Catalytic enantioselective addition of sodium bisulfite to chalcones.

Herein we report the first example of the catalytic enantioselective addition of sodium bisulfite to a,b-unsaturated alkenes 1a–q which was achieved by the selection of an appropriate aminothiourea bifunctional catalyst (Scheme 1). Following the pioneering work carried out by the groups of Soo s, Connon, Deng, and Wang, bifunctional catalysis has matured to become a prominent area of organic synthesis and a key enabling technology for the planning of cascade reactions. Sulfonic acids are among the strongest acids in organic chemistry and are extensively employed as resolving agents. Kellogg et al. reported a few enantiopure sulfonic acids 2, obtained from chalcones, as resolving agents by a “Dutch Resolution” approach. Several multistep syntheses of chiral sulfonic acids have been reported. However, the simple addition of bisulfite to olefins, discovered over a century ago, remains the most straightforward access to aliphatic sulfonic acids. This reaction employs reactants in large excess and requires high temperature, which limits its synthetic utility. We have recently shown that 1) decreasing the concentration of bisulfite and 2) employing organic base catalysis can increase the rate of addition of bisulfite to electrophilic alkenes. Careful adjustment of these conditions resulted in a mild protocol for the addition of bisulfite to electrophilic alkenes. Scheme 1. Enantioselective addition of bisulfite to chalcones.