Rhodium catalysts containing carboxylated

Other recent reports have also indicated that mixed-metal systems, particularly those containing combinations of ruthenium and rhodium complexes, can provide effective catalysts for the production of ethylene glycol or its carboxylic acid esters (5 9). However, the systems described in this paper are the first in which it has been demonstrated that composite ruthenium-rhodium catalysts, in which rhodium comprises only a minor proportion of the total metallic component, can match, in terms of both activity and selectivity, the previously documented behavior (J ) of mono-metallic rhodium catalysts containing significantly higher concentrations of rhodium. Some details of the chemistry of these bimetallic promoted catalysts are described here. 

Various rhodium catalysts, such as [(l,5-hexadiene)RhCl]2, RhCl3, or Rh black, in methanol solution under H2 reduce and esterify unsaturated carboxylic acids. Even the phenyl ring of cinnamic acid derivatives is reduced at ambient conditions as shown in equation (3). The substrate must contain a C=C double... 

Dror and Manassen hydrogenated a- and cyclic olefins such as 1-octene, 1 -dodecene and cyclohexene in micellar systems using rhodium catalysts modified with water-insoluble carboxylated tenside phosphines 45 (Table 3 n=3,5,7,9,l 1) in the presence of conventional tensides such as sodium dodecylsulfate (SDS) or cetyltrimethylammonium bromide (CTAB) and cosolvents e.g. dimethyl sulfoxide. Linear olefins were more reactive than cyclic olefins. Maximum efficiency was observed in the presence of the anionic surfactant SDS when 45 contained a chain of 5-7 carbon atoms. In contrast, using the cationic tenside CTAB the ligand 45 with 5 C atoms was almost inactive but became active again with dodecyl trimethylammonium bromide. 

Reactions of ruthenium catalyst precursors in carboxylic acid solvents with various salt promoters have also been described (170-172, 197) (Table XV, Expt. 7). For example, in acetic acid solvent containing acetate salts of quaternary phosphonium or cesium cations, ruthenium catalysts are reported to produce methyl acetate and smaller quantities of ethyl acetate and glycol acetates (170-172). Most of these reactions also include halide ions the ruthenium catalyst precursor is almost invariably RuC13 H20. The carboxylic acid is not a necessary component in these salt-promoted reactions as shown above, nonreactive solvents containing salt promoters also allow production of ethylene glycol with similar or better rates and selectivities. The addition of a rhodium cocatalyst to salt-promoted ruthenium catalyst solutions in carboxylic acid solvents has been reported to increase the selectivity to the ethylene glycol product (198). 

Ketones and aldehydes, including activated and enolizable substrates and those containing alcohol or carboxylic acid substituents, can be alkynylated using a rhodium(II) catalyst complexed with a bulky phosphine.194... 

Rhodium and palladium catalysts that contain 4 display high enantioselectivities for the asymmetric hydrogenation of enamides, itaconates, P-keto esters, asymmetric hydroboration, and asymmetric allylic alkylation,80 82 but this ligand system distinguishes itself from other chiral bisphos-phines in the asymmetric reduction of tetrahydropyrazines and tetrasubstituted olefins (see also Chapter 15). The reduction of tetrahydropyrazines produces the piperazine-2-carboxylate core,... 

Enantiomerically pure carboxylic acids are routinely obtained from N-acylsultams by Hydrogen Peroxide assisted saponification with Lithium Hydroxide in aqueous THF. 4 Alternatively, transesterification can be effected under neutral conditions in allyl alcohol containing Titanium Tetraisopropoxide, giving the corresponding allyl esters which can be isomerized/hydrolyzed with Wilkinson s catalyst (Chlorotris(triphenylphosphine)rhodium(I)) in Et0H-H20. This provides a convenient route to carboxylic acids containing base-sensitive functionality. Primary alcohols are obtained by treatment with L-Selectride (Lithium Tri-s-butylborohydride) in THF at ambient temperature. ... 

Suitable catalysts for this type of process must be capable of hydrogenating both carboxylic acids and their esters to alcohols, but also of carbonylating these compounds to their homologous acids. The best catalytic systems known contain either Rh or Ru in the presence of iodide. Ruthenium iodide systems are the most active ones in the hydrogenation reaction, but suffer from low activity in the carbonylation step, whereas rhodium iodide systems are very active when carbonylating alcohols to their acids (cf. Section 2.1.2.1). 

Intramolecular C—H insertion, on the other hand, is already a practical alternative for the constmction of cyclobutanols, P-lactams - and of cyclopentane-containing targets. With regard to the latter, diazo transfer can be effected on a large scale with the inexpensive methanesulfonyl azide. The rhodium carboxylate catalysts are effective at very low concentration (<1 mol %) and can easily be recovered from the reaction mixture, if desired. ... 

The dimeric rhodium(II) compounds become enantioselective catalysts if they contain optically active ligands. For this the anions of optically active carboxylic acids seem to be most appropriate. The complex in which the Rhz unit is clamped by four mande-late anions was synthesized and structurally characterized some time ago. [7] As a catalyst, however, this complex results in only small enantiomeric excesses. [8] The reason for this is probably that the asymmetric centers lie in a plane between the two Rh atoms and are thus too far away from the coordination sites directed to the outside, at which the catalysis occurs. Chiral substituents at the nitrogen atoms of carboxamide anions would be considerably closer to these reaction centers, and... 

Baldwin and Widdison prepared ethyl a-deuterodiazoacetate by reacting ethyl-diazoacetate with two different batches of sodium deuteroxide in deuterium oxide containing hexadecyltrimethylammonium bromide as a phase transfer catalyst. A proton NMR analysis showed the product was more than 98% deuterated at the a-carbon. The deuter-ated diazoacetate was then converted into two diastereomeric ethyl-(2-bromo-2-methylcyclopropane-1 -d) carboxylates by reaction with 2-bromopropene and a rhodium(II)... 

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