Hydrogen transfer iridium catalysts

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. ... 

A wide variety of iridium-based hydrogenation catalysts are currently under development, notably for organic syntheses including enantioselective synthesis. Hydrogenation by hydrogen transfer is well known [15], and the reduction of C=0 and C=N double bonds is also possible [16, 17]. 

Henbest and Mitchell [78] have shown that water can be used as hydrogen source with chloroiridic acid (6) as the catalyst through oxidation of phosphorous acid (59) to phosphoric acid (60) in aqueous 2-propanol. Under these conditions, no hydrogen transfer occurs from 2-propanol. However, iridium complexes with sulfoxide or phosphine ligands show the usual transfer from 2-pro-panol [79-81]. 

Transition-metal catalysts are, in general, more active than the MPVO catalysts in the reduction of ketones via hydrogen transfer. Especially, upon the introduction of a small amount of base into the reaction mixture, TOFs of transition-metal catalysts are typically five- to 10-fold higher than those of MPVO catalysts (see Table 20.7, MPVO catalysts entries 1-20, transition-metal catalysts entries 21-53). The transition-metal catalysts are less sensitive to moisture than MPVO catalysts. Transition metal-catalyzed reactions are frequently carried out in 2-propanol/water mixtures. Successful transition-metal catalysts for transfer hydrogenations are based not only on iridium, rhodium or ruthenium ions but also on nickel [93], rhenium [94] and osmium [95]. It has been reported that... 

Scheme 1 A highly active iridium catalyst for transfer hydrogenation...

The mechanism for the iridium-catalyzed hydrogen transfer reaction between alcohols and ketones has been investigated, and there are three main reaction pathways that have been proposed (Scheme 4). Pathway (a) involves a direct hydrogen transfer where hydride transfer takes place between the alkoxide and ketone, which is simultaneously coordinated to the iridium center. Computational studies have given support to this mechanism for some iridium catalysts [18]. 

New catalytic allylation methodologies continue to emerge. For example, iridium-catalyzed transfer hydrogenation of a-(trimethylsilyl)allyl acetate in the presence of aldehydes mediated by isopropanol and employing the iridium catalyst... 

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... 

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... 

In the past, this field has been dominated by ruthenium, rhodium and iridium catalysts with extraordinary activities and furthermore superior enantioselectivities however, some investigations were carried out with iron catalysts. Early efforts were reported on the successful use of hydridocarbonyliron complexes HFcm(CO) as reducing reagent for a, P-unsaturated carbonyl compounds, dienes and C=N double bonds, albeit complexes were used in stoichiometric amounts [7]. The first catalytic approach was presented by Marko et al. on the reduction of acetone in the presence of Fe3(CO)12 or Fe(CO)5 [8]. In this reaction, the hydrogen is delivered by water under more drastic reaction conditions (100 bar, 100 °C). Addition of NEt3 as co-catalyst was necessary to obtain reasonable yields. The authors assumed a reaction of Fe(CO)5 with hydroxide ions to yield H Fe(CO)4 with liberation of carbon dioxide since basic conditions are present and exclude the formation of molecular hydrogen via the water gas shift reaction. H Fe(CO)4 is believed to be the active catalyst, which transfers the hydride to the acceptor. The catalyst presented displayed activity in the reduction of several ketones and aldehydes (Scheme 4.1) [9]. 

Recent mechanistic studies on transition metal-catalysed hydrogen transfer reactions have been reviewed. Experimental and theoretical studies showed that hydrogen transfer reactions proceed through different pathways. For transition metals, hydridic routes are the most common. Within the hydridic family there are two main groups the monohydride and dihydride routes. Experimentally, it was found that whereas rhodium and iridium catalysts favour the monohydride route, the mechanism for ruthenium catalysts proceeds by either pathway, depending on the ligands. A direct hydrogen transfer mechanism has been proposed for Meerwein-Ponndorf-Verley (MPV) reductions.352... 

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. 

Iridium and osmium are little used for ketone to carbinol reduction, Ir mainly as a hydrogen transfer catalyst, and Os in order to minimize ring saturation in keto-com-pounds having an aromatic nucleus. ... 

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