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

The synthesis of a (carbonyl) (valine methyl ester) ruthenium(II) picket-fe nce complex bearing optically active alpha-methoxy-alpha(trifluoromethyl)ph enylacetyl residues on both sides of the porphyrin plane (alpha,beta,alpha, beta-isomer) is described. For various amino esters, chiral recognition was observed for the complexation of the ligand with up to 52% enantiomeric ex cess for tert-leucine methyl ester. The dissociation rate constants of the two enantiomers of valine methyl ester were determined by H-1 NMR using mag netization transfer experiments, showing that the origin of the enantiosele ctivity in favour of the (L)-valine (ca. 2.6:1) resides in the difference b etween the kinetics of the axial ligand dissociation of the two enantiomers . (C) 1999 Elsevier Science Ltd. All rights reserved.