Direct Experimental Observation of the Stereochemistry of the First Propene Insertion Step at an Active Homogeneous Single-Component Metallocene Ziegler Catalyst

Abstract

rac-[Me2Si(1-indenyl)2]ZrCl2 was treated with (butadiene)magnesium to yield the respective ansa-metallocene (s-cis-η4-butadiene) complex, 6. It reacts with the organometallic Lewis acid B(C6F5)3 to give the ansa-metallocene-(μ-C4H6)-B(C6F5)3 betaine 7 that was characterized by X-ray diffraction. Complex 7 contains a distorted (σ,π-type) allyl ligand, bonded to zirconium, that bears a syn-oriented −CH2B(C6F5)3 substituent. An ortho C−F substituent of one of the C6F5 groups at boron coordinates to the electron-deficient zirconium center. Complex 7 is an active single-component Ziegler catalyst that stereoselectively polymerizes propene to give isotactic polypropylene by enantiomorphic site control (up to ca. 90% mmmm). Treatment of 7 with propene at −15 °C in toluene-d 8 leads to a regioselective but stereounselective stoichiometric monoinsertion of the prochiral 1-alkene into the Zr−CH2 bond of the betaine 7 to give a 60:40 mixture of the metallacyclic products 9A and 9B. Cleavage of both the intramolecular C4−C5 olefin coordination to zirconium and the internal C6···Zr ion pair interaction in the 9A/9B pair of diastereoisomers by the addition of THF yields a 60:40 mixture of the respective open-chain THF adducts 10A and 10B. This result shows that the first propene insertion reaction at the rac-[Me2Si(1-indenyl)2]Zr derived single-component Ziegler catalyst system is not stereoselective, whereas all subsequent propene insertion steps show a high degree of stereoselectivity. A stereochemical “relay mechanism” is proposed to account for this behavior, where an intermediately generated stereogenic center at the α-carbon atom of the growing σ-ligand hydrocarbon chain effectively serves to transfer the stereochemical information from the chiral metallocene backbone onto the growing carbon chain. The ansa-metallocene backbone alone seems to have no intrinsic ability for direct stereochemical control. The essential role of the α-carbon stereochemistry is supported by the stereoselective propene insertion into the betaine (16) derived from the regioselective B(C6F5)3 addition to (1,3-pentadiene)ZrCp2.

Syndiotactic Polypropylene: Discovery, Development, and Industrialization via Bridged Metallocene Catalysts

“…The importance of the "head-down" orientation of the monomer with respect to the "upward" orientation of the chain during the π-complex formation and transition state was recognized and proposed early on after extensive molecular mechanics and force field calculations performed by Corradini and coworkers [64,69,71,[73][74][75]. The model underwent later additional refinement and took its current form after experiments conducted by several research groups supported the idea of the formation of an α-hydride agostic Zr bond assisting and stabilizing the chain conformation orientation in the metalacyclobutane transition state geometry, before the actual propylene insertion [98][99][100][101][102][103][104][105][106]. It implies that the insertion transition state relies on the formation of an α-agostic bond between either one of the two available hydrides on the last carbon (α-C) of the growing polymer chain and the transition metal center.…”

“…Moreover, the thermal bonding temperature is lowered by about 5-10 C, whereas the mechanical properties of the nonwovens are retained or slightly improved. 106 A. Razavi…”

The (butadiene)zirconocene route to active homogeneous olefin polymerization catalysts

“…90% mmmm) as expected. At À15°C in d 8 -toluene, the betaine 38 cleanly inserted one propene molecule to give a 60:40 mixture of two metallacyclic stereoisomers (39a, 39b) that were subsequently opened by the addition of d 8 -THF to yield the open monopropene insertion products (40a, 40b) again in a 60:40 mixture [21] (see Scheme 13).…”

Homogeneous Single-Component Betaine Ziegler−Natta Catalysts Derived from (Butadiene)zirconocene Precursors

“…24 Treatment of 15 with ethylene at -15°C in toluene-d 8 led to the formation of a single monoinsertion product (21), which is again characterized by weak internal alkene coordination to zirconium ( 1 H NMR: 4-H, 5-H, δ 5.41 and 5.87, trans 3 J(4-H, 5-H) ) 15.7 Hz) and an internal ion-pair interaction between Zr and the terminal -CH 2 -[B] moiety. The reaction of 15 with ethene can lead, in principle, to the formation of two diastereomers with relative configurations (M*)(4,5,6-pR*) or (M*)(4,5,6-pS*).…”