STEREOCHEMISTRY OF METALLOCENES .43. TORSIONAL-ISOMERIC BIPHENYL TRICARBONYLCHROMIUM COMPLEXES - SYNTHESIS, CONFORMATIONAL-ANALYSIS AND KINETICS OF INTERCONVERSION

Complexation of methyl 3-nitro-2-(o-tolyl)-benzoate (2, an o-trisubstituted biphenyl) with hexacarbonyl chromium gave the isomeric Cr(CO)3 complexes 3 and 4 both existing as two torsional isomers a and b, as seen by nmr. For the main product 3 the preferred conformations of the two torsional isomers a and b were deduced by the lanthanide induced shift (LIS) technique. The activation energy for the interconversion process could be determined from kinetic measurements and especially from1H-nmr coalescence temperature of the methylsignals. The barrier of 91±1.5 kJ/mol (21.7±0.3 kcal/mol) in 3 reflects the interaction of the o-H-atom of the benzene ring passing the Cr(CO)3 moiety of the benchrotrene ring during the interconversion. A partial optical resolution of 3 was achieved by asymmetric reduction with a chiral lithium-aluminium hydride yielding (+)-3 with [α]D20+30° (benzene) and the levorotatory alcohol (-)-7. © 1979 Springer-Verlag.