Platinum(II)-Olefin Complexes Containing Amino Acids. III. Optical Inversion and Substitution of Coordinated Olefins in Optically Active trans(N)-Chloro-l-prolinato-olefinplatinum(II) Complexes in Organic Solvents

Abstract

Optical inversion of coordinated 2-methyl-2-butene and trans-2-butene in trans(N)-chloro-l-prolinato-olefinplatinum(II), trans(N)-[PtCl(l-prol)(olefin)] (nitrogen of l-prolinate ion is trans to the olefin), i.e. the interconversion between diastereoisomers (Fig. 1), has been kinetically studied in acetone by measuring the decrease in the intensity of circular dichroism (CD) in near ultraviolet region. The rate was measurable only in the presence of olefin, and can be expressed by the relation, Rate=k[complex][olefin]. The Ea and the ΔS\eweq values for the two complexes are 7.8±1.0 and 9.0±1.0 kcal/mol, and −44 and −39 entropy unit, respectively. The coordinated trans-2-butene is replaced by other olefins, 2,3-dimethyl-2-butene, 2-methyl-2-butene, trans-2-butene (optical inversion), and styrene, under similar conditions to give the same rate formula. The second order rate constant increases in the given order. The inversion seems to take place via bimolecular mechanism with excessive olefin as nucleophile. The corresponding complexes containing styrene and its para derivatives give such an inversion much faster in the absence of excess of olefin. With change of para substituents, the rate increases in the order, methoxy<methyl<hydrogen<chloro<nitro. This seems to be related with their electrondrawing ability.

Carbon-13 nuclear magnetic resonance studies of platinum(II) olefin bonds

“…These distortions within the sty-Pt bond are in the main consistent with the valence bond description of the bonding given in Figure 3.55 (v) Optical Inversion and Substitution of Coordinated Styrenes in trans (A)-Chloro-L-prolinatostyreneplatinum(II) Complexes. 56 The rate of inversion of coordinated styrenes in the complexes trans(N)-[(p-YC6H4CH=CH2)PtCl(L-pro)] (pro = prolinate) has been found to be quite sensitive to the nature of the para Y substituent increasing in the order Y = MeO < Me < H < Cl < NO2. While the details of the mechanism of this inversion reaction are still unknown, it has been suggested that the reaction proceeds via a solvolytic displacement of the coordinated styrene.…”