Rhodium, ligands

In recent years, much attention has been focused on rhodium-mediated carbenoid reactions. One goal has been to understand how the rhodium ligands control reactivity and selectivity, especially in cases in which both addition and insertion reactions are possible. These catalysts contain Rh—Rh bonds but function by mechanisms similar to other transition metal catalysts. 

The first application of a rhodium-ligand system was realized in the LPO-process (low pressure oxo Fig. 18). Huge stirred tank reactors are used, equipped with internal heat exchangers to control the heat of reaction. The solution of the catalyst recycle is simple but efficient. The catalyst remains in the reactor, products and unconverted propene are stripped by a huge excess of synthesis gas. Because of strong foaming, only a part of the reaction volume is used. After the gas has left the reactor, the products are removed by condensing, the big part of synthesis gas is separated from the liquid products and recycled via compressors. The liquid effluent of the gas-liquid separator... 

Chiu found that the diastereoselectivity had eroded slightly from an earlier synthetic study that included a methyl group in place of the MEM ether side chain of the A ring. Since it had been demonstrated that many of these catalytic systems are metal associated, Chiu attempted to change rhodium ligands from acetate to the... 

Conventional Cobalt Cobalt Ligand Rhodium Ligand... 

The rhodium ligand process, which will be discussed in greater detail in the following sections, operates at lower pressures and temperatures and overcomes the high by-product levels as well as the isomer ratio disadvantage of the conventional cobalt catalyst. 

Catalyst system Rhodium Ligand 17 mmol/L 10 mmol/L 13 mmol/L 10 mmol/L 20 mmol/L Ph2PCH2CH2P(0)Ph2... 

These considerations have been used to design new rhodium ligands [114, 115]. For instance, Achiwa and coworkers replaced the phenyl R groups of ligand 41 with cyclohexyl groups. The usefulness of the corresponding complexes is quite broad ( 7.1.1.1). 

Enantiopure 4-alkyl-1,3-oxazolidin-2-ones 3.62 have also been reported as rhodium ligands in similar cyclopropanations [937, 938, 941], although they are less efficient than 3.61. 

Cationic rhodium(I) complexes catalyze the isomerization of tertiary ally-lamines E-11.17 and Z-11.17 to enamines 11.18. In THF at 80°C, these enamines are readily hydrolyzed to provide aldehydes. When the rhodium ligand is a chiral diphosphine such as (R)- or (6)-binap 3.43 (Ar = Ph), or better yet, (R)- or (S)-tolbinap 3.43 (Ar = 4-MeC6H4), (R) or (5)-enamines 11.18 are obtained with an excellent enantiomeric excess. This method is used in industry for the synthesis of optically active dtronellol and menthol [811, 812, 853, 889], Biphemp 3.45 is also a highly potent ligand for the isomerization of Z-11.17, and ee s as high as 99.5% are observed [905]. The absolute configuration of the enatnine product... 

Hydrogenation of 1-phenylethanone in the presence of a bis(/j4-norbornadiene)dichloro-dirhodium/(S.S)-4,5-bis(diphenvlphosphinomcthy])-2,2-dimethyl-1,3-dioxolane, [Rh(nbd)CI]2/ (Diop), catalyst gives a 64% yield of (S)-l-phenylethanol in 80% ee7. Typically, a substrate,/ rhodium catalyst ratio of 100 1 and, within the catalyst, a rhodium/ligand ratio of 1 1.1 is used. 

Scale Diphos- phanc5 Substrate/ Rhodium/ Ligand Solvent, Amount (mL) Hydrogen Pressure (bar) Temp CC) Time (h) ee (%) Con- fig. Yield (%) ... 

The co-production of isobutyraldehyde has always proved embarrassing, but the introduction of rhodium liganded with a phosphine (Union Carbide) or phosphane (Ruhrchemie/Rhone-Poulenc) as catalyst permits milder reaction conditions and gives -/iso-ratios of 8 1 or more (see section 11.7.8.2). 

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