Biphenyl tricarbonylchromium complexes. Part 8. Optical resolutions, chiroptical properties, kinetics and absolute chiralities of mono-tricarbonylchromium complexes of o,o′-bridged biphenyls

Abstract

Chiral biphenyls and their tricarbonylchromium complexes can be conveniently resolved by medium pressure chromatography on triacetylcellulose in ethanol or ether. Thereby the mono-Cr(CO)₃exo-complexes of 9,10-dihydrophenanthrene (1) and of the corresponding o,o′-bridged ketone (3)[(1m) and (3m), respectively] were separated into enantiomers whereas the mixtures of the dimethylated biphenyls (2) and (4)[(2m) and (4m), respectively] could be separated in one run into all four stereoisomers, i.e. the enantiomers of exo- and endo-forms. The chiroptical properties of these complexes are reported. From kinetic studies based on the relative intensities of diagnostic bands in the circular dichroism spectra a barrier of ΔG^‡≈ 90 kJ mol^–1 was found for the interconversion exo⇌endo-(2m)[i.e. an epimerization (R)_a⇌(S)_a around the biphenyl axis without affecting the metallocene chirality (R)_m or (S)_m]. From the results of the photochemical decomplexation of the optically active complexes (2m) and (4m)(to give the active ligands of known configurations) and by optical comparison with the corresponding complexes (1m) and (3m) the axial chiralities and (on the basis of general stereochemical considerations) also the metallocene chiralities were deduced.