SYNTHESIS OF ME(2)SI-BRIDGED ANSA-ZIRCONOCENES BY AMINE ELIMINATION

Me(2)Si-bridged ansa-zirconocenes of interest for a-olefin polymerizat ion catalysis are prepared in good yield by amine elimination reaction s. The reaction of Me(2)Si(1-C5H4-3-(t)Bu)(2) (3) and Zr(NMe(2))(4) af fords Me(2)Si(1-C5H3-3-(t)Bu)(2)Zr(NMe(2))(2) (4) in 95% NMR yield (ra c/meso = 1/2) and pure meso-4 in 38% isolated yield. The reaction of M e(2)Si(1-C5H3-2-Me-4-(t)Bu)(2) (6) with Zr(NMe(2))(4) affords Me(2)Si( 1-C5H2-2-Me-4-(t)Bu)(2)Zr(NMe(2))(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/me so isomerizations are catalyzed by NMe(2)H via reversible Zr-Cp bond a minolysis. The thermodynamic bias for meso-4 is ascribed to the ease o f distortion of the metallocene framework of this diastereomer by a la teral deformation which reduces steric crowding between the amide liga nds and the Cp substituents. The reaction of 6 and Zr(NEt(2))(4) proce eds only in 1,2-dichlorobenzene at 180 degrees C, affording Me(2)Si(1- C5H2-2-Me-4-(t)Bu)(2)ZrCl2 (rac-1) in 35% NMR yield (rac/meso = 3/1). The reaction of 6 and the piperidide complex Zr(NC5H10)(4) (m-xylene, 140 degrees C, 24 h) affords rac-Me(2)Si(1-C5H2-2-Me-4-(t)Bu)(2)Zr(NC5 H10)(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(NC 4H8)(4) (m-xylene, 90 degrees C, 4h) affords Me(2)Si(1-C5H2-2-Me-4-(t) Bu)(2)Zr(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 deter mined by X-ray crystallography and exhibits severe crowding between th e amide ligands and the Cp substituents. Treatment of rac-7 or rac-10 with Me(3)SiCl results in clean conversion to rac-1 without isomerizat ion.