Model reactions for stereochemistry of Ziegler‐Natta polymerization: Asymmetric hydroisomerization of α‐olefins by chiral titanium tetra‐alkoxides/tri‐isobutyl aluminum catalytic systems

Abstract

AbstractHomogeneous catalytic systems based on optically active low‐molecular‐weight and macromolecular titanium alcoholates and tri‐isobutyl aluminum were used in the isomerization and hydrogenation of chiral and prochiral olefins. The reactions performed in a nitrogen and hydrogen atmosphere show a different behavior in regard to reaction rate, composition of reaction mixture, and stereochemical control. The catalyst based on macromolecular titanium alcoholate displays lower activity and a varying influence on the stereochemical pathway with respect to the catalysts based on low‐ molecular‐weight alcoholates. The data obtained in both cases strongly support the absence of a stereocontrol by the growing chain in the stereoelective polymerization of racemic α‐olefins by Ziegler‐Natta catalysts based on optically active precursors. Finally evidence is provided in favor of the intermediate step responsible for asymmetric isomerization which does not occur via η‐allyl complex.