Tishchenko Reaction Over Solid Base Catalysts

Catalytic behavior of solid bases for mixed Tishchenko reaction in which an equimolar mixture of two different aldehydes is allowed to react was investigated employing the combinations of benzaldehyde and pivalaldehyde, pivalaldehyde and cyclopropanecarbaldehyde, and cyclopropanecarbaldehyde and benzaldehyde. The reactions were performed at 353 K for 4 h in vacuo without solvent using 5 mmol of each aldehyde and 100 mg of solid base catalyst. For all the combinations, the catalytic activity of alkaline earth oxides increased in the order of BaO≪MgO<CaO<SrO, implying that strongly basic site and high surface area are indispensable for exhibiting high activity. Treatment of benzaldehyde and pivalaldehyde with other solid bases such as La2O3, ZrO2, ZnO, γ-alumina, hydrotalcite, KF/alumina, and KOH/alumina produced either no amount or very small amounts of cross-esterification and self-esterification products. Quantum chemical calculations carried out at the PM3-MO level for the positive charges on the carbonyl carbon atoms of aldehydes and the structure parameters of the active species for the ester formations indicated that the selectivities to four Tishchenko dimers over MgO and CaO are determined primarily in the step of the nucleophilic addition of the active species (PhCH2O-, tBuCH2O-, and C3H5CH2O-) to the carbonyl carbon atoms of aldehydes. The reaction of the aldehyde having more positively charged and sterically less hindered carbonyl carbon atom with the active species having less hindered oxygen atom proceeds faster.