Calcium Impregnated Silica Gel in the Domino Reaction Involving Irreversible Aldol Addition, Dehydration, and Michael Addition

An innovative method was developed for the performance of aldol additions in an irreversible fashion by the use of calcium metal impregnated silica gel (Ca@SiO₂) as a remarkable reducing reagent. In this approach, Ca@SiO₂ drove the reaction forward, prevented reversibility, and ensured the formation of the desired products. Thus, in the presence of Ca@SiO₂ (3.0 equiv), aldehydes (1.0 equiv) condensed with ketones (1.0 equiv) in 2-MeTHF to yield α,β-unsaturated enones in 71-90% yields at 25 °C. Additionally, a domino reaction involving successive aldol addition, dehydration, and Michael addition was developed for the preparation of 1,5-diketones. Accordingly, when aldehydes (1.0 equiv) were allowed to react with ketones (2.2 equiv) and Ca@SiO₂(4.0 equiv), 1,5-diketones were produced in 67-88% yields. These reactions involved radical processes, where Ca@SiO₂ abstracted two α hydrogen atoms from ketones and the oxygen atom from aldehydes to form CaH₂@SiO₂ and CaO@SiO₂, respectively. These species were confirmed by powder X-ray diffraction analysis. The resultant impregnated silica gel species were solid and insoluble in the reaction mixtures, which made the addition reactions irreversible. This method represents a significant advancement in aldol condensation reactions and offers the advantages of both atom economy and atom efficiency.