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Chiral Bis(oxazoline)copper(II) Complexes as Lewis Acid Catalysts for the Enantioselective Diels−Alder Reaction
journal contribution
posted on 1999-08-10, 00:00 authored by David A. Evans, Scott J. Miller, Thomas Lectka, Peter von MattBis(oxazoline)copper(II) complexes are highly enantioselective catalysts in Diels−Alder reactions
involving bidentate dienophiles. Cationic [Cu((S,S)-t-Bu-box)]X2 complexes with different counterions have
been used as catalysts, revealing a profound influence of the counterion on the rate and stereoselectivity of the
catalyst. A square-planar catalyst−substrate complex is proposed to account for the high diastereo- and
enantioselectivities observed. Three bis(oxazoline)−Cu(II) X-ray structures have been obtained that support
this model. Double-stereodifferentiating experiments, performed employing chiral dienophiles, afforded results
that are fully consistent with the proposed square-planar transition-state assemblage. In addition to imide-based substrates, α,β-unsaturated thiazolidine-2-thiones have been introduced as a new class of dienophiles
with enhanced reactivity. Kinetics experiments were performed to quantify the role that product inhibition
plays in the course of the reaction. Rate and equilibrium binding constants of various catalyst inhibitors were
also derived from the kinetic analysis. A comparative study was undertaken to elucidate the differences between
the bis(oxazoline)−Cu(II) catalyst and the bis(oxazoline) catalysts derived from Fe(III), Mg(II), and Zn(II).
Catalyst performance was found to be a function of a subtle relationship between bis(oxazoline) structure and
transition metal.