Urea Activation by an External Brønsted Acid: Breaking Self-Association and Tuning Catalytic Performance
Isaac G. Sonsona, Eugenia Marqués-López, Marleen Häring, David Díaz Díaz and Raquel P. Herrera
Catalysts 2018, 8(8), 305; https://doi.org/10.3390/catal8080305
Abstract: In this work, we hypothesize that Brønsted acids can activate urea-based catalysts by diminishing its self-assembly tendency. As a proof of concept, we used the asymmetric Friedel–Crafts alkylation of indoles with nitroalkenes as a benchmark reaction. The resulting 3-substituted indole derivatives were obtained with better results due to cooperative effects of the chiral urea and a Brønsted acid additive. Such synergy has been rationalized in terms of disassembly of the supramolecular catalyst aggregates, affording a more acidic and rigid catalytic complex.
Keywords: Brønsted acids; Friedel-Crafts; organocatalysis; self-association; synergy; urea