Description:
<jats:title>Abstract</jats:title><jats:p>Aldol cyclotrimerization of 1‐indanone afforded truxene in ionic liquid [BMIM][Tf<jats:sub>2</jats:sub>N] in the presence of <jats:italic>p</jats:italic>‐TsOH as a Brønsted acid catalyst. Truxene was easily isolated by filtration, and the ionic liquid containing <jats:italic>p</jats:italic>‐TsOH could be reused. Cyclic ketones such as 2‐indanone, cyclopentanone, and cyclohexanone successfully gave aldol cyclotrimerization products in ionic liquid. The reaction mechanism for the cyclotrimerization of 1‐indanone was assessed by the DFT calculations with the polarizable continuum model. The reaction involves 3 C─C bond forming steps: the first aldol reaction of 1‐indanone, the second aldol reaction of a dimer, and 6π‐electrocyclization of a trimer, whose transition states were computed to be energetically the highest in the entire reaction sequence. The second aldol reaction is considered to proceed through 2 alternative pathways: a nucleophilic addition of inden‐1‐ol to O‐protonated 2‐(1‐indanylidene)‐1‐indanone and a nucleophilic addition of 2‐(1‐indenyl)inden‐1‐ol to O‐protonated 1‐indanone. The transition state of the latter pathway was estimated to be energetically favored over that of the former by 3 kcal/mol, suggesting that the second aldol condensation proceeds by the latter pathway according to the Curtin‐Hammett principle. Stabilization by aromatization at the final step was confirmed to be the driving force for the reaction. Reaction rates of the first and the second aldol reaction process were enhanced by using a polar ionic liquid, but the electrocyclization was found to remain similar.</jats:p>