Stereoselectivity in the substitution reaction of square-planar platinum(II) complexes determined in situ by nuclear magnetic resonance spectroscopy using a chiral solvent

By use of a chiral solvent [(S)-(+)-2,2,2-trifluoro-1-phenylethanol], the stereoselectivity in the associative ligand-substitution reaction of trans-[PtCl2(SR2)(R,S-Val-OMe)](Val-OMe =R,S-valine methyl ester (R = Me, CH2Ph, or But) with R- or S-1-phenylethylamine has been determined in situ by time-sequential 1H n.m.r. As compared with the stereoselection in the formation of stable bis(amino acidate)–platinum(II) complexes without a third binding site in the amino acid, the observed kinetic stereoselectivity is substantial (6–10% excess), which suggests closer arrangement of chiral ligands in the trigonal-bipyramidal state. The importance of this reaction, which is close to an elementary process, is that it can give a detailed understanding of more complex asymmetric reactions.