Synthesis and catalytic applications of chiral monomeric organomolybdenum(VI) and organorhenium(VII) oxides in homogeneous and heterogeneous phase

In recent years several attempts have been made to introduce chirality into organometallic Re(VII) compounds of formula RReO₃ and related organometallic complexes containing the (MoO₂)²⁺ moiety for applications in chiral catalysts. The earliest synthetic procedures applied the addition of chiral organic Lewis base ligands, most conveniently taken from Nature's chiral pool. The major flaw of most of these attempts, however, was the weak coordination of the chiral Lewis base ligands to the metal center, which leads either to high ees only at the very beginning of the catalytic reaction (low conversion) or to generally low optical yields. The heterogenization of the Mo(VI) complexes was, at least in some cases successfully achieved, but with the same drawbacks with respect to the ees as in homogeneous phase occurred. More recent attempts include the addition of chiral ligands via covalent bonds to the metal center and subsequent heterogenization. The stability of the complexes depends in these latter cases crucially on the strength of the metal-carbon interactions involved and the ees obtained also reflect intramolecular mobility of such ligands. The main target of these examinations with respect to applications in catalysis were olefin epoxidation and related reactions.