Iridium catalysts preparation

The reduction of nitrobenzene with cyclohexane, cyclohexene, and n-hexane over a series of catalysts on aluminum oxide afforded aniline in yields greater than 90%. An iridium catalyst prepared by Adams method is useful for the reduction of nitroaromatics to hydroxylamines... 

Iridium catalyst [1,519, before Iron]. An iridium catalyst prepared by the method of R. Adams is useful for the reduction of aromatic nitro compounds to arylhydroxyl-... 

Cyanuric acid can also be prepared from HNCO (100). Isocyanic acid [75-13-8] can be synthesized directiy by oxidation of HCN over a silver catalyst (101) or by reaction of H2, CO, and NO (60—75% yield) over palladium or iridium catalysts at 280—450°C (102). Ammonium cyanate and urea are by-products of the latter reaction. 

Rhodium-on-carbon has also been found to bring about the formation of 2,2 -biquinoline from quinoline, the yield and the percentage conversion being similar to that obtained with palladium-on-carbon. On the other hand, rhodium-on-carbon failed to produce 2,2 -bipyridine from pyridine, and it has not yet been tried with other bases. Experiments with carbon-supported catalysts prepared from ruthenium, osmium, iridium, and platinum have shown that none of these metals is capable of bringing about the formation of 2,2 -biquinoline from quinoline under the conditions used with palladium and rhodium. ... 

Iridium-catalyzed transfer hydrogenation of aldehyde 73 in the presence of 1,1-dimethylallene promotes tert-prenylation [64] to form the secondary neopentyl alcohol 74. In this process, isopropanol serves as the hydrogen donor, and the isolated iridium complex prepared from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and (S)-SEGPHOS is used as catalyst. Complete levels of catalyst-directed diastereoselectivity are observed. Exposure of neopentyl alcohol 74 to acetic anhydride followed by ozonolysis provides p-acetoxy aldehyde 75. Reductive coupling of aldehyde 75 with allyl acetate under transfer hydrogenation conditions results in the formation of homoallylic alcohol 76. As the stereochemistry of this addition is irrelevant, an achiral iridium complex derived from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and BIPHEP was employed as catalyst (Scheme 5.9). 

Asymmetric hydrogenation of nitrones in an iridium catalyst system, prepared from [IrCl(cod)]2, (S)-BINAP, NBu 4 BH4, gives with high enantioselectivity the corresponding A-hydroxylamines which are important biologically active compounds and precursors of amines (480). Further reduction of hydroxylamines to secondary amines or imines can be realized upon treatment with Fe/AcOH (479), or anhydrous titanium trichloride in tetrahydrofuran (THF) at room temperature (481). 

The stereoselective isomerization of unsymmetrical diallyl ethers to allyl ( )-vinyl ethers was also carried out in the presence of a cationic iridium(l) catalyst. The catalyst prepared in situ by treating [Ir(cod)(PPh2Me)2]PF6 with hydrogen was found to be an excellent catalyst for the selective isomerization of a less substituted allyl group to an ( )-vinyl ether (Scheme 44).72... 

A polymer-supported iridium catalyst 4 has been prepared and used in the isomerization of the double bonds in aryl allyl ethers and aryl allylic compounds with excellent trans-scIcctivity and without conventional workup procedures (Scheme 45).73... 

Catalysts other than homogeneous (molecular) compounds such as nanoparticles have been used in ionic liquids. For example, iridium nanoparticles prepared from the reduction of [IrCl(cod)2] (cod = cyclooctadiene) with H2 in [bmim][PF6] catalyses the hydrogenation of a number of alkenes under bipha-sic conditions [27], The catalytic activity of these nanoparticles is significantly more effective than many molecular transition metal catalysts operating under similar conditions. 

Other methods for the preparation of acetic acid are partial oxidation of butane, oxidation of ethanal -obtained from Wacker oxidation of ethene-, biooxidation of ethanol for food applications, and we may add the same carbonylation reaction carried out with a cobalt catalyst or an iridium catalyst. The rhodium and iridium catalysts have several distinct advantages over the cobalt catalyst they are much fester and fer more selective. In process terms the higher rate is translated into much lower pressures (the cobalt catalyst is operated by BASF at pressures of 700 bar). For years now the Monsanto process (now owned by BP) has been the most attractive route for the preparation of acetic acid, but in recent years the iridium-based CATTVA process, developed by BP, has come on stream. 

Bis-allylic oxidation of 23 and related cyclohexa-1,4-dienes provides a convenient and general preparation of cyclohexa-2,5-dien-l-ones (Scheme 7). These cross-conjugated die-nones are substrates for a variety of photochemical rearrangement and intramolecular cycloaddition reactions. Amide-directed hydrogenations of dienones 24a and 24b with the homogeneous iridium catalyst afford cyclohexanones 25a and 25b, containing three stereogenic centers on the six-... 

