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

## TL;DR

A new method using calcium-impregnated silica gel enables irreversible aldol reactions and domino processes to produce unsaturated enones and 1,5-diketones efficiently.

## Contribution

The use of Ca@SiO2 as a reducing reagent to drive irreversible aldol reactions and domino processes is novel.

## Key findings

- Aldol additions produced α,β-unsaturated enones in 71–90% yields using Ca@SiO2.
- A domino reaction created 1,5-diketones in 67–88% yields via aldol addition, dehydration, and Michael addition.
- Ca@SiO2 abstracts hydrogen and oxygen atoms, forming CaH2@SiO2 and CaO@SiO2 confirmed by X-ray diffraction.

## Abstract

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.