OLEFIN-AMINO ACID COMPLEXES OF PLATINUM(II) .3. NOE DIFFERENCE AND LOW-TEMPERATURE NMR DETERMINATIONS OF ROTAMER POPULATIONS OF COORDINATED OLEFINS AND THE STEREOSELECTIVITY OF COORDINATION OF PROCHIRAL OLEFINS

摘要

The exceptional stereoselectivity of coordination of 2-methyl-3-buten-2-ol in cis(N,olefin)-Pt(2-methyl-3-buten-2-ol)(sarcosine)X, 1 (X = Cl), has been attributed to stabilization of one rotamer of the H-2-coordinated olefin by intramolecular hydrogen bonding between the OH oxygen on the olefinic alcohol and the N-H proton of coordinated sarcosine. Nuclear Overhauser enhancement (NOE) difference experiments on 1 have confirmed that the preferred conformation in solution corresponds to that found in the crystalline solid, Both NOE and low-temperature NMR experiments were used to establish approximate rotamer ratios for a series of complexes as a function of olefin structure, X substituent (Cl-, H2O, OH-), and solvent composition (D2O vs CD3OD). Olefins that show strong stereoselectivity in diastereomeric complexes containing sarcosine and proline also show a strong preference for one rotamer in complexes containing the achiral amino acid cr-aminoisobutyric acid, The strong stereoselectivity and rotamer preference noted for 1 is reduced substantially by substituting H2O for Cl- and is almost eliminated by further deprotonation to OH-. A similar reduction in stereoselectivity of binding and of rotamer preference is noted in going from CD3OD to D2O as solvent. These findings all emphasize the importance of hydrogen bonding and olefin conformation to the stereoselectivity of coordination of substituted ethylenes in these species. [References: 12]