Rhodium complexes carbon dioxide reactions

In the past, this field has been dominated by ruthenium, rhodium and iridium catalysts with extraordinary activities and furthermore superior enantioselectivities however, some investigations were carried out with iron catalysts. Early efforts were reported on the successful use of hydridocarbonyliron complexes HFcm(CO) as reducing reagent for a, P-unsaturated carbonyl compounds, dienes and C=N double bonds, albeit complexes were used in stoichiometric amounts [7]. The first catalytic approach was presented by Marko et al. on the reduction of acetone in the presence of Fe3(CO)12 or Fe(CO)5 [8]. In this reaction, the hydrogen is delivered by water under more drastic reaction conditions (100 bar, 100 °C). Addition of NEt3 as co-catalyst was necessary to obtain reasonable yields. The authors assumed a reaction of Fe(CO)5 with hydroxide ions to yield H Fe(CO)4 with liberation of carbon dioxide since basic conditions are present and exclude the formation of molecular hydrogen via the water gas shift reaction. H Fe(CO)4 is believed to be the active catalyst, which transfers the hydride to the acceptor. The catalyst presented displayed activity in the reduction of several ketones and aldehydes (Scheme 4.1) [9]. 

Reactions that combine C-H activation with a C-C bond-forming event are invaluable synthetic tools, allowing concise construction of carbon frameworks. Rhodium(i) catalysts have been shown to catalyze alkane carbonylation [21]. Recently, Sakakura and co-workers succeeded in subjecting methane to a catalytic acetaldehyde synthesis [22], They found that, in dense carbon dioxide, the complex [RhCl(PMe3)3] catalyzed the carbonylation of methane with 77 turnovers. 

Another important example of COj-hydrogenation is the synthesis of form-amides. In 1970, Haynes c/tf/. of Shell Development Co. discovered the reaction of carbon dioxide, hydrogen and certain amines, when catalyzed under mild conditions by cobalt, rhodium, iridium and palladium complexes [ I70. ... 

A side reaction related directly to the carbonylation of methyl acetate is the formation of acetone and carbon dioxide via methylation of the acetyl rhodium complex intermediate [63] through addition of a second molecule of methyl iodide (eqs. (18)-(21)) ... 

In a further variation, the PVP-supported rhodium catalyst was used for methanol carbonylation in supercritical carbon dioxide [100]. This reaction medium has complete miscibility with CO and dissolves high concentrations of methanol and methyl iodide, while being a poor solvent for ionic metal complexes. Catalytic reaction rates up to half of those obtained in conventional liquid-phase catalysis were achieved with minimal catalyst leaching. 

During the hydroformylation of higher alkenes under the Ruhrchemie/Rhone-Poulenc conditions the pH value is controlled and adjusted between 5.5 and 6.2 [26], In discontinuous operation it drops by almost one pH unit from approximately 6.5 at the beginning of the reaction to pH 5.5-6 at the end of the reaction. According to our investigations the pH shift is due to the formation of carbon dioxide, formed via the water-gas shift reaction (WGSR), which is also promoted by rhodium TPPTS complexes but to a lower extent [27]. 

Preliminary results of the reaction between vanadium(iii)-tetrasulpho-phthalocyanine complex with oxygen have been reported these data were compared with those obtained for the corresponding reaction of the hexa-aquo complex ion. The oxidation of methyl ethyl ketone by oxygen in the presence of Mn"-phenanthroline complexes has been studied Mn " complexes were detected as intermediates in the reaction and the enolic form of the ketone hydroperoxide decomposed in a free-radical mechanism. In the oxidation of 1,3,5-trimethylcyclohexane, transition-metal [Cu", Co", Ni", and Fe"] laurates act as catalysts and whereas in the absence of these complexes there is pronounced hydroperoxide formation, this falls to a low stationary concentration in the presence of these species, the assumption being made that a metal-hydroperoxide complex is the initiator in the radical reaction. In the case of nickel, the presence of such hydroperoxides is considered to stabilise the Ni"02 complex. Ruthenium(i) chloride complexes in dimethylacetamide are active hydrogenation catalysts for olefinic substrates but in the presence of oxygen, the metal ion is oxidised to ruthenium(m), the reaction proceeding stoicheiometrically. Rhodium(i) carbonyl halides have also been shown to catalyse the oxidation of carbon monoxide to carbon dioxide under acidic conditions ... 

Rate data are reported for the acid-catalysed aquation of the [M(NH3)5COal+ ions (M = Rh or Ir ), as well as the rates of formation of these carbonato-com-plexes from [M(NH3)50H] + and carbon dioxide. At 298.1 K and p = 0.5 mol 1 , the aquation rate constants are 1.13 and 1.45 s and the formation rate constants 470 and 5901 mol S for the rhodium(m) and iridium(m) complexes respectively. The close similarity of these rate constants for both metal ions indicates carbon-oxygen bond fission for the aquation reactions. 

---