Iridium-based catalysts

The formation of C-C bonds is of key importance in organic synthesis. An important catalytic methodology for generating C-C bonds is provided by carbonylation. In the bulk chemicals arena this is used for the production of acetic acid by methanol carbonylation (Eqn. (9)) in the presence of rhodium- or, more recently, iridium-based catalysts (Maitlis et al, 1998). 

The stereochemistry of reduction by homogeneous catalysts is often controlled by functional groups in the reactant. Delivery of hydrogen occurs cis to a polar functional group. This behavior has been found to be particularly characteristic of an iridium-based catalyst that contains cyclooctadiene, pyridine, and tricyclohexylphosphine as ligands, known as the Crabtree catalyst 6 Homogeneous iridium catalysts have been found to be influenced not only by hydroxy groups, but also by amide, ester, and ether substituents.17... 

Because of the importance of olefin metathesis in the industrial production of olefins and polymers, many different catalysts have been developed. Almost all of these are transition metal-derived, some rare exceptions being EtAlCl2 [758], Me4Sn/Al203 [759], and irradiated silica [760]. The majority of catalytic systems are based on tungsten, molybdenum, and rhenium, but titanium-, tantalum-, ruthenium-, osmium-, and iridium-based catalysts have also proven useful for many applications. 

The above-mentioned complexes are the sole iridium derivatives applied to DCR, and the cycloaddition of nitrones to enals or methacrylonitrile, the unique process studied. We think that iridium-based catalysts are underrepresented in 1,3-dipolar cycloaddition chemistry. For example, no iridium (1) systems have been developed to this end. It can be anticipated that the (bidentate ligand)lr(l) fragment could be active (and stereoselective if chiral bidentate ligands are used) in DCR such as those involving azomethine ylides. 

In summary, the development of 1,3-dipolar cycloadditions will continue during the next years and the potential of iridium-based catalysts in this field is far from being exhausted. 

Iridium complexes are known to be generally less active in hydrosilylation reactions when compared to rhodium derivatives, although iridium-based catalysts with bonded chiral carbene ligands have been used successfully in the synthesis of chiral alcohols and amines via hydrosilylation/protodesilylation of ketones [46-52] and imines [53-55], The iridium-catalyzed reaction of acetophenone derivatives with organosubstituted silanes often gives two products (Equation 14.3) ... 

Abstract The purpose of this chapter is to present a survey of the organometallic chemistry and catalysis of rhodium and iridium related to the oxidation of organic substrates that has been developed over the last 5 years, placing special emphasis on reactions or processes involving environmentally friendly oxidants. Iridium-based catalysts appear to be promising candidates for the oxidation of alcohols to aldehydes/ketones as products or as intermediates for heterocyclic compounds or domino reactions. Rhodium complexes seem to be more appropriate for the oxygenation of alkenes. In addition to catalytic allylic and benzylic oxidation of alkenes, recent advances in vinylic oxygenations have been focused on stoichiometric reactions. This review offers an overview of these reactions... 

The platinum-catalyzed reaction of alkanes with chlorine leads to alkyl chlorides and alcohols (Table 6, entry 46) with modest rates and conversions [50], Cydooctane can be easily dehydrogenated (Table 6, entry 47) in the presence of a stabilized vinylalkane by use of the neutral rhenium compound ReH7(PR3)2 [51]. By employing an iridium-based catalyst, the photochemical dehydrogenation of methylcydohexane to methylenecyclohexane is performed at room temperature... 

It is worth noting that, contrary to x-olefin polymerisation systems, cycloolefin polymerisation systems are not restricted to dry, oxygen-free reaction conditions and hydrocarbon monomers. For example, iridium-based catalysts are efficient in the polymerisation of exo-bicyclo[2.2.1]hept-5-ene-2-carboxylic acid in aqueous or ethanolic media [48],... 

Iridium,204,205 together with osmium, has been not widely used in catalytic hydrogenation. Recently, however, iridium or iridium-based catalysts have been shown to be effective in various hydrogenations, such as in selective hydrogenation of a,P-unsaturated aldehydes to allylic alcohols (Section 5.2), of aromatic nitro compounds to the corresponding hydroxylamines (Section 9.3.6), of halonitrobenzenes to haloanilines without loss of halogen (Section 9.3.2), and in the stereoselective hydrogenation of carbon to carbon double bonds (see, e.g., eqs. 3.25-3.27 and Table... 

Ru Availability of a wide range of chiral dia mines and diphosphines simplifies com binatorial style searches for the most suitable catalyst for a given substrate. In general, less enantioselective than iridium based catalysts. 

Even oxygen nucleophiles have been introduced with good enantioselectivity using both palladium- and iridium-based catalysts. The conditions of the reaction need to be sufficiently mild that the product does not become a substrate for the allylic substitution, since this will ultimately lead to racemisation. Pivalate ( BuC02 ) and phenols have been used as nucleophiles, in the presence of palladium catalysts, with good results, while linear allylic carbonates are converted into chiral branched products with high ee using phenolates, aUcoxides and also hydroxylamines with iridium complexes. Sulfur nucleophiles have also been used in enantioselective allylic substitution reactions. ... 

In addition to rhodium-based catalysts, iridium-based catalysts have also been developed for carbonylation of methanol. The iridium system, known as the Cativa process, follows a cycle similar to the rhodium system in Figure 14.20, beginning with oxidative addition of... 

L. Badia-Bou, E. Mas-Marza, P. Rodenas, E.M. Barea, F. Fabregat-Santiago, S. Gimenez, E. Peris, J. Bisquert, Water oxidation at hematite photoelectrodes with an iridium-based catalyst. J. Phys. Chem. C 117, 3826-3833 (2013)... 

Imines. The hydrogenation of imines is challenging because is difficult to find catalysts providing high enantioselectivity, broad applicability and acceptable rates at moderate pressures of hydrogen. Additional complications are the interconversions between the E and Z isomers in case of the acyclic substrates, the tendency to racemisation of the secondary amines obtained after the reaction and the inhibition of the catalyst by amine coordination. Iridium-based catalysts are finding increased application in this reaction. The most common substrates are ( )-imines derived from acetophenone (35, Scheme 7.15). 

The synthesis of monomers derived from fatty acids incorporating end-functions susceptible to classical poly addition reactions has been revived recently and yielded interesting results. Vinyl oleate (VO) and vinyl linoleate (VL) were synthesised by a transvinylation reaction of the corresponding fatty acids with an excess of vinyl acetate (VAc) in bulk using a safe iridium-based catalyst instead of previously used mercury-based counterparts (which are not acceptable today because of safety concerns) [91]. Scheme 4.21 summarises this synthesis in the case of OA. 

Fig. 53 Iridium-based catalysts for the transfer hydrogenation of imines...

A systematic study by Kennedy and coworkers was performed to find a catalyst that promoted the oxidation of insulin at physiological pH but was more stable than mvRuOx in these solutions [13]. Polynuclear hexacyanometallates, binary metal oxides, and metallophthalocyanines with various metal centers were tested as electrode modifiers using Ru, Cr, Fe, Co, and Os alone or in combination with Pb, Pd, and Ir. Of the modifiers tested, only with films originating by oxidation of RuClj or (NH4)RuClg was oxidation of insulin promoted (an extension of this work, detailed later, described an iridium-based catalyst for the oxidation of insulin [14]). Voltammetric deposition of a ruthenium oxide film from a 0.2 mM RuClj, 10 mM HCIO, mixture onto glassy carbon yielded a modified... 

Fig. 19 The iridium-based catalysts of Bernhard (1), Crabtree (2,4, and associated intermediate 5), and Macchioni (3).

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