Chiral Iridium Complexes

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-... 

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. 

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... 

During recent years, substantial progress has been made in the hydrogenation of unfunctionalized alkenes. With iridium complexes derived from chiral phos-phino-oxazolines and related ligands, excellent enantioselectivities and high TON/TOF values can now be obtained for a wide range of unfunctionalized olefins. Most substrates studied to date have at least one aryl substituent at the... 

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. 

Reactions Catalyzed by Iridium Complexes of Chiral Phosphite Ligands... 

Carreira et al. reported the kinetic resolution of branched allylic carbonates catalyzed by an iridium complex derived from a chiral [2.2.2]-bicyclooctadiene [48]. Reactions of allylic carbonates with phenol were run to 50% conversion of the carbonate, leaving unreacted allylic carbonate in high enantiomeric excess (Scheme 32). The phenyl ether products were also isolated in mid-to-high enantiomeric excess. 

No examples have been reported of enantioselective, iridium-catalyzed allylic substitutions of linear allylic esters to generate 1,1-disubstituted or 2-substituted 7i-allyl intermediates. Takeuchi published reactions in which the proposed allylir-idium intermediates are 1,1- or 1,3-disubstituted, but these substrates have not been shown to undergo reactions catalyzed by chiral iridium complexes. No reactions of 1,2-disubstituted substrates have been published (Scheme 34). 

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) ... 

A resolution of racemic CHIRAPHOS ligand has been achieved using a chiral iridium amide complex (Scheme 8.3). The chiral iridium complex (- -)-l reacts selectively with (S.S -CHIRAPHOS to form the inactive iridium complex 2. The remaining (R,R)-CHIRAPHOS affords the catalytically active chiral rhodium complex 3. The system catalyzes asymmetric hydrogenation to give the (5)-product with 87% ee. The opposite enantiomer (—)-l gives the (R)-product with 89.5% ee, which is almost the same level of enantioselectivity obtained by using optically pure (5,5)-CHlRAPHOS. 

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... 

Ruthenium complexes mediate the hydroamination of ethylene with pyridine.589 The reaction, however, is not catalytic, because of strong complexation of the amine to metal sites. Iridium complexes with chiral diphosphine ligands and a small amount of fluoride cocatalyst are effective in inducing asymmetric alkene hydroamination reaction of norbomene with aniline [the best enantiomeric excess (ee) values exceed 90%].590 Strained methylenecyclopropanes react with ring opening to yield isomeric allylic enamines 591... 

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