Chiral recognition of amino esters by a ruthenium porphyrin complex: kinetics of the exchange process determined by 1H NMR

Abstract

The synthesis of a (carbonyl) (valine methyl ester) ruthenium(II) picket-fence complex bearing optically active α-methoxy-α(trifluoromethyl)phenylacetyl residues on both sides of the porphyrin plane (α,β,α,β-isomer) is described. For various amino esters, chiral recognition was observed for the complexation of the ligand with up to 52% enantiomeric excess for tert-leucine methyl ester. The dissociation rate constants of the two enantiomers of valine methyl ester were determined by 1H NMR using magnetization transfer experiments, showing that the origin of the enantioselectivity in favour of the ( l)-valine (ca. 2.6:1) resides in the difference between the kinetics of the axial ligand dissociation of the two enantiomers.