Aldolizations on Hydrotalcites First Calcined then Rehydrated

4 Aldolizations on Hydrotalcites First Calcined then Rehydrated 

As discussed above rehydration enables the restoration of the hydroxyls and it can be assumed that the compensating anions are OH , as in meixnerite. This solid has been described only recently [30] and few of its potential applications have been studied. 

The specific properties of hydrated hydrotalcites appear not only in the aldoli-zation of acetone, but in many other aldolization reactions. For example, in the aldol condensation of benzaldehyde with acetone the hydrated form catalyzes the reaction at 273 K, yielding aldol as the main product instead of benzalacetone, obtained on the calcined sample. Competitive adsorption kinetics are still observed, with a much greater adsorption coefficient for benzaldehyde. As suggested earlier from Hammett relationships, this reaction can be generalized with success to many substituted benzaldehydes [32], although the reaction could be performed selectively at 273 K with benzaldehyde only, and substituted benzaldehydes required a reaction temperature of 333 K. Because of this high temperature the reaction usually gives a, unsaturated ketones isolated yields are 95 %. 

Because the mechanism of this reaction is supposed to be ionic, it should be affected by the nature of the solvents. This is indeed so for the condensation of benzaldehyde and acetone [33]-rate changes by a factor of 50 are observed when heptane or toluene are substituted by DMF (Table 3). The initial rate, unaffected by possible inhibition by the products, is higher in polar solvents. Acetone also is highly polar and thus a good solvent for these reactions. 

Good catalytic properties have also been reported by Teissier and Kervenal [34] for the synthesis of isophorone on an hydrotalcite first calcined then rehydrated. The superiority of the rehydrated solid in the experiment is apparent from the observation that the reaction performed in a batch reactor at 473 K reaches about 

---