NMR AND CRYSTALLOGRAPHIC DETERMINATION OF STEREOSELECTIVITY IN THE COORDINATION OF PROCHIRAL OLEFINIC ALCOHOLS IN MIXED OLEFIN AMINO-ACID COMPLEXES OF PLATINUM(II)

NMR spectroscopy (H-1, C-13, and Pt-195) was employed to determine the effect of metal-centered stereochemistry on the stereoselectivity of coordination of prochiral olefinic alcohols by comparing the equilibrium distribution of diastereomers of cis- and trans(N,olefin) isomers for mixed olefin-amino acid complexes of general formula Pt(amino acid)(olefin)Cl for amino acid = glycine, alpha-aminoisobutyric acid, sarcosine (sar), and (S)-proline (S-pro) and olefin = allyl alcohol, 3-buten-2-ol, and 2-methyl-3-buten-2-ol (2-mb). In agreement with earlier reports, the stereoselectivity of olefin coordination is not significant (ratio of diastereomers 1.0-1.3) for any of the trans(N,olefin) species. However, for cis(N,olefin) isomers of N-chiral sarcosine and (S)-proline, the stereoselectivity is substantial, especially for 2-mb for which the preferred diastereomers predominate by > 40/1. X-ray structures of the preferred isomers of cis(N-olefin)-Pt(sar)(2-mb)Cl (space group Pcab with a = 10.705 (3) angstrom, b = 13.671 (3) angstrom, c = 15.908 (3) angstrom, Z = 8, and R = 0.046), of trans(N,olefin)-Pt(S-pro)(2-mb)Cl (space group P2(1)2(1)2(1) with a = 6.955 (4) angstrom, b = 9.971 (4) angstrom, c = 19.060 (9) angstrom, Z = 4, and R = 0.069), and of the related dimethyl sulfoxide complex cis(N,S)-Pt-(sar)(Me2SO)Cl (space group Pbca with a = 10.290 (3) angstrom, b = 12.398 (3) angstrom, c = 16.231 (6) angstrom, Z = 8, and R = 0.041) were determined in order to establish the stereochemistry of the preferred isomers (olefin-R, amino acid N-S in both cases) and to identify intramolecular interactions, including hydrogen bonding, that contribute to the large stereoselectivities observed for cis(N,olefin) species.