Aromatization iridium catalysts

Extensive investigations in our laboratories on the deactivation of rhodium and iridium catalysts has shown there to be a number of different mechanisms involved. Both, rhodium and iridium catalysts are generally less stable at higher temperatures, and have more labile ligands than their ruthenium counterparts. All of the catalysts are affected by pH, but the ruthenium catalysts seem to be more readily deactivated by acid. Indeed, these reactions are often quenched with acetic acid, whilst stronger acids are used to quench the rhodium reactions. Each of the catalysts can be deactivated by product inhibition, the ruthenium catalyst with aromatic substrates such as phenylethanol, and the rhodium and iridium ones by bidentate chelating products. 

The treatment of [Cp MCl2]2 (M = Rh and Ir) with (S,S)-TsDPEN gave chiral Cp Rh and Cp Ir complexes (12a and 12b Scheme 5.9). An asymmetric transfer hydrogenation of aromatic ketones using complex 12 was carried out in 2-propanol in the presence of aqueous KOH (1 equiv.) the results obtained are summarized in Table 5.4. In all of the reactions, the (S)-alcohols were obtained with more than 80% enantiomeric excess (ee) and in moderate to excellent yields. The rhodium catalyst 12a was shown to be considerably more active than the iridium catalyst... 

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]. 

Not surprisingly, these rhodium and iridium carbene complexes were tested for their catalytic behaviour in the transfer hydrogenation of benzophenone and acetophenone (M +3), the hydrosilylation of alkynes (M +1) and also the catalytic cyclisation of acetylenic carboxylic acids (M +1). Hydrogenation works better for iridium than rhodium and for aromatic than for aliphatic ketones [40,43,44]. The iridium(I) complex is the first iridium catalyst showing activity for the cyclisation of acetylenic carboxylic acids [40]. The results for the hydrosilylation reactions were very moderate. 

Selective reduction to hydroxylamine can be achieved in a variety of ways the most widely applicable systems utilize zinc and ammonium chloride in an aqueous or alcoholic medium. The overreduction to amines can be prevented by using a two-phase solvent system. Hydroxylamines have also been obtained from nitro compounds using molecular hydrogen and iridium catalysts. A rapid metal-catalyzed transfer reduction of aromatic nitroarenes to N-substituted hydroxylamines has also been developed the method employs palladium and rhodium on charcoal as catalyst and a variety of hydrogen donors such as cyclohexene, hydrazine, formic acid and phosphinic acid. The reduction of nitroarenes to arylhydroxyl-amines can also be achieved using hydrazine in the presence of Raney nickel or iron(III) oxide. ... 

The mechanism of thallation appears to be complex, with electrophilic and electron-transfer mechanisms both taking place. Transient metalated aryl complexes can be formed that react with another aromatic compound. Aryl iodides reacted with benzene to form a biaryl in the presence of an iridium catalyst. Aniline derivatives reacted with TiCLj to give the para-homo coupling product (RaN-Ar-Ar-NRj). ... 

In Figures 5.5 and 5.6, data on the platinum-iridium and platinum-rhenium catalysts are shown for the reforming of a 70-190 C boiling range Persian Gulf naphtha to produce 98 research octane number product at a pressure of 28.2 atm and a temperature of 490 C (33). The naphtha contained (on a liquid volume percentage basis) 69.7% alkanes, 18.5% cycloalkanes, and 11.8% aromatic hydrocarbons. The density of the naphtha was 0.7414 g/cm3. The data in Figure 5.5 show that the platinum-iridium catalyst is almost twice as active as the platinum-rhenium catalyst. 

The attractive features of platinum-rhenium and platinum-iridium catalysts can be combined in a reforming operation. The data for the reactions of selected hydrocarbons considered earlier for platinum-rhenium and platinum-iridium catalysts indicate that the former catalyst is more selective for the conversion of cycloalkanes to aromatics, while the latter is more selective for the dehydrocyclization of alkanes. Since cycloalkane conversion occurs primarily in the initial part of a reforming system while dehydrocyclization is the predominant reaction after the cycloalkanes have reacted, it is reasonable to use a platinum-rhenium catalyst in the front of the system and to follow it with a platinum-iridium catalyst (32). 

In comparing the platinum-iridium and platinum-rhenium catalysts, we see that the yields of benzene, toluene, and total aromatic hydrocarbons in the C5+ reformate are consistently higher for the platinum-iridium catalyst. For the combined catalyst system, the yields of benzene and toluene are equivalent to those observed with the platinum-iridium catalyst. 

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-... 

Progress on the addition of aromatic C-H bonds to olefins has been made by Periana with iridium catalysts - - and Gunnoe with ruthenium catalysts. - Both systems illustrate that the anti-Markovnikov addition products can be generated in larger quantities than the Markovnikov products, although mixtures of regioisomers are still observed. Intramolecular additions of the C-H bonds of electron-rich heterocycles to electron-deficient alkenes have also been reported (Equation 18.65). Most recently, Tilley has reported the addition of the C-H bond of methane across an olefin catalyzed by scandocene complexes. This reaction occurs, albeit slowly, with Markovnikov regiochemistry. 

The well-known iridium catalyst [Ir(COD)(Cy3P)-(Py)]PF, where COD = 1,5-cyclooctadiene and Py = pyridine, demonstrates excellent regioselectivity in isotopic exchange reactions of acetanilides and other substituted aromatic substrates. NMR spectroscopy is invaluable in identifying the site(s) of tritium incorporation - there are many instances where the broad signals in the corresponding NMR spectra are much less informative. 

Direct silylation of aromatic compounds is carried out with 1,2-di-tert-butyl-1,1,2,2-tetrafluorodisilane (165) that serves as a silylating reagent in the presence of an iridium catalyst [60]. For example, the Irotalyzed C-H activation reaction of o-xylene selectively proceeds at the aromatic C-H bond rather than the benzylic one to give 166 in a high yield (Scheme 5.43). The synthetic utility of the products... 

More recently, the same type of hgand was used to form chiral iridium complexes, which were used as catalysts in the hydrogenation of ketones. The inclusion of hydrophihc substituents in the aromatic rings of the diphenylethylenediamine (Fig. 23) allowed the use of the corresponding complexes in water or water/alcohol solutions [72]. This method was optimized in order to recover and reuse the aqueous solution of the catalyst after product extraction with pentane. The combination of chiral 1,2-bis(p-methoxyphenyl)-N,M -dimethylethylenediamine and triethyleneglycol monomethyl ether in methanol/water was shown to be the best method, with up to six runs with total acetophenone conversion and 65-68% ee. Only in the seventh run did the yield and the enantioselectivity decrease slightly. 

Much research has been carried out into direct amination of aromatic substrates, typified by the direct conversion of benzene to aniline using ammonia and a catalyst. Although there have been many patented routes conversions, are normally low, making them uneconomic. Modem catalysts based on rhodium and iridium, together with nickel oxide (which becomes reduced), have proved more active,and such is the research activity in this area that it is only a matter of time before such processes become widely used. 

---