Brønsted Acid Catalyzed Asymmetric Pinacol Rearrangement

Significance

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.