Reaction Control by Catalyst Design: 1,2- versus 1,4-Addition

Significance

The authors report the reaction of N-methylindoles 1 with α-keto esters 2 to atropisomeric bisindoles 3 (with an ee up to 62%) as a 1,2-addition reaction catalyzed by Brønsted acids such as 4. The initially desired 1,4-indole addition product 5 was only observed as side product. N-Triflyl­phosphoramide catalyst 6 gave products 5 in high yields (88%) and excellent enan­tioselectivities (up to 92%). A mechanistical discussion for the unexpected Friedel-Crafts-type alkyl­ation of 3 is given via a chiral ion pair 1a ⁺ 4 ^-.