Hydroboration of Bis(alkenylcyclopentadienyl)zirconium Dichlorides

The reaction of allylcyclopentadienyllithium with 0.5 molar equivalents of MCl4(thf)2 (MZr, Hf) furnished the (η5‐C5H4CH2CHCH2MCl2 complexes 3a (Zr) and 3b (Hf), which cleanly added 9‐borabicyclo[3.3.1]nonane (9‐BBN) to form [η5‐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 [η5‐C5H4CHC(CH3)R]2ZrCl2 9b (RCH3) and 9c (RPh) added 9‐BBN to give [η2‐C5H4CH(9‐BBN)CH(CH3)R]2ZrCl2 12 (RCH3, rac;‐/meso;‐ ≈︁ 1:1) and 13 (RPh, threo;‐/erythro;‐ ≈︁ 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°C the catalyst from the achiral meso;‐12 precursor produced isotactic polypropylene only by means of “chain end control” (σ ≈︁ 0.7), whereas the use of rac;‐12 led to a pronounced participation (≈︁ 30%) of effective “enantiomorphic site control” (α=0.95) in the CC‐coupling reaction.