ENANTIOSPECIFIC SYNTHESIS BY TRANSFORMATIONS OF CHIRAL POOL-DERIVED METAL PI-COMPLEXES - A STRATEGY FOR THE INTRODUCTION OF SUBSTITUENTS ON A PYRANOSE-DERIVED LATERAL PI-LIGAND EITHER SYN OR ANTI TO THE COORDINATING METAL

In order to extend the use of stoichiometric transition metal pi-complexes to the stereocontrolled synthesis of organic target structures bearing substituents either cis or trans to each other or a cyclic unsaturated pi-ligand, new strategies for the functionalization of pi-complexes are required. A first-generation approach to the stereocontrolled introduction of substituents syn or anti relative to an eta-CpMo(CO)2 unit coordinated to a lateral pi-ligand is reported. The pyranone ring was chosen as the first template upon which the strategy for stereocontrolled introduction of substituents would be studied. The stereocontrol of the process relies on (1) the ability of the CpMo(CO)2 Unit to direct all nucleophilic attack away from itself, (2) the observation that cationic diene complexes, CpMo(CO)2(eta4-diene)+, can be generated from (eta3-allyl)molybdenum species bearing an adjacent alkoxy substituent syn to the molybdenum by treatment With Ph3C+PF6-, and (3) the precedented introduction of nucleophiles adjacent to the oxygen atom in CpMo(CO)2(2H-pyran) cations. Using these stereocontrol features, the pyranone carbonyl group was replaced by two different substituents, R1 and R2. By changing the order of substituent introduction, the stereochemistry of the substituents R1 and R2 relative to the molybdenum unit can be influenced. This can be translated into a method for controlling the introduction of multiple substituents either cis or trans to each other on a cyclic pi-template.