh4F2ll, that are distinct from one another (Fig. 7.7). The environment about each rhodium atom, an octahedron of six fluonne atoms, is what we... 

Rhodium(IIl) sulfate exists in yellow and red forms. If Rh(OH)3 is dissolved in cold H2SO4, the product is the yellow form, in which the sulfate is ionic. If this solution is evaporated in hot H2SO4. the product is a red, noiiiouic sulfate. When Rh is treated with F2 at 500-600cC, RI1F3 is slowly formed. This compound is practically insoluble in water, concentrated HO, I1NO(t Il>SO T HF. or NaOH. 

Tetrakis(trimethylphosphine)rhodium(I) chloride loses two trimethylphosphine ligands when allowed to react with PPh3 at elevated temperatures (equation 52). The ionic compound also undergoes a more complex reaction with benzene and sodium. In this a trimethylphosphine ligand is converted to a PMe2Ph ligand (equation 53).186 The geometry adopted by both these complexes... 

Chiral rhodium-DuPHOS complexes are highly efficient catalyst for the enantioselective hydrogenation of enamides. One drawback of these catalysts is that they are easily oxidised and inert conditions are required for optimal results. The methyl- and ethyl substituted Rh-DuPHOS compounds, 3a and 3b, have been successfully applied in the reduction of a-acetamidocinnamic acids in [C4Ciim][PF6], Scheme 3.7.[7,39] While activities and selectivities are slightly lower compared to the homogeneous reaction in 2-propanol, the ionic liquid-immobilised catalyst is less prone to oxidation and recycling is feasible at least three times. 

A series of rhodium and platinum compounds have been tested in the hydroamination of norbomene with aniline, as shown in Scheme 9.36.[141] Selectivity and activity were highly dependent on the nature of the ionic liquid, but were always superior to those observed in THF. Solvents with chloride anions led to essentially no catalytic activity, whereas [PF6] or Br afforded some catalysis. Nonetheless, even with the best solvent/catalyst combination, less than 40 turnovers are achieved after 6 days at 140°C. 

In some cases, it is possible to couple an alcohol with an organometallic compound. Allylic alcohols are coupled with alkylmagnesium bromides in the presence of Ti(OiPr)4, for example. Allylic alcohols can be coupled with arylboronic acids in ionic liquid solvent and a rhodium catalyst. The palladium-catalyzed... 

The use of chiral additives with a rhodium complex also leads to cyclopropanes enantioselectively. An important chiral rhodium species is Rh2(5-DOSP)4, which leads to cyclopropanes with excellent enantioselectivity in carbene cyclopro-panation reactions. Asymmetric, intramolecular cyclopropanation reactions have been reported. The copper catalyzed diazoester cyclopropanation was reported in an ionic liquid. ° It is noted that the reaction of a diazoester with a chiral dirhodium catalyst leads to p-lactones with modest enantioselectivity Phosphonate esters have been incorporated into the diazo compound... 

Other organometallic compounds, including aUylic stannanes, allylic samarium, allylic germanium, and allylic indium compounds add to aldi-mines in the same manner. Aryltrialkylstannanes also add the aryl group to Al-tosyl imines using a rhodium catalyst and sonication. Catalytic enantioselective addition reactions are well known, including reactions in an ionic liquid.Allylic... 

The reactions illustrated in equations (62-64) are each catalyzed by rhodium acetate. Diazo compounds, especially diazoalkanes, can also react by a simple ionic mechanism. The esterification of a carboxylic acid with diazomethane is a familiar example. The ionic pathway is especially likely when the intermediate carbocation would be stabilized. Thus, diazine (158) couples smoothly with phenols, presumably by thermal rearrangement to the corresponding diazo sugar, followed by acid-catalyzed N2 loss... 

Favorable thermodynamics (AG° (298K) <0) for obtaining acetylene diolates from M-M bonded complexes occurs when 2(M-0)>(M-M)+ 147 kcal while single metal units require that the M-0 bond energy exceed 78 kcal (Table I, entries u,v). This limiting type of CO coupling is best known for reactions of alkali metals with CO which form solid ionic acetylene diolate compounds.Rhodium macrocycle complexes have Rh-0 bond energies 50-60 kcal and thus are excluded as potential candidates for acetylene diolate formation. 

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