New Concepts for Organocatalysis
29
as asymmetric catalysts, we hypothesized that catalytic salts of achiral
amines and chiral phosphoric acids could induce asymmetry in these
processes (Scheme 23).
We thought to start with the metal-free biomimetic transfer hydrogenation of α,β-unsaturated aldehydes as a model reaction which has
been earlier discovered in our laboratory and independently in that of
MacMillan et al. (Scheme 24). We have prepared a large number of ammonium salts as crystalline solids by mixing different primary and secondary amines with a chiral phosphoric acid. In particular, the ammonium salts of sterically hindered chiral phosphoric acids could catalyze
Scheme 24. Asymmetric counteranion-directed catalysis: catalyst synthesis and
screening
29
as asymmetric catalysts, we hypothesized that catalytic salts of achiral
amines and chiral phosphoric acids could induce asymmetry in these
processes (Scheme 23).
We thought to start with the metal-free biomimetic transfer hydrogenation of α,β-unsaturated aldehydes as a model reaction which has
been earlier discovered in our laboratory and independently in that of
MacMillan et al. (Scheme 24). We have prepared a large number of ammonium salts as crystalline solids by mixing different primary and secondary amines with a chiral phosphoric acid. In particular, the ammonium salts of sterically hindered chiral phosphoric acids could catalyze
Scheme 24. Asymmetric counteranion-directed catalysis: catalyst synthesis and
screening