Efficient Catalytic Condensation of Methyl Ketones with Indole to Form All‐Carbon Quaternary Center Bisindolylmethanes Mediated by Strong Dual Brønsted Acid

Abstract This study develops a protocol for synthesizing bisindolylmethanes (BIMs) with all‐carbon quaternary centers using a dual Brønsted acid catalyst ( 1a ) containing two meta‐positioned perfluorobutylsulfonimide groups. The catalyst activates aliphatic and aromatic methyl ketones in methanol at 30 °C, yielding 59%–99% BIMs products, with acetophenone showing >90% selectivity. Under harsher conditions (e.g., high acetone concentration or catalyst loading), BIMs formation from acetone drops to <16%, while tetrahydrocyclopenta[b]indole ( 4 ) becomes dominant (54%–77% yield), indicating indole substituents’ electronic control over pathway selection. Solvent‐dependent 1 H NMR studies demonstrate hydrogen‐bond‐driven modulation of 1a 's acid strength, explaining solvent‐dependent selectivity. These findings highlight substrate‐catalyst electronic complementarity in governing product selectivity, providing a framework for designing acid strength‐tunable systems for BIMs synthesis.