Iridium complexes hydrogenation

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. 

In 2004, Bolm et al. reported the use of chiral iridium complexes with chelating phosphinyl-imidazolylidene ligands in asymmetric hydrogenation of functionalized and simple alkenes with up to 89% ee [17]. These complexes were synthesized from the planar chiral [2.2]paracyclophane-based imida-zolium salts 74a-c with an imidazolylidenyl and a diphenylphosphino substituent in pseudo ortho positions of the [2.2]paracyclophane (Scheme 48). Treatment of 74a-c with t-BuOLi or t-BuOK in THF and subsequent reaction of the in situ formed carbenes with [Ir(cod)Cl]2 followed by anion exchange with NaBARF afforded complexes (Rp)-75a-c in 54-91% yield. The chela-... 

The iridium complex 35 has been also used as catalyst for the transfer hydrogenation of substituted nitroarenes [34]. Good to very good conversions were observed (2.5 mol%, in refluxing isopropanol, 12 h). A mixture of two products was obtained, the relative ratio of which depends on the concentration of added base (KOH) and catalyst. (Scheme 2.5)... 

In 1998, Ruiz et al. reported the synthesis of new chiral dithioether ligands based on a pyrrolidine backbone from (+ )-L-tartaric acid. Their corresponding cationic iridium complexes were further evaluated as catalysts for the asymmetric hydrogenation of prochiral dehydroamino acid derivatives and itaconic acid, providing enantioselectivities of up to 68% ee, as shown in Scheme 8.18. 

Scheme 8.18 Hydrogenations of olefins with iridium complexes containing dithio-ether ligands with a pyrrolidine backbone.

Scheme 8.22 Hydrogenation of acetamidoacrylic acid with iridium complexes containing dithioether ligands.

In addition, Peruzzini et al. developed, in 2007, iridium complexes of planar-chiral ferrocenyl phosphine-thioether ligands that were tested in the hydrogenation of simple alkyl aryl ketones.These complexes were diastereoselec-tively generated in high yields (85-90%) by addition of the corresponding... 

To our knowledge, the first published report of a photocatal-ytic reaction at elevated pressure was W. Strohmeyer1s hydrogenation of 1,3-cyclohexadiene under hydrogen at 10 atm /22/. On photolysis, the iridium complex 8 formed a very active catalyst, probably by dissociation of a phosphine ligand (Equation 17). At 70 C, with hydrogen at 10 atm, and a catalyst/substrate ratio of 1/100,000, the activity was 196 per minute and the turnover number was 96,000 mol of product/mol catalyst. 

Iridium-catalyzed transfer hydrogenation of aldehyde 73 in the presence of 1,1-dimethylallene promotes tert-prenylation [64] to form the secondary neopentyl alcohol 74. In this process, isopropanol serves as the hydrogen donor, and the isolated iridium complex prepared from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and (S)-SEGPHOS is used as catalyst. Complete levels of catalyst-directed diastereoselectivity are observed. Exposure of neopentyl alcohol 74 to acetic anhydride followed by ozonolysis provides p-acetoxy aldehyde 75. Reductive coupling of aldehyde 75 with allyl acetate under transfer hydrogenation conditions results in the formation of homoallylic alcohol 76. As the stereochemistry of this addition is irrelevant, an achiral iridium complex derived from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and BIPHEP was employed as catalyst (Scheme 5.9). 

Another area of high research intensity is the catalytic dehydrogenation of alkanes to yield industrially important olefin derivatives by a formally endothermic (ca. 35 kcal mol-1) loss of H2. Recent results have concentrated on pincer iridium complexes, which catalytically dehydrogenate cycloalkanes, in the presence of a hydrogen accepting (sacrificial) olefin, with turnover numbers (TONs) of >1000 (Equation (23)) (see, e.g., Ref 33,... 

The iridium complex [Ir(cod)(//2-,PrPCH2CH2OMe)]+BF4 (22) in dichloro-methane at 25 °C at 1 bar H2 is a particularly active catalyst for the hydrogenation of phenyl acetylene to styrene [29]. In a typical experiment, an average TOF of 50 mol mol-1 h-1 was obtained (calculated from a turnover number, TON, of 125) with a selectivity close to 100%. The mechanism of this reaction has been elucidated by a combination of kinetic, chemical and spectroscopic data (Scheme 14.10). 

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]. 

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