Chiral iridium complex catalysts

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. 

Based on the concept mentioned above, Brown realized the asymmetric deactivation of a racemic catalyst in asymmetric hydrogenation (Scheme 9.18) [35]. One enantiomer of (+)-CHIRAPHOS 28 was selectively converted into an inactive complex 30 with a chiral iridium complex 29, whereas the remaining enantiomer of CHIRAPHOS forms a chiral rhodium complex 31 that acts as the chiral catalyst for the enantioselective hydrogenation of dehydroamino acid derivative 32 to give an enantio-enriched phenylalanine derivative... 

Whilst non-racemic catalysts can generate non-racemic products with or without the NLE, racemic catalysts (0% ee) inherently produce only racemic (0% ee) products. A strategy whereby a racemic catalyst is enantiomer-selectively deactivated by a chiral molecule as a catalyst poison has recently been reported to yield non-racemic products (Fig. 7-3) [46 8]. A unique resolution of racemic CHIRAPHOS has been attained with a chiral iridium complex to give a deactiva-... 

Suzuki T, Morita K, Matsuo Y, Hiroi K. Catalytic asymmetric oxidative lactonizations of meso-diols using a chiral iridium complex. Tetrahedron Lett. 2003 44 2003—2006. Moritani J, Hasegawa Y, Kayaki Y, Ikariya T. Aerobic oxidative desymmetrization of meso-diols with bifunctional amidoiridium catalysts bearing chiral N-sulfonyldiamine ligands. Tetrahedron Lett. 2014 55 1188-1191. 

After screening various catalysts, a chiral iridium complex prepared in situ from [IrCl(cod)]2 (10 mol%) and Me-Duphos [1,2-bis (2,5-dimetilphospholano)benzene] was selected for the study of the [2+2+2] cycloaddition reaction (Table 2.3). Excellent diastereoselectiv-ity and enantioselectivity were achieved [59]. The oxygen-, nitrogen- or... 

Iridium The intermolecular hydroamination of unactivated C=C bonds in ct-olefins (RCH=CH2) and bicycloalkenes (norbornene and norbornadiene) with arylamides (ArCONH2) and sulfonamides has been attained upon catalysis by chiral iridium complexes (PP)IrHCl(NHCOAr)(NH2COAr) [PP = chiral bidentate diphosphine]. Mechanistic studies identified the product of N-H bond oxidative addition and coordination of the amide as the resting state of the catalyst. Rapid, reversible dissociation of the amide precedes reaction with the alkene, but an intramolecular, kinetically significant rearrangement of the species occurs before the reaction with alkene. ... 

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. 

Initial studies with iridium complexes derived from chiral phosphinooxazolines (PHOX ligands) and (E)- ,2-diphenyl-1-propene as substrate gave encouraging results (Scheme 1) [5, 15]. With 4 mol% of catalyst (X = PFs ) at 10-50 bar hydrogen pressure, up to 98% ee could be obtained. However, the turnover numbers were disappointingly low. 

Over the last years, one of the most studied DCR has been the asymmetric version of the cycloaddition of nitrones with alkenes. This reaction leads to the construction of up to three contiguous asymmetric carbon centers (Scheme 4). The resulting five-membered isoxazolidine derivatives may be converted into amino alcohols, alkaloids, or p-lactams. Several chiral metal complexes have been used as catalysts for this process [13-15, 18-22]. However, the employment of iridium derivatives is very scarce. 

The above-mentioned complexes are the sole iridium derivatives applied to DCR, and the cycloaddition of nitrones to enals or methacrylonitrile, the unique process studied. We think that iridium-based catalysts are underrepresented in 1,3-dipolar cycloaddition chemistry. For example, no iridium (1) systems have been developed to this end. It can be anticipated that the (bidentate ligand)lr(l) fragment could be active (and stereoselective if chiral bidentate ligands are used) in DCR such as those involving azomethine ylides. 

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 iridium complexes used as precatalysts are air-stable and easy to handle. A further attractive feature is the modular nature of the chiral ligands, which makes it possible to tailor the catalyst structure for a specific substrate. So far several unfunctionalized and functionalized olefins have been hydrogenated with good to... 

Recently, Pfaltz and co-workers reported that iridium complexes with chiral P,N-ligands are highly selective catalysts for asymmetric hydrogenation of several... 

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