om970434b_si_001.pdf (1.26 MB)
Enantiomerically Pure Organolanthanides for Asymmetric Catalysis. Synthesis, Structures, and Catalytic Properties of Complexes Having Pseudo-meso-Me2Si(η5-C5H3R)(η5-C5H3R*) Ancillary Ligation
journal contribution
posted on 1997-09-30, 00:00 authored by Peter W. Roesky, Uwe Denninger, Charlotte L. Stern, Tobin J. MarksAs established by NMR, circular dichroism, and X-ray
diffraction, organolanthanides of
the new chelating ligand
Me2Si(tBuCp)[(+)-neo-Men-Cp]2-
(Men = menthyl; Cp =
η5-C5H3)
preferentially adopt a single chiral configuration of the asymmetric
metal−ligand template.
Metallocene dichloro complexes
(R,S)-Me2Si(tBuCp)[(+)-neo-Men-Cp]Ln(μ-Cl2)Li(OEt2)2
(Ln
= Y, Lu) are synthesized by alkylation of the corresponding
lanthanide trichlorides with
the ligand dilithium salt and are isolated isomerically pure by
crystallization from diethyl
ether. Alkylation of the (R,S)-epimers with
MCH(SiMe3)2 (M = Li, K) proceeds with
retention
of configuration at the lanthanide center, affording chiral hydrocarbyl
complexes in high
yield. Reaction of the Lu−hydrocarbyl with hydrogen affords
diastereomerically pure {(R,S)-Me2Si(tBuCp)[(+)-neo-Men-Cp]LuH}2,
an active catalyst for asymmetric olefin hydrogenation.
X-ray diffraction reveals a
pseudo-C2-symmetric dimer with a
pseudo-meso Cp ring
substituent arrangement. The two
Me2Si(tBu-Cp)[(+)-neo-Men-Cp]LuH
fragments are
slightly twisted with respect to each other. These hydrocarbyls
are effective precatalysts
for asymmetric hydrogenation of unfunctionalized olefins. The ee
values obtained for
deuteration of 1-pentene (up to 63%) are the highest reported to date
for this reaction, and
in all reactions, the (R)-product enantiomer is favored.
These results together with those
obtained using other chiral organolanthanides provide better insight
into lanthanocene
stereochemical preferences and chirality transfer mechanisms.
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Keywords
Enantiomerically Pure Organolanthanideshydrogenationorganolanthanidechiral hydrocarbyl complexeschirality transfer mechanismsC 5 H 3configurationmeso Cp ring substituent arrangementNMRMetallocene dichloro complexesdiffractionLiLuBuCpligand dilithium saltMCH2 Silanthanocene stereochemical preferences
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