Chiral ferrocenyl diphosphines for asymmetric transfer hydrogenation of acetophenone

The synthesis of new optically pure ferrocenyl diphosphines have been realized from (R)-(+)-N,Ndimethylaminoethylferrocene. Particularly, dissymmetric ferrocenyl diphosphines have been synthesized. The diphosphines have been used as ligands in asymmetric transfer hydrogenation of acetophenone in the presence of Ru catalysts. © 2006 Elsevier Science. All rights reserved ——— * Corresponding author. Tel.: +33-3-20434893; fax: +33-3-20436585; e-mail: [email protected]. Catalytic asymmetric hydrogenation of prochiral ketones to chiral alcohols using transition metal complexes has gained increasing interest during recent years. In particular, ruthenium-catalyzed asymmetric transfer hydrogenation using 2-propanol under basic conditions presents the advantages of a low cost, ease of handling and high solubility of 2-propanol as hydrogen donor reagent. Noyori developed an efficient and highly enantioselective ruthenium catalyst using diamines as chiral ligands. Other types of ligands such as amino alcohols, aminooxazolines, aminophosphines, diureas, and phosphine oxides have also been used with various levels of rates, yields and selectivities. In particular, only ferrocenyl ligands possessing oxazoline/phosphines, triphosphines, amino alcohols imine/phosphines, and diamines have been studied. Also, to our best knowledge, the involvement of ferrocenyl diphosphines has not been reported so far. Moreover, Genêt reported a series of dibromodiphosphinoruthenium catalysts ([RuP*2Br2], where P*=diphosphine) for transfer hydrogenation of ketones, achieving good conversion in short reaction times. However, only moderate enantioselectivities (7– 52% ee) has been obtained. We have had an ongoing interest in the synthesis and use of optically active ligands in asymmetric catalysis, especially ferrocenyl amino alcohols. Herein we present the synthesis of new ferrocenyl diphosphines 1-5 (Figure 1) and the first results for asymmetric transfer hydrogenation of acetophenone catalyzed by Ru(II) complexes of these bidentate ligand system. Our initial efforts focused on screening a variety of ferrocenyl compounds in which chelating groups and stereogenic centers were varied in order to ascertain their effects on the reaction. R = H, R' = R'' = Ph R = H, R' = R'' = Cy R = H, R' = Ph, R'' = Cy 1 2 3 5 Fe PR''2 Me