ChemInform Abstract: Stereoselective Synthesis with Zirconium Complexes

Abstract

Two types of electrophilic zirconium based catalyst systems are described in several variants. The organometallic Lewis acid CpZrCl is easily available. It catalyzes a variety of organic reactions quite selectively. Starting from &e camphor enantiomers the chiral Lewis acid (diborna-Cp)ZrClg was prepared. It catalyzes the enantioselective ortho-hydroxylation of a- naphthol. In separate experiments the enantiomeric a-naphthoVethylpyruvate addition products were obtained on a preparative scale with 285% ee using 1 to 5 mol% of the optically pure (+) or (-)(diborna-Cp)zirconium mchloride catalyst, respectively. Bis(cyclopentadieny1)zirconium dihalides (R*Cp)2ZrCl bearing chiral substituents at the Cp-rings were used for generating homogeneous group 3 metallocene/alumoxane Ziegler catalysts for the stereoselective polymerization of propene. Systematic variation of substituents at the chiral Cp-side chains revealed a substituent additivity effect of the stereoselective CC-coupling process. Non-bridged bis( 1-alkylindeny1)zirconium complexes exhibiting planar chirality can very easily be prepared optically pure in an asymmemc organometallic synthesis using terpene- or steroid-derived indenyl substituents. Some of these complexes produce homogeneous Ziegler catalysts that are nearly as stereoselective propene polymerization catalysts as the commonly used ansa- metallocene complexes. Bis(isopropylcyclopentadieny1)titanium dichloride/alumoxane is an achiral organometallic catalyst system that produces isotactic block polypropylene at -50°C by means of chain end control. At +lO°C this is changed to slightly syndiotactic polymer formation. A reaction scheme is proposed that may account for the observed linear lg o/(l-o) vs l/r relationship observed for this specific catalyst system.