Rhodium catalysts phosphates

Muller has explored enantioselective C-H insertion using optically active rhodium complexes, NsN=IPh as the oxidant, and indane 7 as a test substrate (Scheme 17.8) [35]. Chiral rhodium catalysts have been described by several groups and enjoy extensive application for asymmetric reactions with diazoalkanes ]46—48]. In C-H amination experiments, Pirrung s binaphthyl phosphate-derived rhodium system was found to afford the highest enantiomeric excess (31%) of the product sulfonamide 8 (20equiv indane 7, 71% yield). 

The hT-benzenesulfonylprolinate catalyst, (3) of Fig. 1, provided the highest levels of stereocontrol. The binaphthyl hydrogen phosphate rhodium (II) catalyst also promoted chromanone formation and while the cis-diastereoselectivity was excellent (94%), the enantiocontrol was modest (33% ee) application of this catalyst to the P-lactam-forming C-H insertion reaction in Eq. (39) revealed high chemoselectivity (93% yield of trans),hut low enantioselectivity (26% ee) [28]. 

A process for the coproduction of acetic anhydride and acetic acid, which has been operated by BP Chemicals since 1988, uses a quaternary ammonium iodide salt in a role similar to that of Lil [8]. Beneficial effects on rhodium-complex-catalyzed methanol carbonylation have also been found for other additives. For example, phosphine oxides such as Ph3PO enable high catalyst rates at low water concentrations without compromising catalyst stability [40—42]. Similarly, iodocarbonyl complexes of ruthenium and osmium (as used to promote iridium systems, Section 3) are found to enhance the activity of a rhodium catalyst at low water concentrations [43,44]. Other compounds reported to have beneficial effects include phosphate salts [45], transition metal halide salts [46], and oxoacids and heteropolyacids and their salts [47]. 

The direct phenylation of l//-pyrrole (1) using a variety of metal catalysts has been reported and key examples will be discussed herein (Scheme 10.1). Sames et al. employed a rhodium catalyst to obtain 2-phenyl pyrrole (2) in 78% yield. The choice of CsOPiv proved to be crucial in this site-selective transformation as the carbonates and phosphates of alkali metals were ineffective. The authors believe that the mildly basic pivalate ligand catalyzes the rhodium-mediated C—H activation via a... 

In one patent (31), a filtered, heated mixture of air, methane, and ammonia ia a volume ratio of 5 1 1 was passed over a 90% platinum—10% rhodium gauze catalyst at 200 kPa (2 atm). The unreacted ammonia was absorbed from the off-gas ia a phosphate solution that was subsequently stripped and refined to 90% ammonia—10% water and recycled to the converter. The yield of hydrogen cyanide from ammonia was about 80%. On the basis of these data, the converter off-gas mol % composition can be estimated nitrogen, 49.9% water, 21.7% hydrogen, 13.5% hydrogen cyanide, 8.1% carbon monoxide, 3.7% carbon dioxide, 0.2% methane, 0.6% and ammonia, 2.3%. 

Activation of a C-H bond requires a metallocarbenoid of suitable reactivity and electrophilicity.105-115 Most of the early literature on metal-catalyzed carbenoid reactions used copper complexes as the catalysts.46,116 Several chiral complexes with Ce-symmetric ligands have been explored for selective C-H insertion in the last decade.117-127 However, only a few isolated cases have been reported of impressive asymmetric induction in copper-catalyzed C-H insertion reactions.118,124 The scope of carbenoid-induced C-H insertion expanded greatly with the introduction of dirhodium complexes as catalysts. Building on initial findings from achiral catalysts, four types of chiral rhodium(n) complexes have been developed for enantioselective catalysis in C-H activation reactions. They are rhodium(n) carboxylates, rhodium(n) carboxamidates, rhodium(n) phosphates, and < // < -metallated arylphosphine rhodium(n) complexes. 

