Brønsted Acid Catalyzed Asymmetric Pinacol Rearrangement
T. Liang, Z. Zhang, J. C. Antilla*
University of South Florida, Tampa, USA
21. Dezember 2010 (online)
Antilla and colleagues report a Brønsted acid catalyzed asymmetric pinacol rearrangement of indolyl diols 1. Using only 2.5 mol% of catalyst 3, various α-indolyl ketones 2 were obtained with excellent yields and enantioselectivities. Mechanistically, the authors propose a Brønsted acid promoted dehydration of 1 to form carbocation B, which is stabilized as a better indolinium ion. This charged species is presumably tightly coordinated to the phosphate anion catalyst via hydrogen bonding and electrostatic attraction, and undergoes the enantioselective [1,2]-aryl shift.