5026-95-9Relevant academic research and scientific papers
Kinetic resolution of chiral α-olefins using optically active ansa-zirconocene polymerization catalysts
Baar, Cliff R.,Levy, Christopher J.,Min, Endy Y.-J.,Henling, Lawrence M.,Day, Michael W.,Bercaw, John E.
, p. 8216 - 8231 (2007/10/03)
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.
Silylformylation of chiral 1-alkynes, catalysed by solvated rhodium atoms
Aronica, Laura Antonella,Terreni, Silvia,Caporusso, Anna Maria,Salvadori, Piero
, p. 4321 - 4329 (2007/10/03)
Solvated rhodium atoms, prepared by the metal vapour synthesis technique, promote the silylformylation reaction of variously substituted alkynes R1R2CH(CH2)nC≡CH, with catalytic activities comparable with and even higher than more common species such as Rh4(CO)12. Z-Silylalkenals are exclusively formed in high yields (60-95%) indicating syn addition both of CO and of the silane (Me2PhSiH) to the triple bond. The chemoselectivity of the process (silylformylation vs. hydrosilylation) is highly affected by the amount of catalyst employed (mmol of Rh species with respect to the alkyne reagent), by the steric requirements of the acetylenic substrates and by the hydrosilane/alkyne molar ratio. When optically active acetylenes are treated in the presence of Me2PhSiH under carbon monoxide pressure, the silylformylation reaction occurs with total retention of stereochemistry of the stereogenic centre, even if it is at the α-position of the unsaturated moiety, to afford enantiomerically enriched β-silylalkenals.
