TiO₂, however, proved particularly interesting as a milling auxiliary. Despite being neutral in acid-base terms, the reaction still occurred in its presence, even without a base (entry 17). Remarkably, no indigo byproduct was detected (unlike in the other reactions where indigo formed as a side product). We hypothesize that in the presence of TiO₂, the Knoevenagel reaction proceeds via aldehyde activation (Scheme 1b). TiO₂ binds to oxygen, increasing the electrophilicity of the carbonyl carbon, facilitating the attack by the indole unit, and preventing the formation of the indoxyl anion intermediate, which leads to indigo formation when oxidized in the pr esence of oxygen. Increasing the amount of TiO₂ in the reaction improved conversion rates, with the highest yield obtained using 50 equivalents (entry 18). Additionally, in cases where total conversion was below 90%, the starting materials could be recovered and reused, enhancing the environmental sustainability of this method. The activating effect of TiO₂, however, proved dependent on the functional groups present in the benzaldehyde.