ja210946z_si_003.cif (45.65 kB)
Enol Ethers as Substrates for Efficient Z- and Enantioselective Ring-Opening/Cross-Metathesis Reactions Promoted by Stereogenic-at-Mo Complexes: Utility in Chemical Synthesis and Mechanistic Attributes
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posted on 2012-02-08, 00:00 authored by Miao Yu, Ismail Ibrahem, Masayuki Hasegawa, Richard
R. Schrock, Amir H. HoveydaThe first examples of catalytic enantioselective ring-opening/cross-metathesis
(EROCM) reactions that involve enol ethers are reported. Specifically,
we demonstrate that catalytic EROCM of several oxa- and azabicycles,
cyclobutenes and a cyclopropene with an alkyl- or aryl-substituted
enol ether proceed readily in the presence of a stereogenic-at-Mo
monopyrrolide-monoaryloxide. In some instances, as little as 0.15
mol % of the catalytically active alkylidene is sufficient to promote
complete conversion within 10 min. The desired products are formed
in up to 90% yield and >99:1 enantiomeric ratio (er) with the disubstituted
enol ether generated in >90% Z selectivity. The
enol ether of the enantiomerically enriched products can be easily
differentiated from the terminal alkene through a number of functionalization
procedures that lead to the formation of useful intermediates for
chemical synthesis (e.g., efficient acid hydrolysis to afford the
enantiomerically enriched carboxaldehyde). In certain cases, enantioselectivity
is strongly dependent on enol ether concentration: larger equivalents
of the cross partner leads to the formation of products of high enantiomeric
purity (versus near racemic products with one equivalent). The length
of reaction time can be critical to product enantiomeric purity; high
enantioselectivity in reactions that proceed to >98% conversion
in as brief a reaction time as 30 s can be nearly entirely eroded
within 30 min. Mechanistic rationale that accounts for the above characteristics
of the catalytic process is provided.