Iridium synthesis activity

An even more impressive example of catalytic efficiency has recently been disclosed by Novartis (Bader and Bla.ser, 1997). The key step in a proce.ss for the synthesis of the optically active herbicide, (S)-metolachlor involves asymmetric hydrogenation of a prochiral imine catalysed by an iridium-ferrocenyldipho-sphine complex (see Fig. 2.36). 

Et4N]2[Fe2lr2(CO)i2] cluster precursor, which exhibit a high activity in the synthesis of methanol from CO and H2, were studied by Ir and Fe Mossbauer spectroscopy. The study extends from the precursors via the fresh to the aged catalysts. The presence of iridium in the metallic state as well as the presence of trivalent, divalent and alloyed iron is detected. Representative Ir and Fe Mossbauer spectra are shown in Fig. 7.69. Information about the adsorption on the surface of MgO... 

A new class of heterogeneous catalyst has emerged from the incorporation of mono- and bimetallic nanocolloids in the mesopores of MCM-41 or via the entrapment of pro-prepared colloidal metal in sol-gel materials [170-172], Noble metal nanoparticles containing Mex-MCM-41 were synthesized using surfactant stabilized palladium, iridium, and rhodium nanoparticles in the synthesis gel. The materials were characterized by a number of physical methods, showed that the nanoparticles were present inside the pores of MCM-41. They were found to be active catalysts in the hydrogenation of cyclic olefins such as cyclohexene, cyclooctene, cyclododecene, and... 

The synthesis of pure metalacycle la from [Ir(COD)Cl]2 and LI requires only amine base and heat, followed by precipitation and removal of amine hydrochloride. However, this complex was typically generated in situ during early studies by the treatment of a combination of [lr(COD)Cl]2 and LI with an amine base, such as uPrNH2, l,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), or l,8-diazabicyclo[5.4.0] undec-7-ene (DBU). If a 1 1 ratio of iridium to ligand is used, a mixture of la and [Ir(COD)Cl]2 is produced. Helmchen et al. have reported that catalyst activation in the presence of tetrahydrothiophene (THT) prevents coordination of the k -phosphoramidite [71]. 

The use of ethylene adduct lb is particularly important when the species added to activate catalyst la is incompatible with one of the reaction components. Iridium-catalyzed monoallylation of ammonia requires high concentrations of ammonia, but these conditions are not compatible with the additive [Ir(COD)Cl]2 because this complex reacts with ammonia [102]. Thus, a reaction between ammonia and ethyl ciimamyl carbonate catalyzed by ethylene adduct lb produces the monoallylation product in higher yield than the same reaction catalyzed by la and [Ir(COD)Cl]2 (Scheme 27). Ammonia reacts with a range of allylic carbonates in the presence of lb to form branched primary allylic amines in good yield and high enantioselectivity (Scheme 28). Quenching these reactions with acyl chlorides or anhydrides leads to a one-pot synthesis of branched allylic amides that are not yet directly accessible by metal-catalyzed allylation of amides. 

To date, only a few iridium catalysts have been applied to industrially relevant targets, especially on the larger scale. It is likely that several types of Ir catalyst are, in principle, feasible for technical applications in the pharmaceutical and agrochemical industries. At present, the most important problems are the relatively low catalytic activities of many highly selective systems and the fact, that relatively few catalysts have been applied to multifunctional substrates. For this reason, the scope and limitations of most catalysts known today have not yet been explored. For those in academic research, the lesson might be to employ new catalysts not only with monofunctional model compounds but also to test functional group tolerance and-as has already been done in some cases-to apply the catalysts to the total synthesis of relevant target molecules. 

A large number of reports have concerned transfer hydrogenation using isopropanol as donor, with imines, carbonyls-and occasionally alkenes-as substrate (Scheme 3.17). In some early studies conducted by Nolan and coworkers [36], NHC analogues of Crabtree catalysts, [Ir(cod)(py)(L)]PF,5 (L= Imes, Ipr, Icy) all proved to be active. The series of chelating iridium(III) carbene complexes shown in Scheme 3.5 (upper structure) proved to be accessible via a simple synthesis and catalytically active for hydrogen transfer from alcohols to ketones and imines. Unexpectedly, iridium was more active than the corresponding Rh complexes, but... 

Perhaps most dramatically of all, for the first time, bis(carbene)-substituted iridium complexes, such as [Ir(cod)(NHC)2] (NHC = 1,3-dimethyl- or 1,3-dicyclohexylimidazolin-2-ylidene] were successfully used by Herrmann and coworkers as C—H-activation catalysts in the synthesis of arylboronic acids starting from pinacolborane and arene derivatives [46]. 

This chapter describes the application of iridium-catalyzed reactions in the synthesis of molecular and macromolecular organosilicon compounds and related sihcon derivatives. Some mechanistic implications are introduced which illustrate the specific catalytic activation of organic and silicon compounds by iridium... 

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) ... 

The direct silylation of arenes through C—H bond activation provides an attractive route for the synthesis of useful aromatic compounds [64]. Vaska s complex was the first of the iridium catalysts to be reported for activation of the C—H bond in benzene by Si—H of pentamethyldisiloxane to yield phenylsubstituted siloxane [65]. However, a very attractive method for the aromatic C—H silylation with disilanes has been recently reported by the groups of Ishiyama and Miyaura [66-68]. 

Carbonylation of Methyl Acetate on Ni/A.C. Catalysts. Table II shows the catalytic activities of nickel and platinum group metals supported on activated carbon for the carbonylation of methyl acetate. Ruthenium, palladium, or iridium catalysts showed much lower activity for the synthesis of acetic anhydride than the nickel catalyst. In contrast, the rhodium catalyst, which has been known to exhibit an excellent carbonylation activity in the homogeneous system (1-13), showed nearly the same activity as the nickel catalyst but gave a large amount of acetic acid. 

Quite a wide range of substrates 100 could be converted into products 101 with high ee values since it is known that the N-protecting group of 101 can easily be cleaved, the approach represents a formal synthesis of optically active amines. It remains to be seen if this iridium/sulfoximine combination also opens up an alternative access to industrially relevant products such as the herbicide (S)-metolachlor produced by Syngenta [80]. 

As mentioned above in connection with the acetic acid synthesis, iridium complexes catalyze the water-gas shift reaction (equation 70). From IrCl3-3H20 and sulfonated derivatives of bipy and phen, water-soluble catalysts were obtained.444 Using dioxane as solvent, complexes of the type [Ir(cod)L2]+ (L= PMePh2, PPh3), [Ir(cod)L ]+ (L = diphos, phen, 4,7-Me2-phen, 4,7-Ph2-phen, 3,4,7,8-Me4-phen) and [Ir(cod)X] (X = 4,7-diphenylphenanthroline disulfonate) also catalyzed the reaction, with the anionic species being most active.470 The mechanism was thought... 

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