Iridium catalysts ketones

Hydrogenation of substrates having a polar multiple C-heteroatom bond such as ketones or aldehydes has attracted significant attention because the alcohols obtained by this hydrogenation are important building blocks. Usually ruthenium, rhodium, and iridium catalysts are used in these reactions [32-36]. Nowadays, it is expected that an iron catalyst is becoming an alternative material to these precious-metal catalysts. 

The isomerization of propargylic alcohols to a,/3-enones was also developed by using an iridium catalyst. A variety of secondary propargylic alcohols can be isomerized to the corresponding a,/3-unsaturated ketones in the presence of 1 mol% IrHs(Pr 3P)2 (Scheme 54).93... 

Transition metals can display selectivities for either carbonyls or olefins (Table 20.3). RuCl2(PPh3)3 (24) catalyzes reduction of the C-C double bond function in the presence of a ketone function (Table 20.3, entries 1-3). With this catalyst, reaction rates of the reduction of alkenes are usually higher than for ketones. This is also the case with various iridium catalysts (entries 6-14) and a ruthenium catalyst (entry 15). One of the few transition-metal catalysts that shows good selectivity towards the ketone or aldehyde function is the nickel catalyst (entries 4 and 5). Many other catalysts have never been tested for their selectivity for one particular functional group. 

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]. 

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]. 

An iridium catalyst (52) has been found to catalyse H-D exchange in a variety of unsaturated carboxylic acids, ketones and amines.155 The mechanism presumably involves displacement of cyclooctadiene by a solvent molecule, which later on is replaced by the a,p-unsaturated compound. 

Solodar (41) has also observed the asymmetric hydrogenation of ketones with related iridium catalysts but much more slowly than with rhodium catalysts. 

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. 

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. 

In early patents by Halcon, molybdenum carbonyls are claimed to be active catalysts in the presence of nickel and iodide [23]. Iridium complexes are also reported to be active in the carbonylation of olefins, in the presence of other halogen [24] or other promoting co-catalysts such as phosphines, arsines, and stibines [25]. The formation of diethyl ketone and polyketones is frequently observed. Iridium catalysts are in general less active than comparable rhodium systems. Since the water-gas shift reaction becomes dominant at higher temperatures, attempts to compensate for the lack of activity by increasing the reaction temperature have been unsuccessful. 

These low yields may be attributed either to a specially high rate of hydrogenation of the ketones on iridium or to the fact that the direct route is by far the major one. The first possibility can be discarded, because the iridium catalyst has a relatively low activity in the reduction of pure menthone between 333 and 373 K it is lower for instance than that of platinum catalyst. Therefore the direct hydrogenation of thymol to the menthols is the most favored route on iridium catalyst. 

Selective reduction of the carbonyl group of benzylideneacetone has been achieved with an iridium catalyst (Scheme 2).11 This represents the first highly selective catalytic hydrogenation of the carbonyl group of an a,B-unsaturated ketone. 

The chiral iridium catalyst (258) bearing a tridentate spiro aminophosphine ligand catalysed the asymmetric hydrogenation of ketones in EtOH and KOBu with excellent enantioselectivities (up to 99.9% ee) and extremely high turnover numbers. ... 

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