- Kinetic resolution of chiral α-olefins using optically active ansa-zirconocene polymerization catalysts
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A series of enantiopure C1-symmetric metallocenes, {(SiMe 2)2[η5-C5H(CHMe 2)2][η5-C5H2((S) CHMeCMe3)]}ZrCl2, (S)-2, {(SiMe2) 2[η5-C5H(CHEt2) 2][η5-C5H2((S)-CHMeCMe 3)]}ZrCl2, (S)-6, and {(SiMe2) 2[η5-C5HCy2][η5- C5H2((S)-CHMeCMe3)]}ZrCl2), (S)-7 (Cy = cyclohexyl), zirconocene dichlorides that have an enantiopure methylneopentyl substituent on the "upper" cyclopentadienyl ligand, and diastereomerically pure precatalysts, {(SiMe2) 2[η5-C5H((S)-CHMeCy)(CHMe 2)][η5-C5H3]}ZrCl2, (S)-8a and (S)-8b, which have an enantiopure, 1-cyclohexylethyl substituent on the "lower" cyclopentadienyl ligand, has been synthesized for use in the polymerization of chiral α-olefins. When activated with methylaluminoxane, these metallocenes show unprecedented activity for the polymerization of bulky racemic monomers bearing substitution at the 3- and/or 4-positions. Due to the optically pure nature of these single site catalysts, they effect kinetic resolution of racemic monomers: the polymeric product is enriched with the faster reacting enantiomer, while recovered monomer is enriched with the slower reacting enantiomer. The two components are easily separated. For most olefins surveyed, a partial kinetic resolution was achieved (s = kfaster/kslower ≈ 2), but, in one case, the polymerization of 3,4-dimethyl-1-pentene, high levels of separation were obtained (s > 15). 13C NMR spectroscopy of poly (3-methyl-1-pentene) produced with (S)-2 indicates that the polymers are highly isotactic materials. X-ray crystal structure determinations for (S)-2, {(SiMe2)2[η 5-C5H(CHMe2)2][η5- C5H2((S)-CHMeCMe3)]}Zr(SC6H 5)2, (S)-6, and (S)-7 have been used in combination with molecular mechanics calculations to examine the prevailing steric interactions expected in the diastereomeric transition states for propagation during polymerization. Precatalysts (S)-8a and (S)-8b are less selective polymerization catalysts for the kinetic resolution of 3-methyl-1-pentene than are (S)-2, (S)-6, and (S)-7.
- Baar, Cliff R.,Levy, Christopher J.,Min, Endy Y.-J.,Henling, Lawrence M.,Day, Michael W.,Bercaw, John E.
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p. 8216 - 8231
(2007/10/03)
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- Synthesis and properties of sterically congested cyclopentadienes and their transition metal complexes
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The synthesis of 1,3-dialkyl substituted cyclopentadienes, C5H4RR', Cp'H (Va-g) (where R and R' are tert-butyl, iso-propyl, neo-pentyl, cyclohexyl, or 1-methylcyclohexyl) is reported.These are synthesized by the nucleophilic addition of methyl or hydride anions to the corresponding 2-alkyl-6,6-dialkylfulvenes (IIIa-f) followed by hydrolysis.These substituted 1,3-cyclopentadienes have been converted to organometallic derivatives such as 2, (M=Mo or W) and Cp'2Fe.The spectroscopic and electrochemical properties of selected complexes have been investigated to probe the steric and electronic properties of the substituted cyclopentadienyl ligands.
- Clark, T. Jeffrey,Killian, Christopher M.,Luthra, Sanjay,Nile, Terence A.
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p. 247 - 258
(2007/10/02)
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