72377-41-4Relevant academic research and scientific papers
Optically active transition-metal complexes. 65. Conformational analysis in diastereoisomer equilibria of square-pyramidal dicarbonylcyclopentadienylmolybdenum-pyridine-2-carbaldimine complexes using the Ruch/Ugi rules
Brunner, Henri,Rastogi, Devendra K.
, p. 891 - 895 (2008/10/08)
Ten chiral amines and amino acids H2NCH(R1)(R2) were used to prepare the corresponding pyridine-2-carbaldimines, abbreviated NN*. The unsymmetrical chelate ligands NN* were introduced into the complexes [C5H5Mo(CO)2NN*]PF6, chiral at the Mo atom. Because racemic amines were used and the optically active amino acids racemized during the reaction, mixtures of two diastereoisomeric pairs of enantiomers were obtained. The diastereoisomers differ in their 1H NMR spectra and were separated by fractional crystallization. On heating in acetone-d6 at 80°C the diastereoisomers interconvert. In this epimerization the labile Mo configuration changes under the influence of the stable asymmetric carbon atom in the ligand NN*. A gauge for the asymmetric induction in this equilibration is the equilibrium ratio of the diastereoisomers, obtained by 1H NMR integration. The asymmetric inductions, ranging from 8 to 62%, can be interpreted on the basis of the stereochemical model of Ruch and Ugi, by using γ values distinctly different from those found for organic systems. The negative phenyl value indicates a weak attraction between the phenyl substituent at the asymmetric carbon atom and the MC5H5 group which accounts for the high optical inductions observed and the high chemical shift differences of the diastereoisomers.
