Chapter 2
Downloaded by NORTH CAROLINA STATE UNIV on August 20, 2013 | http://pubs.acs.org Publication Date: August 13, 1996 | doi: 10.1021/bk-1996-0641.ch002
New Developments in Chiral Ruthenium (II) Catalysts for Asymmetric Hydrogenation and Synthetic Applications Jean Pierre Genet Laboratoire de Synthèse Organique Associé au Centre National de la Recherche Scientifique, Ecole Nationale Supérieure de Chimie de Paris, 11 rue Pierre et Marie Curie, 75231 Paris, France
This chapter covers catalytic asymmetric synthesis effected by chiral ruthenium (II) complexes. New general and useful methods for the synthesis of diphosphine ruthenium (II) complexes : (P*P)RuX (X = carboxylato, 2-methylallyl, halide), P*P =, BIPHEMP, MeO-BIPHEP, BINAP DIPAMP, DIOP, CHIRAPHOS, DUPHOS etc., as well as hydrido and dinuclear chiral ruthenium complexes are reviewed. These catalysts were evaluated in asymmetric hydrogenation reactions of prochiral substrates. Allylic alcohols, α- and β- acylamino acrylic acids, enamides, α- and β-keto esters, diketones were easily reduced to give the corresponding saturated products in good yields. High efficiency is displayed by Ru catalysts having atropisomeric ligands and C symmetric bis phospholanes (e.g. Me-DUPHOS, Et-DUPHOS and iPr-BPE). Chirally labile compounds capable of undergoing in situ racemization were hydrogenated with high diastereoselectivity (syn or anti) and high enantioselectivity. This reaction provides a powerful tool, dynamic kinetic resolution, for the synthesis of chiral compounds. Applications of these processes, particularly significant in the preparation of synthetic intermediates and pharmaceuticals, are summarized. 2
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In the last few years the demand for optically pure compounds has grown rapidly. Asymmetric synthesis using transition metal catalysis is an ideal method for preparing optically active materials. The development of this method started in the 1970's with chiral rhodium (I) catalysts which have been used with great success in homogeneous hydrogenation of prochiral olefins. To date synthetic organic chemists have designed a number of selective catalysts. Many reviews have been published on asymmetric synthesis in general (1-12). A wide range of compounds contain a hydrogen atom at the stereogenic centre. As this hydrogen atom can be introduced into an appropriate prochiral substrate by a hydrogenation reaction, asymmetric hydrogenation provides a major route to highly enantiomerically-pure compounds. Spectacular enantioselectivities(up to 99 %) were obtained with rhodium (I) catalysts. There are many reviews covering the field (13-16). In this chapter we limit the topic to the homogeneous asymmetric reactions catalyzed by chiral ruthenium catalysts. What follows is a survey of the present state of the
0097-6156/96/0641-0031$15.25/0 © 1996 American Chemical Society
In Reductions in Organic Synthesis; Abdel-Magid, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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REDUCTIONS IN ORGANIC SYNTHESIS
Downloaded by NORTH CAROLINA STATE UNIV on August 20, 2013 | http://pubs.acs.org Publication Date: August 13, 1996 | doi: 10.1021/bk-1996-0641.ch002
field, with special emphasis on development of new chiral ruthenium catalysts containing a large variety of chiral diphosphines. Chiral Ligands. Increases in stereoselectivity can be related to their structure. They differ in size and position of the chiral information (17-18). The most efficient phosphines are diphosphines ; the chirality can either be located on the phosphorous atom (DIPAMP) or on the carbon skeleton of the diphosphine (CHIRAPHOS, SKEWPHOS, B P P M , NORPHOS, C B D , PROPHOS, DUPHOS, etc.). One other important class of ligands is atropisomeric ligands such as BINAP, BIPHEMP, MeO-BIPHEP, BICHEP. Some representative chiral ligands used for the preparation of chiral ruthenium catalysts are shown scheme 1. scheme 1. Chiral diphosphines for asymmetric hydrogenation with Ru(II) catalysts
>cc
PPh
2
r-i^PPh
PPh
2
' — k ^PPh
(S.S)-DIOP
(R^)-CBD
PPh
2
Me
