NOVEL CONCEPTS IN DIRECTED BIARYL SYNTHESIS .15. CHIRAL RHENIUM COMPLEXES [(ETA-5-C5H5)RE(NO)(PPH3)(R,R-LACTONE)]+BF4- OF LACTONE-BRIDGED BIARYLS AS LIGANDS - SYNTHESIS, STRUCTURE, AND STEREOCHEMICAL PROPERTIES

The preparation is reported of the first series of (racemic) chiral rhenium complexes with "axially prostereogenic" biaryl lactone ligands, which are potential substrates for atropisomer-selective biaryl syntheses. NMR spectroscopy reveals the presence of both helimeric forms of the twisted biaryllactone ligand for the more hindered representatives (R = Me, Et), whereas the less hindered compounds (R = H, OMe) show rapid interconversion of the two helimeric diastereomers. In the crystal, by contrast, one of the complexes (R = OMe) occurs as only one of the two possible diastereomers, as shown by an X-ray diffraction study.