Bis(η-methylcyclopentadienyl)(η4-butadiene)zirconium adds 1 equiv of the organometallic Lewis acid B(C6F5)3 to yield the metallocene−(μ-C4H6)−borate betaine 5, which is an active, homogeneous, one-component Ziegler catalyst for the polymerization of 1-alkenes. The metallacyclic metallocene betaine 5 undergoes a degenerate π ⇌ σ ⇌ π-allyl interconversion on the NMR time scale in toluene solution (ΔG ⧧ m(obs)(toluene) = 19.8 kcal mol-1) which becomes markedly faster in the presence of added reactive 1-alkenes (ΔG ⧧ m(obs) = 18.9 (1-hexene), 17.7 (1-pentene), 17.5 (1-butene), 17.2 kcal mol-1 (propene)). This lowering of the activation barrier is probably due to an increased stabilization of the (σ-alkyl)(π-alkene)metallocene betaine intermediate 7, which at the same time is passed as the essential intermediate stage of the insertion of these alkenes into the reactive zirconium carbon bond of 5 to yield the mono olefin insertion products 9. The Gibbs activation energies of this chemical insertion reaction (ΔG ⧧ chem = 20.1 (1-hexene), 18.8 (1-pentene), 18.5 (1-butene), 17.3 kcal mol-1 (propene)) are similar in magnitude to the activation energies of magnetical exchange (ΔG ⧧ m(obs)). This creates the interesting situation that in the presence of these reactive 1-alkenes, the (reversible) magnetical exchange rate (k m (obs)), as determined by the dynamic NMR experiment, is dependent on the rate of the competing (irreversible) overall chemical addition reaction (k chem). The rate constants k m (obs) and k chem were measured in the presence of these 1-alkenes, which allowed for a determination of the height of the first, i.e., the complexation barrier (ΔG ⧧ 1 = 18.5 (1-hexene), 17.3 (1-pentene), 17.1 (1-butene), 16.4 kcal mol-1 (propene); standard state c ⊖ = 1 mol L-1) and its difference (ΔΔG ⧧ 2) to the top of the actual insertion barrier (ΔΔG ⧧ 2 = 1.6 (1-hexene), 1.5 (1-pentene), 1.3 (1-butene), 0.7 kcal mol-1 (propene)). These values, together with the activation energy of the degenerate allyl ligand interconversion of the model system (σ-allyl)(π-allyl)zirconocene (10) (ΔG ⧧ 3 = 7 ± 0.5 kcal mol-1), allowed for a good experimentally based estimate of the intrinsic activation energy (reaction 7 → 9) of the insertion of these 1-alkenes into the zirconium carbon bond at this group 4 bent metallocene unit. The thus obtained insertion barrier is ΔG ⧧ ins ≈ 10−11 kcal mol-1 for the 1-alkenes used in this study. The alkene decomplexation barrier (reaction 7 → 5) is lower by ca. 1−2 kcal mol-1.

The organometallic Lewis acid B(C 6 F 5 ) 3 adds to the terminal dCH 2 group of the (butadiene)metallocene complexes 5a and 5b to give the ansa-metallocene betaine systems [Me 2 Si-(C 5 H 4 ) 2 ]Zr[C 4 H 6 -B(C 6 F 5 ) 3 ] (6a) and [Me 2 Si(3-MeC 5 H 3 ) 2 ]Zr[C 4 H 6 -B(C 6 F 5 ) 3 ] (6b), respectively, in high yield. Both complexes were characterized by X-ray diffraction. They both contain a substituted η 3 -allyl ligand F of E configuration, and they show a characteristic (ortho aryl)-C-F‚‚‚Zr interaction that stabilizes the electron-deficient metal center inside the dipolar structure. B(C 6 F 5 ) 3 also adds to one butadiene terminus of (s-cis-η 4 -C 4 H 6 )[Me 2 C(C 5 H 4 )-(indenyl)]Zr to give a high yield of a single isomer of the respective ansa-metallocene [C 4 H 6 -B(C 6 F 5 ) 3 ] betaine complex 9. The X-ray crystal structure analysis of 9 has revealed that in this case a (Z)-η 3 -allyl-CH 2 B(C 6 F 5 ) 3 ligand is formed. This precluded the (aryl)C-F‚‚‚Zr coordination. Instead, the zirconium center in 9 forms a stabilizing internal ion pair interaction between the negatively polarized [B]-C(4)H 2 methylene group and the positive zirconium center. The analogously structured ansa-metallocene [(Z)-C 4 H 6 -B(C 6 F 5 ) 3 ] betaine complex 12 is obtained in high yield from B(C 6 F 5 ) 3 addition to (s-cis-η 4 -butadiene)[Me 2 C-(C 5 H 4 )(fluorenyl)]Zr. In solution the complexes 6, 9, and 12 exhibit structures that are analogous to those found in the solid state. However, treatment of (butadiene)[Me 2 Si(C 5 H 4 ) 2 ]-Zr (5a) with B(C 6 F 5 ) 3 under kinetic control (233 K in toluene-d 8 ) quantitatively yields the [Me 2 Si(C 5 H 4 ) 2 ]Zr[(Z)-C 4 H 6 -B(C 6 F 5 ) 3 ] betaine isomer 13, which contains the stabilizing [B]-C(4)H 2 ‚‚‚Zr internal ion pair interaction. Subsequent thermally induced rearrangement of the kinetic product 13 (∆G q rearr (298 K) ) 21.5 ( 0.5 kcal mol -1 ) then results in the formation of the eventually observed thermodynamic ansa-metallocene betaine product 6a, that contains the (E)-C 4 H 6 -B(C 6 F 5 ) 3 ligand and exhibits internal (aryl)C-F‚‚‚Zr coordination. A similar reaction sequence was observed during the addition of B(C 6 F 5 ) 3 to the parent (butadiene)zirconocene system 1: at 213 K the kinetic Cp 2 Zr[(Z)-(1-3η),κC 4 -C 4 H 6 -B(C 6 F 5 ) 3 ] betaine product 14 is formed, which rapidly rearranges at temperatures above 253 K to yield the previously observed stable Cp 2 Zr[(E)-C 4 H 6 -B(C 6 F 5 ) 3 ] betaine system 2, which is characterized by an internal C-F‚‚‚Zr bond. The ansa-metallocene betaines 6, 9, and 12 are all active homogeneous single-component Ziegler catalysts for ethene and propene polymerization. They are similarly effective as the usually employed ansa-metallocene dichloride/methylalumoxane catalyst systems.

