Hydroboration of Bis(alkenylcyclopentadienyl)zirconium Dichlorides

The reaction of allylcyclopentadienyllithium with 0.5 molar equivalents of MCl 4 (thf) 2 (MZr, Hf) furnished the (η 5 ‐C 5 H 4 CH 2 CHCH 2 MCl 2 complexes 3a (Zr) and 3b (Hf), which cleanly added 9‐borabicyclo[3.3.1]nonane (9‐BBN) to form [η 5 ‐C 5 H 4 (CH 2 ) 3 (9‐BBN)] 2 MCl 2 ( 10a, b ). Treatment of 3 with butadiene‐magnesium led to coupling of one Cp‐bonded allyl unit with butadiene to give 5 ; the remaining CpCH 2 CHCH 2 group underwent clean H[B(C 8 H 14 )] addition when exposed to the 9‐BBN hydroboration reagent. (Alkenyl‐Cp) 2 ZrCl 2 complexes ( 9a – d ) were prepared starting from fulvenes by deprotonation with the help of LDA and reaction with ZrCl 4 (thf) 2 . The complexes [η 5 ‐C 5 H 4 CHC(CH 3 )R] 2 ZrCl 2 9b (RCH 3 ) and 9c (RPh) added 9‐BBN to give [η 2 ‐C 5 H 4 CH(9‐BBN)CH(CH 3 )R] 2 ZrCl 2 12 (RCH 3 , rac ;‐/ meso ;‐ ≈ 1:1) and 13 (RPh, 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.