Enantioselective hydrogenation ferrocene-based ligands

Enantioselective Hydrogenation of Alkenes with Ferrocene-Based Ligands... 

In the area of asymmetric hydrogenation chiral dighosphines have played a center role since and many applications have been developed. Important new ligands that have been introduced comprise Noyori s BINAP [28], DuPhos (Burk) [29], Takaya s BINAPHOS [30], and Ci-symmetric ferrocene-based ligands introduced by Togni [31]. Industrial products, of which the synthesis uses enantioselective phosphine-derived metal-catalysts are for instance menthol, metolachlor, biotin, and several alcohols, e.g. (R)-1,2-propanedioI, For details about the applications the reader is referred to reviews and references therein [32, 33]. Substituents and backbones have an enormous influence on the performance of the ligands, but usually rationalizations are lacking. 

Fu has reported a planar-chiral bisphosphorus ligand 45 with a phosphaferrocene backbone. The ligand has provided enantioselectivity up to 96% ee in the hydrogenation of a-dehydroamino acid derivatives.99 Another planar-chiral ferrocene-based bisphosphorus ligand 46 has been reported by Kagan recently and enantioselectivity up to 95% ee has been obtained in the reduction of dimethyl itaconate.100... 

Ferrocene-based complexes have some potential for the enantioselective reduction of ketones, but compared to other ligand classes this is relatively limited [3]. Rh complexes of bppfa, bophoz and josiphos are among the most selective catalysts for the hydrogenation of a-functionalized ketones (Table 25.9 Fig. 25.18, 30-32). Ru complexes of walphos and ferrotane are quite effective for... 

Some excellent bisphosphonite ligands have also been developed. For example, Re-etzfs binaphthol-derived ferrocene-based bisphosphonite hgand L12 has demonstrated to have excellent reactivity and enantioselectivity in the rhodium-catalyzed hydrogenation of itaconates and a-dehydroamino acid derivatives [76]. Zanotti-Gerosa s bisphosphonite ligand L13 has also been successfully apphed to the asymmetric hydrogenation of a-dehydroamino acid derivatives with up to 99% ee [77]. 

Ferrocene-based aminophosphines are shown to be effective ligands in the Ru(II)- catalysed asymmetric hydrogenation of ketones. The enantioselectivity is mainly determined by the C-centred chirality of the ligands, but the planar chirality is also important, and (Rc,SFc)- or (Sc,Rfc)- is the matched combination of chiralities.334 Dimethyl oxalate is selectively hydrogenated to methyl glycolate with Ru(acac)3,... 

Chen WP, McCormack PJ, Mohammed K, Mbafor W, Roberts SM, Whittall J (2007) Stereoselective synthesis of ferrocene-based c2-symmetric diphosphine ligands application to the highly enantioselective hydrogenation of a-substituted cinnamic acids. Angew Chem Int Ed 46 4141 144... 

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