Aldolizations on Other Solid Catalysts

MgAl hydrotalcite impregnated with potassium t-butylate has recently been used for aldolization reactions [35]. The results are summarized in Table 4 for the condensation of acetone on benzaldehydes of general formula R2CHO. 

This modified catalyst is more active than decarbonated and rehydrated HOT because it catalyzes the reaction very quickly at 273 K whereas HDT required a slightly higher temperature and a longer time. As a result of operation at 273 K the aldol can be formed selectively. 

Nucleophile Electrophile Time (h) Conv. (%) Selectivity (%) Aldol adduct Dehydrated product 

Aldolization with MCM 41, a mesoporous material modified by anchoring of 3-trimethoxysilylpropylethylenediamine, has also been reported [36]. This solid is expected to have the medium basicity of a diamine. Results for some aldolization reactions are reported in Table 5. The temperature was 323 K and relatively long times are required to reach high conversions, so these solids seem to be weaker bases than activated hydrotalcites. The high selectivity for dehydrated products is, moreover, evidence of the residual acidity of the solid. 

The aldol condensation of benzaldehyde with acetophenone has been used to compare hydrated hydrotalcites with many solids claimed to be strong solid bases-KF and KNO3 supported on alumina, X zeolites containing excess Cs or Mg, and lanthanum oxide. Only hydrated HDT and KF/AI2O3 could be used to perform this reaction at 273 K-HDT with 100% selectivity for chalcone and KF with lower selectivity, because of secondary Michael addition of the chalcone to acetophenone. The other solids were inactive, but could induce comparable conversions at 423 K. 

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