CP-SUBSTITUENT ADDITIVITY EFFECTS CONTROLLING THE STEREOCHEMISTRY OF THE PROPENE POLYMERIZATION REACTION AT CONFORMATIONALLY UNRESTRICTED (CP-CHR1R2)2ZRCL2/METHYLALUMOXANE CATALYSTS

Four chiral bent metallocene complexes (Cp-CHR1R2)2ZrCl2 (1-4, Cp = C5H4) were prepared and used to generate homogeneous Ziegler catalyst systems for the stereoselective polymerization of propene. 6-Cyclohexyl-6-methylfulvene was reduced by intermolecular beta-hydride transfer from primary alkyllithium reagents LiCH2CHRR' (6; R, R' = H, alkyl, aryl) to give [Cp-CH(CH3)Cy]Li (7a). Subsequent reaction with zirconium tetrachloride resulted in a 1:1 mixture of the [Cp-CH(CH3)Cy]2ZrCl2 diastereomers (1), from which the chiral complex rac-1 was obtained > 98% isomerically pure by fractional crystallization. rac-[Cp-CH(CH3)Ph]2ZrCl2 (rac-2) was obtained analogously from 6-methyl-6-phenylfulvene. Regioselective alpha-deprotonation of 6-cyclohexyl-6-methylfulvene with lithium diisopropylamide followed by treatment with ZrCl4 gave [Cp-C(Cy) = CH2]2ZrCl2 (9a), which was characterized by X-ray diffraction. Complex 9a crystallizes in space group C2/c with cell constants a = 28.044 (6) angstrom, b = 6.627 (1) angstrom, c = 13.150 (2) angstrom, beta = 108.59 (1)-degrees, Z = 4, R = 0.024, and R(W) = 0.031. Hydroboration of 9a gave a 1:1 mixture of the [Cp-CH(Cy)CH2(9-BBN)]2ZrCl2 diastereomers (3). Isomerically pure rac-3 was recovered by fractional crystallization. The chiral metallocene complex rac-[Cp-CH(Ph)CH2(9-BBN)]2ZrCl2 (rac-4) was similarly prepared by means of a regioselective 9-BBN addition to the C = C double bonds of [Cp-C(Ph) = CH2]2ZrCl2 (9b). The activation of the metallocene dihalide rac-1 with excess oligomeric methylalumoxane (Al:Zr almost-equal-to 900) produced a propene polymerization catalyst that gave isotactic polypropylene at -50-degrees-C. C-13 NMR pentad analysis in combination with a statistical treatment using a two-parameter model revealed a combined influence of ''enantiomorphic-site control'' (statistical descriptor-alpha, statistical weight fraction-omega) and ''chain-end control'' (sigma, 1 - omega) similar to what is observed as double stereodifferentiation in conventional organic synthesis. The effectiveness of chirality transfer from the chiral metallocene backbone of this catalyst system was expressed by a ''relative enantioselectivity'' [ee* = (2-alpha - 1)omega] of 13%. Systematic variation of the metallocene Cp substituents revealed a remarkable additivity effect. Exchange of the cyclohexyl groups in 1 for phenyl doubled the asymmetric induction of the C-C coupling process (the ee* of the rac-2-derived catalyst at -50-degrees-C was 25%) as did the formal exchange of the substituent methyl group for the bulkier -CH2(9-BBN) moiety (rac-3: ee* = 30% at -50-degrees-C). Both amendments combined in a single catalyst system quadrupled the efficiency of the metallocene chirality transfer (rac-4: ee* = 60% at -50-degrees-C). This observation should be helpful in the continuing efforts toward a rational design of catalyst systems combining high stereoselectivity with high reaction rates.