Intramolecular Crossed-Benzoin Reactions Catalyzed by Chiral Triazolium Salts

Significance

Novel polycyclic triazolium salts were employed in an intramolecular crossed-benzoin reaction with high yields and excellent enan­tioselectivities. The authors describe the synthesis of the catalysts in four to eight steps (9-47% overall yields). The triazolium salts were used as precatalysts, which generate the catalytically active heterocyclic carbene species upon in situ deprotonation using a base cocatalyst. Several keto aldehydes 1 could be converted into the corresponding acyloins 2 in moderate to high yields (43-93%) and good to excellent enantioselectivities (61-98%) within 24 hours at room temperature. Decreasing the temperature did not improve the enantioselectivities further but increased the reaction time. Five-membered cyclic acyloins are produced in excellent yield (95%) but with only moderate enantioselectivity (75%).