The preparation of polymer-supported iridium catalysts (61) and (62) for the stereoselective isomerization of double bonds using polystyrene based immobilized triphenyl phosphine were recently reported by Ley and coworkers (Fig. 4.5). The immobilized catalyst is potentially useful for deprotection strategies of aUyl ethers [130]. 

Iridium nanoparticles prepared in imidazolium-based ILs have been also used in the catalytic hydrogenation of ketones under mild conditions [50]. Firstly, cyclohexanone was chosen as the model substrate to optimize the reaction conditions (temperature, hydrogen pressure, catalyst concentration). Initially, isolated lr(0) nanoparticles were tested in a solventless system for the hydrogenation of cyclohexanone the prehminarily results are listed in Table 15.6. 

Iridium nanopartides also catalyze the hydrogenation of benzyhnethylketone, with high selectivity in reduction of the aromatic ring (92% selectivity in saturated ketone, 8% in saturated alcohol at 97% benzylmethylketone conversion). This preferential coordination of the aromatic ring can be attributed to steric effects that make carbonyl coordination difficult. Therefore, metallic iridium nanoparticles prepared in ILs may serve as active catalysts for the hydrogenation of carbonyl compounds in both solventless and biphasic conditions. 

Catalysts Prepared from Metal Carbonyls of Croup 9 Cobalt, Rhodium and Iridium... 

Catalysts prepared from iridium neutral binary carbonyl compounds and several supports have been studied extensively. Small Ir (x = 4, 6) clusters supported on several oxides and caged in zeolite, and their characterization by EXAFS, have been prepared [159, 179, 180, 194-196]. The nuclearity of the resulting metallic clusters has been related with their catalytic behavior in olefin hydrogenation reactions [197]. This reaction is structure insensitive, which means that the rate of the reac-hon does not depend on the size of the metallic particle. Usually, the metallic parhcles are larger than 1 nm and consequently they have bulk-like metallic behavior. However, if the size of the particles is small enough to lose their bulk-like metallic behavior, the rate of the catalytic reaction can depend on the size of the metal cluster frame used as catalyst. 

Iridium carbynes, preparation and characteristics, 7, 361 Iridium catalysts... 

Iridium catalysts have been employed only infrequently in asymmetric hydroformylation with a catalytic system prepared in situ from [Ir(CO)3(PPh3)2]BPh4 and DIOP, a low optical yield (0.5 %) was obtained in the synthesis of 2-acetoxy-propanal from vinyl acetate32). 

II. 5).204,205 Unsupported iridium catalysts have been prepared by reducing an iridium oxide of Adams type at 165°C under a stream of hydrogen206 or by reducing iridium hydroxide, prepared by addition of lithium hydroxide to an aqueous solution of irid-ium(III) chloride, at 80-90°C and 8 MPa H2.204 Unsupported and supported iridium catalysts may also be prepared by reduction of iridium(IV) chloride with sodium boro-hydride.207 It is noted that the catalytic activity of deactivated iridium can be almost completely regenerated by treatment with concentrated nitric acid.205... 

The substrate (0.5 ml) was hydrogenated over 5 mg of iridium black in 7 ml t-BuOH at 80°C and 4.9 MPa H2. The same catalyst, prepared from the iridium hydroxide precipitated at pH 7, was used throughout the experiments. 

An interesting variation of hydroformylation with a great potential for the industrial preparation of primary amines is hydroaminomethylation. In this process two catalytic reactions are combined, a hydroformylation and a reductive amination of the resulting aldehyde. Although first described more than 60 years ago a really successful procedure was only published recently [78]. To ensure the success of this sequence a rhodium catalyst for the hydroformylation was combined with an iridium catalyst for the imine reduction in a two-phase system, similar to the Ruhrchemie/Rhone-Poulenc process for the hydroformylation. It was demonstrated that less polar solvents such as toluene in combina-... 

The iridium catalyst used in this preparation may be regenerated by reducing the volume of the aqueous residue to about 200 ml. at diminished pressure. This solution is then used instead of the iridium tetrachloride and water called for in the procedure. 

The Oppenauer Oxidation. When a ketone in the presence of an aluminum alkoxide is used as the oxidizing agent (it is reduced to a secondary alcohol), the reaction is known as the Oppenauer oxidation. This is the reverse of the Meerwein-Ponndorf-Verley reaction (19-36) and the mechanism is also the reverse. The ketones most commonly used are acetone, butanone, and cyclohexanone. The most common base is aluminum ferf-butoxide. The chief advantage of the method is its high selectivity. Although the method is most often used for the preparation of ketones, it has also been used for aldehydes. An iridium catalyst has been developed for the Oppenauer oxidation, and also a water-soluble iridium catalyst An uncatalyzed reaction under supercritical conditions was reported. 

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