A site-isolated Lewis acidic aluminium and Brønsted basic amine sites in the dimeric silsesquioxane cage as a reusable homogeneous bifunctional catalyst for one-pot tandem deacetalization/deketalization-Knoevenagel condensation reactions

Abstract

The development of multifunctional catalysts for one-pot tandem reactions is significantly required to attain multiple sequential transformations in a single reactor, which would considerably decrease the number of manipulations demanded for chemical manufacturing in industries. Herein, dimeric silsesquioxane Al-POSS-NH₂ (2), a homogenous bifunctional acid–base catalyst containing environmentally friendly robust silica and high chemical and thermal stabilities, permanent catalytic activity, and reusability, was synthesized by the reaction of trisilanol aminopropyl hexaisobutyl-POSS (1) with trimethylaluminium. Al-POSS-NH₂ was successfully used as a bifunctional catalyst for one-pot tandem reactions because of the synergism and effective compartmentalization between Lewis acidic aluminium and Brønsted basic amine sites (>10.0 Å) in the dimeric silsesquioxane cage, which was confirmed by DFT and QTAIM studies. Subsequently, different acetals were tested to obtain their corresponding benzylidene malononitrile derivatives using Al-POSS-NH₂ for the one-pot tandem deacetalization-Knoevenagel condensation reactions and showed high efficiency (>90%) under optimized conditions (DMF, 0.3 mol% catalyst loading and 80 °C) with different reaction times. Furthermore, the bifunctional Al-POSS-NH₂ catalyst was separated from the reaction mixture via the precipitation method by adding acetonitrile into the reaction mixture and reusing it for five consecutive cycles without losing activity considerably, thus providing the inherent advantage over traditional homogeneous catalysts. In a one-pot tandem deketalization-Knoevenagel condensation reaction for various ketals, the reaction condition was slightly modified by increasing the catalyst loading (0.6 mol%) and reaction time (16 to 24 hours) to acquire better conversion and yield of their desired products. Finally, the present study suggests that the bifunctional POSS might facilitate the rapid development of environmentally friendly and economically feasible catalysts for multistep reactions.