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Ruthenium Catalysts Containing Rigid Chiral Diamines And Achiral Diphosphanes For Highly Enantioselective Hydrogenation Of Aromatic Ketones.
Published 2011 · Chemistry, Medicine
The development of practical and efficient chiral catalytic systems for asymmetric organic reactions is one of the most important subjects in synthetic organic chemistry. In general, conformationally rigid chiral ligands proved to be extremely useful for the construction of a good oriented and fixed chiral environment around the metal center, which could efficiently impart high enantioselectivity and activity to some important reactions. The catalytic asymmetric reduction of ketones is a fundamental transformation in organic synthesis for making chiral alcohols. For asymmetric hydrogenation of simple aromatic ketones, the use of [(BINAP)Ru ACHTUNGTRENNUNG(diamine)]-based ternary catalytic systems pioneered by Noyori and co-workers is the most powerful and reliable procedure. The general definitive feature of this catalyst is that the ruthenium is coordinated with a chiral diphosphane and a chiral diamine, forming a good oriented chiral trans-octahedral complex. As the excellent enantioselectivity was proposed to be a result of the excellent chiral environment impact by synergistic effect of the chiral diphosphane and chiral diamine ligands, tremendous efforts have been dedicated to developing efficient phosphane ligands such as chiral phosphane, chirally flexible phosphane and even achiral bulky monophosphane ligands. However, most phosphanes have been used in conjunction with chiral DPEN and DAIPEN to mimic the Noyori catalyst. There are very few reports focused on developing efficient chiral diaminea that could be combined with commercial available inexpensive achiral phosphane ligands to construct an efficient Ru-catalyst for the asymmetric hydrogenation of ketones, 6] although the nitrogen-containing ligands are more favorable in terms of preparation. Herein, we report a ruthenium catalytic system consisting of a new rigid chiral diamine and achiral commercially available diphosphane, which hydrogenates aromatic ketones with high enantioselectivity and activity. We have recently introduced the chiral diamine 1 a ((1S,1’S)-1,1’-biisoindoline, named BIDN), a new and intriguing structural motif as a chiral amine ligand, and demonstrated that it can be more effective for some asymmetric reactions. The fused benzene rings not only make this amine more conformationally rigid compared with DPEN, but also render it more electron-deficient due to the p-electron acceptability of the fused benzene ring. Given that the conformationally rigid structure would have good chiral induction ability and inspired by the elegant work of Mikami and Ding, we envisaged that 1 could be evolved into a good chiral ruthenium catalyst for asymmetric hydrogenation of ketones upon combination with an appropriate achiral phosphane. Furthermore, the tunable modular structure of this newly developed chiral diamine can provide us an opportunity to systematically turn the electronic feature by introducing different substituents, and thus investigate the catalytic properties of the corresponding ruthenium catalyst on the asymmetric hydrogenation of ketones. We expected that this key modification, if successful, would not only establish a practical chiral catalyst for asymmetric hydrogenation of ketones but also provide some new insights into the mechanism for this reaction. To test this hypothesis, a series of rigid chiral diamines 1 a–1 h with different electronic features were successfully prepared according to our previously reported procedure and a series of commercially available structurally similar achiral diphosphane ligands 2 a–2 h with different bite angle and steric hindrance were applied for pursuing the practical catalysts for the asymmetric hydrogenation of ketones (Scheme 1). Acetophenone was used as a model substrate