The synthesis of fused aromatic carbocycles from aryl iodides and difunctional acceptors is outlined. This methodology is based on a palladium-catalyzed aromatic substitution followed by an intramolecular Heck sequence. Under the optimized conditions (Pd(OAc)(2) (10 mol %), tri-2-furylphosphine (20-30 mol %), norbornene (2 equiv), Cs(2)CO(3) (2 equiv), CH(3)CN, reflux), bromoenoates react with aryl iodides bearing numerous substituents (F, Cl, CF(3), Me, etc.). The expanded description of our initial work as well as the use of polysubstituted aryl iodides is described.

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.

Treatment of the ansa-metallocene complex [Me 2 Si(C 5 H 4 ) 2 ]Zr(butadiene) (s-cis-/s-trans-4) with B(C 6 F 5 ) 3 yields the corresponding ansa-zirconocene[C 4 H 6 B(C 6 F 5 ) 3 ] betaine 6. Complex 6 is a homogeneous single component Ziegler catalyst that actively polymerizes ethene and propene, respectively. With the olefins ethene, propene, 1-butene, 1-pentene, 1-hexene, or 1,5-hexadiene complex 6 undergoes a stoichiometric insertion reaction at -20 °C to generate the metallacyclic carbon-carbon coupling products 9a-9f, which feature an internal C4dC5 alkene coordination to the metal center and an intramolecular C6-Zr ion pair interaction. The rate of the overall 1-alkene insertion process 6 f 9 (k chem ) was measured, and the observed rate constant of the degenerate allyl inversion process of the starting material (6 f ent-6, k m(obs) ). This allows for a mathematical kinetic deconvolution of the two-step reaction sequence, namely the initial alkene addition process to 6 to generate the reactive (π-alkene)(σ-allyl)metallocene-type intermediate 8 (rate of formation (k 1 [alkene], rate of alkene dissociation: k -1 ) and the subsequent insertion reaction 8 f 9 (k 2 ) to give k 1 ()). This procedure quantitatively determines the two transition states involved. In each case of the 6 f 9b-f series the second transition state is higher than the first one: a general energy profile is observed in which the actual insertion step is rate-determining and is preceded by the alkene addition/alkene dissociation preequilibrium. For example, the rate of 1-pentene dissociation at the stage of the intermediate 8 to reform the starting material 6 is ∼70 times higher than the competing actually product forming alkene insertion reaction to yield 9d. The difference of transition-state energies ranges from ∆∆G q 2 ) 1.7 ( 0.4 kcal mol -1 for 1-butene insertion to ∆∆G q 2 ) 2.1 ( 0.4 kcal mol -1 for 1-pentene and 1-hexene insertion, respectively. The kinetic analysis of the alkene insertion reaction at the single component "constrained geometry" catalyst [Me 2 Si(C 5 H 4 )N t Bu]Zr[C 4 H 6 B(C 6 F 5 ) 3 ] 11 was carried out analogously. B(C 6 F 5 ) 3 addition to [Me 2 Si(C 5 H 4 )N t Bu]Zr(butadiene) 10 yields a 1.8:1 mixture of the stereoisomeric betaines 11A/ 11B ("supine/prone" orientation of the ligand). The 11A/11B complex mixture actively polymerizes ethene. At low temperature the 11A/11B mixture reacts stoichiometrically with the series of olefins listed above to give the mono-alkene insertion products 14a-f, each found in solution as a single diastereoisomer. The kinetic analysis shows an even more pronounced alkene addition/alkene dissociation preequilibrium step (k -1 /k 2 ≈ 130 for 1-pentene) followed by rate-determining insertion (∆∆G q 2 ranging from +2.2 to +3.0 kcal mol -1 ). Reaction profiles featuring the actual alkene insertion step as the kinetically controlling activation barrier could be characteristic for group 4 metallocene Ziegler catalysts and related systems.