HYDROBORATION OF BIS(ALKENYLCYCLOPENTADIENYL)ZIRCONIUM DICHLORIDES

The reaction of allylcyclopentadienyllithium with 0.5 molar equivalents of MCl4(thf)2 (M = Zr, Hf) furnished the eta-5-C5H4CHcomplexes 3a (Zr) and 3b (Hf), which cleanly added 9-borabicyclo[3.3.1]nonane (9-BBN) to form [eta-5-C5H4(CH2)3(9-BBN)]2MCl2 (10a, b). Treatment of 3 with butadiene-magnesium led to coupling of one Cp-bonded allyl unit with butadiene to give 5; the remaining CpCH2CH = CH2 group underwent clean H[B(C8H14)] addition when exposed to the 9-BBN hydroboration reagent. (Alkenyl-Cp)2ZrCl2 complexes (9a - d) were prepared starting from fulvenes by deprotonation with the help of LDA and reaction with ZrCl4(thf)2. The complexes [eta-5-C5H4CH = C(CH3)R]2ZrCl2 9 b (R = CH3) and 9c (R = Ph) added 9-BBN to give [eta-2-C5H4CH(9-BBN)CH(CH3)R]2ZrCl2 12 (R = CH3, rac-/meso- almost-equal-to 1:1) and 13 (R = Ph, threo-/erythro- almost-equal-to 1:1). The mixtures of diastereoisomers were in each case separated by fractional crystallization. The stereochemical assignments of complexes rac-12 and meso-12 were achieved by means of the stereochemistry of polypropylene formed at the 12/methylalumoxane-derived homogeneous Ziegler-type olefin polymerization catalysts. At 50-degrees-C the catalyst from the achiral meso-12 precursor produced isotactic polypropylene only by means of "chain end control" (sigma almost-equal-to 0.7), whereas the use of rac-12 led to a pronounced participation (almost-equal-to 30%) of effective "enantiomorphic site control" (alpha almost-equal-to 0.95) in the CC-coupling reaction.