Switchable Synthesis of Bis- and Monoindolyl Oxindoles on Water Using B(C ₆ F ₅ ) ₃ -Activated H ₂ O as a Brønsted Acid Catalyst

to activate water, generating an in situ Brønsted acid that not only accelerates the transformation but also creates a notable kinetic differentiation between the first and second indole addition steps. This allows the product distribution to be strategically controlled by the catalyst loading. This strategy provides a practical, metal-free, and environmentally benign route to access these valuable heterocyclic scaffolds, with broad substrate scope and scalability demonstrated. Mechanistic studies, including Hammett analysis and control experiments, support the proposed catalytic cycle and the role of water in enabling selectivity.