Stereoselective synthesis with zirconium complexes

Abstract

Two types of electrophilic zirconium based catalyst systems are described in several variants. The organometallic Lewis acid CpZrCl3is easily available. It catalyzes a variety of organic reactions quite selectively. Starting from the camphor enantiomers the chiral Lewis acid (diboma-Cp)ZrCl3 was prepared. It catalyzes the enantioselective ortho-hydroxylation of α-naphthol. In separate experiments the enantiomeric α-naphthol/ethylpyruvate addition products were obtained on a preparative scale with ≧85% ee using 1 to 5 mol% of the optically pure (+) or (-)(diborna-Cp)zirconium trichloride catalyst, respectively. Bis(cyclopentadienyl)zirconium dihalides (R*Cp)2ZrC2bearing chiral substituents at the Cp-rings were used for generating homogeneous group 4 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(l-alkylindenyl)zirconium complexes exhibiting planar chirality can very easily be prepared optically pure in an asymmetric 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(isopropylcyclopentadienyl)titanium dichloride/alumoxane is an achiral organometallic catalyst system that produces isotactic block polypropylene at -50°C by means of chain end control. At +10°C this is changed to slightly syndiotactic polymer formation. A reaction scheme is proposed that may account for the observed linear lg σ/(l-o) vs 1/T relationship observed for this specific catalyst system. © 1992 IUPAC