Asymmetric Addition of Terminal Acetylenes to Aldehydes, Catalyzed by Zn(OTf)2-Cinchona Alkaloid Complexes

Cinchona alkaloids in combination with zinc triflate ­catalyze the addition of acetylenes to aldehydes at room temperature with ee up to 89% for aliphatic aldehydes. Cinchonidine has proven to be the ligand of choice for the addition reaction whose outcome is demonstrated to be very sensitive to the alkaloid structure. Aromatic aldehydes are less reactive and give adducts of low optical purity. The more sterically hindered 2-bromo- and 2-nitro­benzaldehydes are considerably more reactive in the reaction as compared to their 4-substituted congeners.