Brønsted acid‐catalyzed aldol cyclotrimerization of 1‐indanones in ionic liquid: An experimental and DFT study of substituent effect

Abstract

Reactions of 5‐R‐1‐indanones (R = Me, t‐Bu, Cl, F) in ionic liquids in the presence of Brønsted acid afforded substituted truxenes as aldol cyclotrimerization products, whose yields were found to be better in [BMIM][Tf2N] than in [BMIM][BF4]. Their cyclotrimerizations were slower and gave lower yields than that of the parent 1‐indanone. Furthermore, the reaction mixtures contained some amounts of intermediate products, 5‐R‐2‐(5‐R‐1‐indanylidene)‐1‐indanone, formed by the initial aldol condensation. These results suggested that both the electron‐donating and the electron‐withdrawing substituents at the C(5) position reduced the rate of cyclotrimerization. This rate retardation was supported by DFT/B3LYP calculations with the PCM solvation model. The activation energies of the reaction of the protonated indanones and the indenols in the initial aldol condensation were increased by both the electron‐donating group (EDG) and the electron‐withdrawing group (EWG). The transition states in the second aldol condensation and in the electron‐cyclization were also destabilized by both the EDG and the EWG.