Asymmetric hydrogenation iridium complex

Ramies, O., Net, G., Ruiz, A. and Claver, C. (2000) Chiral diphosphites as ligands for the rhodium- and iridium-catalysed asymmetric hydrogenation Precatalyst complexes, intermediates and kinetics of the reaction. Eur. J. Inorg. Chem., 1287-1294. 

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-... 

An even more impressive example of catalytic efficiency has recently been disclosed by Novartis (Bader and Bla.ser, 1997). The key step in a proce.ss for the synthesis of the optically active herbicide, (S)-metolachlor involves asymmetric hydrogenation of a prochiral imine catalysed by an iridium-ferrocenyldipho-sphine complex (see Fig. 2.36). 

The asymmetric hydrogenation of C=N (Eqn. (23)), in contrast with C=0 and C=C bonds, is much less developed. Hexahydrowoquinoline was used as its phosphoric acid salt. Iridium-ferrocenyl complexes were found to be sati.sfactory. After optimisiation, Meyer et al. (1997) were able to realize an enantioselectivity of 89% ee. 

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. 

Enantioselectivities of up to 47% ee were reported by Ruiz et al. in 1997 for the asymmetric hydrogenation of various prochiral dehydroamino acid derivatives and itaconic acid by using iridium cationic complexes of the novel chiral... 

Compared with the Osborn-type cationic rhodium complexes (Section III,A,3), the iridium analogs are much less active for asymmetric hydrogenation of ketones (280). 

In situ-generated iridium complexes containing bulky 3,3 -substituted H8-phosphoroamidites 20 as ligands catalyzed the asymmetric hydrogenation of... 

Despite the very low number of examples describing effective iridium-based asymmetric catalysts, there are some which deserve to be mentioned at this point. The preparation of hybrid NHC—oxazoUne ligands, allowed Burgess and coworkers to prepare Ir(l) complexes for the asymmetric hydrogenation of aryl alkenes (Scheme 3.19) [43]. These catalysts are among the most efficient in terms of yield and asymmetric inductions for this reaction. 

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