Enantioselective catalysis with transition metal compounds: Right or left, that is the question
New methods to prepare optically active phosphines for enantioselective transition metal catalysts are presented. The transfer hydrogenation of itaconic acid to give methylsuccinic acid using HCOOH is optimized to optical purity. The enantioselective monophenylation of meso-diols with Cu(0Ac) lpyridineoxazoline catalysts is introduced as a new reaction. The stereodemi stry of the sto i ch i ometr i c cyc 1 opropanat i on of styrene with the carbene ligand of a Fe complex is determined by the metal configuration, the optically active phosphine ligand making only a small contribution. in the Khand-Pauson reaction of phenylacetylene with norbornene to give optically pure 3a,4,5,6,7,7a-hexahydro-2-phenyl-4,7-methano-lH-inden~-l-one, the stereochemistry is determined by the cluster configuration and not by the optically active phosphine ligand. The conclusion is drawn that a control of the metal configuration during enantioselective catalysis would be of major importance for the optical induction. Similarly, INTRODUCTION Enantioselective catalysis with transition metal compounds is a new approach to the synthesis of optically active compounds, needed in economy, starting from prochiral precursors. As the optically active catalyst reenters each catalytic cycle anew with its chiral information, it is a method to prepare large amounts of optically active compounds using only a small quantity of an optically active catalyst. situ catalyst, consisting of a stable and commercially available metal compound (the procatalyst) and a stable and commercially available optically active ligand (the cocatalyst). The most celebrated example for this concept is the enantioselective hydrogenation of dehydroamino acids yielding amino acids. In Scheme 1 the most frequently used standard system is shown, the hydrogenation of (Z)-a-N-acetamidocinnamic acid 1 to give N-acetylphenylalanine 2 (ref. 1-3). Advantageously, the optically active catalyst is an in