Chiral Brønsted Acid-Base Pyridinium Disulfonate Catalysts

disulfonic acids - Mannich-type reaction - pyridinium salts

Significance

By combining chiral disulfonic acid 1 and achiral 2,6-diphenylpyridine the authors obtain tailor-made Brønsted acid-base organocatalysts for a direct Mannich-type reaction. Excellent yields and enantioselectivities were achieved with this concept.

Comment

The authors use an original and elegant strategy for high enantioinduction in comparison to the generally applied strategy of substitution at the 3,3′-positions with bulky substituents in analogous binaphthyl phosphoric acid catalysts (e.g., see: Akiyama et al. Angew. Chem. Int. Ed. 2004, 43, 1566). By forming a salt of chiral disulfonic acid 1, which has been tested earlier as organocatalyst by List and co-workers (Chem. Asian J. 2008, 3, 430; Adv. Synth. Catal. 2008, 350, 962), and bulky pyridines, the authors circumvent the often tedious synthesis of the 3,3′-substituted derivatives. On the other hand, the acidity is reduced this way and therefore the substrate scope might be limited to easily activated compounds.