In addition to the catalysts listed in Table 2, several rhodium(I) complexes of the various diphosphines prepared by acylation of bis(2-diphenylphosphinoethyl)amine were used for the hydrogenation of unsaturated acids as well as for that of pyruvic acid, aUyl alcohol and flavin mononucleotide [59,60]. Reactions were mn in 0.1 M phosphate buffer (pH = 7.0) at 25 °C under 2.5 bar H2 pressure. Initial rates were in the range of 1.6-200 mol H2/molRh.h. 

Much less severe conditions can be used with the Wilkinson homogeneous catalyst rhodium tricarbonyl triphenyl phosphate, HRh(CO)3(PC6H5)3. Pressures equal to 225 psig and temperatures of 100°C selectively produce the more useful linear form.34 The milder conditions more than compensate for the more expensive Rh (1000 times that of Co). The aldehyde product is distilled leaving the catalyst in the solvent ready for reuse. 

Enones are reduced to saturated ketones by catalytic hydrogenation provided the reaction is stopped following the absorption of 1 mol of hydrogen. " A number of catalysts were found useful for this, including platinum, platinum oxide,Pt/C, " Pd/C, - Rh/C, " tris(triphenylphosphine)rhodium chloride, - nickel-aluminum alloy in 10% aqueous NaOH, and zinc-reduced nickel in an aqueous medium. Mesityl oxide is formed from acetone and reduced in a single pot to methyl isobutyl ketone using a bifunctional catalyst which comprised palladium and zirconium phosphate (Scheme 20). 

The use of ionic liquids in combination with CO2 has the potential to produce cleaner processes with improved selectivity. The negligible miscibility of the ionic liquid in CO2 compared with appreciable amounts of CO2 that can be found in the liquid phase make the use of CO2 as a green solvent attractive for continuous reaction processes. Sellin, Webb, and Cole-Hamilton conducted a hydroformylation of hex-l-ene and 1-octene catalyzed by rhodium based catalyst in l-butyl-3-methylimidazolium hexafluoro-phosphate (BMIMHF) in contact with CO2. Improved n iso product selectivity was obtained, compared with that using toluene with similar selectivity, but substantially lower yield (40% compared to >99%). Using 1-octene as a substrate and [Rh2(OAc)4]/[1-propyl-3-methylimidazolium] [PhP(C6H4S03)2] as catalyst, over 20 hr of continuous operation was achieved with minimal catalyst leaching at 373 K. 

For the hydrosilylation reaction various rhodium, platinum, and cobalt catalysts were employed. For the further chain extension the OH-functionalities were deprotected by KCN in methanol. The final step involved the enzymatic polymerization from the maltoheptaose-modified polystyrene using (z-D-glucose-l-phosphate dipotassium salt dihydrate in a citrate buffer (pH = 6.2) and potato phosphorylase (Scheme 59). The characterization of the block copolymers was problematic in the case of high amylose contents, due to the insolubihty of the copolymers in THF. 

Muller and co-workers [52] have studied the intermolecular rhodium (11) catalyzed C-H insertion of a nitrene derived from [M-(p-nitrobenzenesulfonyl)imi-nojphenyliodinane into cycloalkanes, cycloalkenes, and cyclic ethers. In one example involving indane as the substrate with Pirrimg s chiral phosphate catalyst, the product showed an enantioenrichment of 31% ee, Eq. (34). 

Carbon monoxide carbonylates methane at 573-673 K in the presence of nitrous oxide on a rhodium-doped iron phosphate catalyst to produce methyl acetate [88a]. Vapor-phase carbonylation of toluene yields p-tolualdehyde, which can be easily oxidized to terephthalic acid [88b]. 

The following metal compounds are used for the preparation of the catalysts oxides, metal carbonyls, halides, alkyl and allyl complexes, as well as molybdenum, tungsten, and rhenium sulfides. Oxides of iridium, osmium, ruthenium, rhodium, niobium, tantalum, lanthanum, tellurium, and tin are effective promoters, although their catalytic activity is considerably lower. Oxides of aluminum, silicon, titanium, manganese, zirconium as well as silicates and phosphates of these elements are utilized as supports. Also, mixtures of oxides are used. The best supports are those of alumina oxide and silica. 

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