Olefin−Amino Acid Complexes of Platinum(II). 2. NMR Study of the Effect of Olefin Structure on the Stability and on the Stereoselectivity of Binding for η2-Coordinated Prochiral Olefins

The exceptional stereoselectivity of coordination ( RS isomer favored) of the prochiral olefinic alcohol 2-methyl-3-buten-2-ol in cis ( N, olefin )-chloro(2-methyl-3-buten-2-ol)(sarcosinato)platinum(II), 1, prompted this investigation of the effect of olefin structure on the equilibrium constant for platinum binding ( K d ) and on the stereoselectivity of coordination, as measured by K i = [ RR ]/[ RS ], for 32 other monosubstituted ethylenes with the same chiral template. The only other olefins which show significant preference for the RS diastereomer ( K i < 0.5) contain hydrogen-bond acceptor atoms in close proximity to the N−H proton of the coordinated olefin which is cis to that nitrogen. Increasing the distance between the N−H proton and the proton acceptor atom by one C−C link has a dramatic effect on K i . Almost no correlation was found between K d and K i, but a significant correlation between the formation constant, K f, for formation of PtCl 3 (olefin) - species of 14 of the olefins and the 195 Pt chemical shift suggested a way to separate electronic effects from steric and hydrogen-bonding effects on the relative stabilities of the mixed olefin−amino acid complexes. Deviations from a simple correlation between log K d and log K f are discussed in terms of this model, but a clean separation of electronic effects from other effects cannot be claimed.