Synthesis of Me2Si-Bridged ansa-Zirconocenes by Amine Elimination

Abstract

Me2Si-bridged ansa-zirconocenes of interest for α-olefin polymerization catalysis are prepared in good yield by amine elimination reactions. The reaction of Me2Si(1-C5H4-3-tBu)2 (3) and Zr(NMe2)4 affords Me2Si(1-C5H3-3-tBu)2Zr(NMe2)2 (4) in 95% NMR yield (rac/meso = 1/2) and pure meso-4 in 38% isolated yield. The reaction of Me2Si(1-C5H3-2-Me-4-tBu)2 (6) with Zr(NMe2)4 affords Me2Si(1-C5H2-2-Me-4-tBu)2Zr(NMe2)2 (7) in 90% NMR yield (rac/meso = 2.5/1) and pure rac-7 in 52% isolated yield. The 1/2 rac-4/meso-4 and 2.5/1 rac-7/meso-7 ratios are the thermodynamic ratios, and the rac/meso isomerizations are catalyzed by NMe2H via reversible Zr−Cp bond aminolysis. The thermodynamic bias for meso-4 is ascribed to the ease of distortion of the metallocene framework of this diastereomer by a lateral deformation which reduces steric crowding between the amide ligands and the Cp substituents. The reaction of 6 and Zr(NEt2)4 proceeds only in 1,2-dichlorobenzene at 180 °C, affording Me2Si(1-C5H2-2-Me-4-tBu)2ZrCl2 (rac-1) in 35% NMR yield (rac/meso = 3/1). The reaction of 6 and the piperidide complex Zr(NC5H10)4 (m-xylene, 140 °C, 24 h) affords rac-Me2Si(1-C5H2-2-Me-4-tBu)2Zr(NC5H10)2 (rac-9) in 35% NMR yield (no meso detected) and pure rac-9 in 7% isolated yield. The reaction of 6 and the pyrrolidide complex Zr(NC4H8)4 (m-xylene, 90 °C, 4h) affords Me2Si(1-C5H2-2-Me-4-tBu)2Zr(NC4H8)2 (10) in 80% NMR yield (rac/meso = 3/1) and pure rac-10 in 39% isolated yield. The molecular structure of rac-10 was determined by X-ray crystallography and exhibits severe crowding between the amide ligands and the Cp substituents. Treatment of rac-7 or rac-10 with Me3SiCl results in clean conversion to rac-1 without isomerization.