Abstract
Several strategies are available for enantioselective catalysis, including heterogeneous catalysis, Brønsted or Lewis acid and base catalysis, homogeneous transition-metal catalysis, and biocatalysis. One remarkable molecule, the amino acid proline, has become a crucial component in examples of all of the catalytic strategies listed above. Proline can be a ligand in asymmetric transition-metal catalysis, a chiral modifier in heterogeneously catalyzed hydrogenations, and, most importantly, proline itself can be an effective organocatalyst of several powerful asymmetric transformations, such as the aldol, Mannich, and Michael reactions.
In the focus of this review are proline-catalyzed asymmetric reactions. Covered are those reactions that are either catalyzed by proline alone, or by proline in combination with cocatalysts such as metal salts. Selected non-enantioselective proline-catalyzed reactions have also been included. However, the present review does not cover the use of important proline-derived auxiliaries or catalysts including the Enders-hydrazones or the Corey–Bakshi–Shibata-catalyst, which have been reviewed elsewhere. Furthermore, other remarkable and useful amino acid based catalysts such as MacMillan's iminium catalysts and Miller's peptide catalysts are beyond the scope of this review.
