Nitroaldol reaction over solid base catalysts

Nitroaldol reaction of a nitro compound with a carbonyl compound was carried out over a variety of solid base catalysts to elucidate the activity-determining factors in the nature of the catalysts and in the nature of nitro and carbonyl compounds. Among the catalysts examined, MgO, CaO, Ba(OH)(2), KOH/alumina, KF/alumina, Sr(OH)(2), hydrotalcite, and MgCO3 exhibited high activity for nitroaldol reaction of nitromethane with propionaldehyde, the activities being in this order. Over these catalysts, the yields exceeded 20% at a reaction temperature of 313 K and a reaction time of 1 h. Mg(OH)(2), gamma-alumina, SrO, Ca(OH)(2). BaCO3, SrCO3, BaO, and La2O3 exhibited moderate activites; the yield were in the range 20-2%. CaCO3, ZrO2, and ZnO scarcely showed the activity. It is suggested that strongly basic sites are not required for the reaction because the abstraction of a proton from a nitro compound is easy. The reactivities of the nitro compounds were nitroethane > nitromethane > 2-nitropropane, and those of carbonyl compounds were propionaldehyde > isobutyraldehyde > pivalaldehyde > acetone > benzaldehyde > methylpropionate. On the basis of IR study of adsorbed reactants and the reactivities of the reactants, the reaction mechanisms are proposed. The reaction proceeds by the nucleophilic addition of the carbanion formed by the abstraction of a proton from nitro compounds to the cationic species formed by the adsorption of carbonyl compounds on the acidic sites (metal cations). The nitroaldol reaction of nitromethane with propionaldehyde over MgO was scarcely poisoned by carbon dioxide and water; nitromethane is so acidic that it is able to be adsorbed on the catalyst on which carbon dioxide or water was preadsorbed. (C) 2003 Elsevier Science B.V. All rights reserved.