Addition of a large excess of trans-2-butene (tbn) or 2-methyl-2-butene (mbn) to an acetone solution of cis- or trans(N, ethylene)[PtCl(l-am)(C2H4)] (l-am, 9 kinds of l-aminocarboxylate) gave first increase and then gradual decrease in CD strength in the region \simeq26500 cm−1. The kinetic optical yield amounted to 53% whereas that at the equilibrated state was much less (up to 27% for cis(N, olefin) complexes and 6 to 12% for trans complexes), trans-2-Butene gives larger yields than mbn. Kinetic analysis of the growth and decay curve of the CD strength disclosed that the first fast increase in CD reflects the greater rate of substitution of the prochiral olefins for ethylene in S-configuration than that in R-configuration. The second step seems to involve the exchange of coordinated tbn or mbn catalyzed by ethylene which was made free in the first step. The mechanism of asymmetric induction has been discussed from both steric and electronic viewpoints. Asymmetry of the coordinated nitrogen seems to give a dominating effect.
The circular dichroism (CD) peaks of trans(N, ethylene)[PtCl(l-am)(C2H4)] (l-am, l-amino carboxylate) in acetonitrile (AN) at 33000 and 37000 cm−1 are characteristic of those complexes with asymmetric nitrogen. The CD sign depends on the substituents on the asymmetric nitrogen, and the additivity law holds for methyl and benzyl derivatives of l-prolinato, l-hydroxyprolinato, and l-valinato complexes. The quadrant rule is applicable to the contribution of the substituents on the nitrogen. On the preparation of the benzyl-l-valinato complex from Zeise’s salt in a weakly acid medium the nitrogen exhibits a marked stereoselectivity to give R configuration.
Replacement of the coordinated trans-2-butene in trans(N,olefin)[PtCl(l-pro)(S,S-trans-2-butene)] (l-pro=l-prolinate anion) by various olefins in acetone has been studied at −20 to +23 °C. The rate is proportional to concentrations of the complex and the olefin nucleophile. No solvent path is observed. The second order rate constant k2 (10−4 to 10−1 M−1 s−1 at 8.0 °C) can be the measure of each substitution. The ΔH\eweq is around 10 kcal/mol, and ΔS\eweq ranges from −25 to −46 cal/mol K. Substitution of cis-2-butene for the asymmetrically coordinated trans-2-butene-3H gives equal k2’s on the measurement of CD strength and on that of isotopic exchange. Similar substitution of trans-2-butene gives significantly larger k2 on isotopic exchange than that on CD measurement, indicating sterically selective substitution with retention of configuration. Substitution of ethylene derivatives with various groups on the carbon atoms gives different k2’s depending on the kind of substituents. The steric interaction between the olefin nucleophile and the coordinated ligands seems to be a very important factor in determining the ease of substitution.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.