24
S.C. Pan, B. List
Alder reactions, 1,3-dipolar cycloadditions (Jen et al. 2000), and conjugate additions of electron rich aromatic and heteroaromatic compounds
can be catalyzed using chiral amino acid derived imidazolidinones as
catalysts (Scheme 17; Eqs. 35–38; Paras and MacMillan 2001, 2002;
Austin and MacMillan 2002; Brown et al. 2003b). In addition, highly
enantioselective epoxidations (Marigo et al. 2005b) and cyclopropanations (Kunz and MacMillan 2005) have recently been developed.
4.1
Organocatalytic Conjugate Reduction of α,β-Unsaturated
Aldehydes
In 2001, we reasoned that this catalysis strategy might be applicable
to the conjugate reduction of α,β-unsaturated carbonyl compounds if
a suitable hydride donor could be identified (Scheme 18). Hantzsch
ester 11 seemed to be particularly promising since its reaction with
preformed α,β-unsaturated iminium ions had already been established
(Makino et al. 1977; Baba et al. 1980).
This process was published in 2004 and constitutes the first metalfree organocatalytic transfer hydrogenation of α,β-unsaturated aldehyScheme 18. Iminium catalytic transfer hydrogenation of α,β-unsaturated aldehydes