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S.C. Pan, B. List
termediates can be conducted highly enantioselectively if a chiral counteranion is introduced into the catalyst, as a result of the generation of
a chiral ion pair. We termed this new strategy as asymmetric counteranion-directed catalysis (ACDC) (Scheme 21).
Although efficient asymmetric catalytic transformations involving
anionic intermediates with chiral, cationic catalysts have been realized
(for several reviews, see Houk and List 2004), analogous versions of
inverted polarity with reasonable enantioselectivity, despite attempts,
have been illusive (Scheme 22; Eqs. 39–40; for a review, see Lacour
and Hebbe-Viton 2003; see also: Llewellyn and Arndtsen 2005; Dorta
et al. 2004; Carter et al. 2003).
5.1
Asymmetric Counteranion-Directed Catalysis:
Application to Iminium Catalysis
In iminium catalysis, both we and the group of MacMillan had observed
a strong counteranion effect on the yield and enantioselectivity of the
reactions. Inspired by recent use of chiral phosphoric acid derivatives
Scheme 23. Asymmetric counteranion-directed catalysis: application to
iminium catalysis