Complex Rh

Cartnnylalion, hydrolormylaiion (CO-H2] of olefins catalyzed by metal (Pd or Rh) complexes. 

N O Y O R I Chiral homogeneous hydrogenation Homogeneous chiral hydrogenation ol unsaluraled alcohols, or cartMxyNc acids, enamides, ketones in the presence ol BINAP Ru or Rh complex 8 as catalyst. 

It was recently found that the modification of neutral phosphine ligands with cationic phenylguanidinium groups represents a very powerful tool with which to immobilize Rh-complexes in ionic liquids such as [BMIM][PFg] [76]. The guani-dinium-modified triphenylphosphine ligand was prepared from the corresponding iodide salt by anion-exchange with [NH4][PFg] in aqueous solution, as shown in Scheme 5.2-15. The iodide can be prepared as previously described by Stelzer et al. [73]. 

The rhodium complexes are excellent catalysts for hydrogenation of NBR. At low temperature and pressure, high catalyst concentrations are used to obtain a better rate of reactions. Due to higher selectivity of the reaction, pressure and temperature can be increased to very high values. Consequently the rhodium concentration can be greatly reduced, which leads to high turnover rates. The only practical drawback of Rh complex is its high cost. This has initiated the development of techniques for catalyst removal and recovery (see Section VU), as well as alternate catalyst systems based on cheaper noble metals, such as ruthenium or palladium (see Sections IV.A and B). 

Hydrogenation of olefinic unsaturation using ruthenium (Ru) catalyst is well known. It has been widely used for NBR hydrogenation. Various complexes of Ru has been developed as a practical alternative of Rh complexes since the cost of Ru is one-thirtieth of Rh. However, they are slightly inferior in activity and selectivity when compared with Rh catalyst. 

Recently, Cardin et al. 89) reported the metathesis of electron-rich alkenes catalyzed by Rh complexes ... 

Metal-free poly-4-vinyl-4 -methyl-2,2 -bipyridine films on electrodes have been prepared by the electroreductive polymerization of a Rh complex and subsequent leaching of the metal by a strong complexand. The films can incorporate a variety of transition metals... 

The d5Tiamic stereochemistries of M(dtc)3 and [M(dtc)3] (M = Fe, Co, or Rh) complexes have been studied (315). The cobalt complex is non-rigid, but the mechanism of optical inversion could not be determined. The Rh complex is stereochemically rigid up to 200°. The optical inversion of (-l-)546 [Colpyr-dtcla] in chloroform has been studied, by loss of optical activity, by polarimetry (316). 

Recent studies on isolated BVMOs using Rh-complexes as NADPH substitutes for facile cofactor recycling suggested a pivotal role of the native cofactor to generate the proper environment within chiral induction in sulfoxidation reactions. While biooxidation was still observed in the presence of the metal complex, stereoselectivity of the enzyme was lost almost completely [202]. 

A series of Rh and Ir complexes is also basic enough to add boron halides, although the metal-boron bonds are weak, e.g., dppe and PPh3-Rh complexes serve as electron-pair bases when they react with BF3 and BCI3 . Table 1 summarizes the results. 

One Rh complex with a 2,3-dihydro-l,3-diborolyl ligand is prepared by dehalo-genation of [(Et2C2B2Et2CMe)RhCl]2 with K in the presence of free 2,3-dihydro-1,3-diborole (Et2C2B2Et2CHMe)Rh(Et2C2B2Et2CMe) possesses one borole and one borolyl ring ligand. ... 

Some chiral mono-, acyl- and di-thioureas have been used as ligand for the Rh-catalysed asymmetric hydroformylation of styrene. Although thiourea ligands form inactive systems with [Rh(COD)Cl]2 as the catalyst precursor, in standard conditions (40 °C, 40 bar CO -l- H2 1/1), the cationic Rh complex [Rh(COD)2]Bp4 combined with monothioureas as the ligand showed moderate to good activity (Scheme 29) [114]. 

The Rh complexes (cod)Rh[ jt-OSi(0 Bu)3] 2 and (nbd)Rh[ jt-OSi (0 Bu)3] 2 thermally decompose primarily via loss of HOSi(O Bu)3 and formation of Rh metal particles, rendering them ineffective for use as precursors to Rh/Si/0 materials however, these complexes are potentially useful as sources of Rh nanoparticles or site-isolated Rh species via grafting methods [99]. 

The linear aldehyde usually is the desired product. Often, this aldehyde is converted to the corresponding alcohol. Both Co and Rh complexes are used in hydroformylation. In general, Rh catalysts are more active and produce a higher n/wo-ratio. Co catalysts have higher... 

An unusual enhancement of catalytic activity in a two-phase system has been reported by Fremy et al. (1998) for the hydroformylation of acrylic esters using Rh complex of TPTS as catalyst. Even though acrylic esters have reasonable solubility in water, rate enhancements in two-phase systems by a factor of 2 to 14 have been reported. It seems that water is not an inert solvent but also acts as a reactant or a co-ordinating solvent which can modify elementary steps of the catalytic cycle (Cornilis, 1997). 

Bortoletto et al. (1997) have used neutral and cationic triphenylphosphine trisulphonate or triphenylphosphine mono-sulphate Rh complexes. Monosulphonate is less readily oxidized and can replace trisulphonate in industrial processes. The presence of a quaternary ammonium counter ion associated with the trisulphonate confers phase-transfer properties to the catalytic. species, which makes use of co-solvents unnecessary. 

The proposed reaction mechanism (Scheme 7-2) comprises (1) oxidative addition of ArSH to RhCl(PPh3)3 to give Rh(H)(Cl)(SPh)(PPli3)n, (2) coordination ofalkyne to the Rh complex, (3) ris-insertion of alkyne into the Rh-H bond with Rh positioned at terminal carbon and H at internal carbon, (4) reductive elimination of 16 from the Rh(III) complex to regenerate the Rh(I) complex. 

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