Rhodium oxidative addition

The catalytic cycle (Fig. 5) (20) is well estabUshed, although the details of the conversion of the intermediate CH COI and methanol into the product are not well understood the mechanism is not shown for this part of the cycle, but it probably involves rhodium in a catalytic role. The CH I works as a cocatalyst or promoter because it undergoes an oxidative addition with [Rh(CO)2l2]% and the resulting product has the CO ligand bonded cis to the CH ligand these two ligands are then poised for an insertion reaction. 

Complex [(CXI )Ir(/j,-pz)(/i,-SBu )(/j,-Ph2PCH2PPh2)Ir(CO)] reacts with iodine to form 202 (X = I) as the typical iridium(II)-iridium(II) symmetrical species [90ICA(178)179]. The terminal iodide ligands can be readily displaced in reactions with silversalts. Thus, 202 (X = I), upon reaction with silver nitrate, produces 202 (X = ONO2). Complex [(OC)Ir(/i,-pz )(/z-SBu )(/i-Ph2PCH2PPh2)Ir(CO)] reacts with mercury dichloride to form 203, traditionally interpreted as the product of oxidative addition to one iridium atom and simultaneous Lewis acid-base interaction with the other. The rhodium /i-pyrazolato derivative is prepared in a similar way. Unexpectedly, the iridium /z-pyrazolato analog in similar conditions produces mercury(I) chloride and forms the dinuclear complex 204. 

The reaction with Mel proceeds in two stages. Initial reaction is oxidative addition to give a rhodium(III) species, isolated as a Mel adduct... 

Such a complex, cw-Rh(CO)2I2, is the active species in the Monsanto process for low-pressure carbonylation of methanol to ethanoic acid. The reaction is first order in iodomethane and in the rhodium catalyst the rate-determining step is oxidative addition between these followed by... 

Like other planar rhodium(I) complexes, Rh(RNC)4 undergoes oxidative addition with halogens to form 18-electron rhodium(III) species and also add other small molecules (S02, NO+) (Figure 2.31). 

As shown in Scheme 168, oxidative addition reactions with either methyl chloride or methyl iodide proved successful and yielded the corresponding octahedral rhodium(III) complexes. ... 

The most interesting work on the isocyanide complexes of the elements in this subgroup has been done with rhodium and iridium. For the most part, the work is involved with the oxidative addition reactions of d square-planar metal complexes. 

Rhodium(I) complexes are effective reagents and/or catalysts for the decarbonylation of acyl halides and aldehydes 9 11,34,195,230,231,236). The compound Rh(PPh3)3Cl, especially, has received considerable attention. The first step in such reactions involves oxidative addition to Rh(I) of the organic molecule, exemplified by the following ... 

Oxidative addition of RX to square-planar Ir(CO)L2X or to CpIr(CO)L (L = PR3 or ASR3) invariably affords RIr(CO)L2X2 (71, 85, 86) and CpIr(CO)LR X (106, 198), respectively. Unlike some of their rhodium analogs, these complexes do not rearrange to the acyls. 

Scheme 6-3 Oxidative addition of water to rhodium phosphine complexes...

The most recent catalysts that operate under thermal conditions were then based on the premise that a Cp M fragment with ligands that dissociate under thermal conditions could be a catalyst for alkane borylation. After a brief study of Cp IrH4 and Cp Ir(ethylene)2, Dr. Chen studied related rhodium complexes. Ultimately, he proposed that the Cp Rh(ri" -C6Me6) complex would dissociate CeMce as an iimocent side product, and that Cp Rh(Bpin)2 from oxidative addition of pinBBpin (pin=pinacolate) would be the active catalyst. The overall catalytic... 

After the precatalyst is completely converted to the active catalyst Xq, three steps are required to form the desired reduction product. The first step is the coordination of dehydroamino acid (A) to the rhodium atom forming adducts (Xi) and (Xi ) through C=C as well as the protecting group carbonyl. The next step is the oxidative addition of hydrogen to form the intermediate (X2). The insertion of solvent (B) is the third step, removing the product (P) from X2 and regenerating Xq. Hence, the establishment of the kinetic model involves these three irreversible steps. 

S. J. Pool and K.H. Shaughnessy. Effects of ionic liquids on oxidative addition to square planar iridium and rhodium complexes. Abstracts of Papers, 231st ACS National Meeting, Atlanta, GA, USA, March 26-30,2006 (2006). 

The mechanism of alkene hydrogenation catalyzed by the neutral rhodium complex RhCl(PPh3)3 (Wilkinson s catalyst) has been characterized in detail by Halpern [36-38]. The hydrogen oxidative addition step involves initial dissociation of PPI13, which enhances the rate of hydrogen activation by a factor... 

The reactions in Equations (5) and (6) involve oxidation of rhodium(I) to rhodium(III). Equations (6) can also be written as an oxidative addition of I2 (formed thermally from 2 HI) to (1). Rhodium(III) iodides may precipitate from the reaction medium. They have to be converted to rhodium(I) again by water and carbon monoxide. 

Other companies (e.g., Hoechst) have developed a slightly different process in which the water content is low in order to save CO feedstock. In the absence of water it turned out that the catalyst precipitates. Clearly, at low water concentrations the reduction of rhodium(III) back to rhodium(I) is much slower, but the formation of the trivalent rhodium species is reduced in the first place, because the HI content decreases with the water concentration. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilization of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives.8 The kinetics of the title reaction with respect to [MeOH] change if H20 is used as a solvent instead of AcOH.9 Kinetic data for the Rh-catalyzed carbonylation of methanol have been critically analyzed. The discrepancy between the reaction rate constants is due to ignoring the effect of vapor-liquid equilibrium of the iodide promoter.10... 

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