Effects of Brønsted acid cocatalysts on the activities and selectivities of charge‐enhanced thiourea organocatalysts in Friedel–Crafts and oxa‐Pictet–Spengler reactions

Abstract

AbstractCharge‐enhanced chiral thioureas were used in the organocatalysis of the Friedel–Crafts alkylation of indole with trans‐β‐nitrostyrene and the oxa‐Pictet–Spengler reaction of tryptophol and benzaldehyde. The effects of substoichiometric Brønsted acidic additives on the reaction conversions and enantiomeric ratios of these transformations were examined, and the role of these species in the mechanism of the latter reaction was explored using variable time normalization kinetics to elucidate the reaction order of the catalyst, cocatalyst, and reagents. A proposed pathway for the oxa‐Pictet–Spengler cyclization that involves matched and mismatched catalyst and cocatalyst pairs and an off‐cycle racemization of the latter species is provided.