Selective nitroaldol condensations over heterogeneous catalysts in the presence of supercritical carbon dioxide

Article abstract of DOI:10.1021/jo801650p

(Chemical Equation Presented) At 40-60°C, in the presence of heterogeneous catalysts based on Al₂O₃, supercritical carbon dioxide not only acts as a good solvent for the reaction of aromatic and aliphatic aldehydes with 1-nitroalkanes but, most importantly, it also allows the selectivity to be tuned between the competitive formation of β-nitroalcohols and nitroalkenes (from the Henry reaction and the nitroaldol condensation, respectively). In particular, when the pressure (and the density) of the supercritical phase is enhanced from 80 to 140 bar, the nitroalkene's selectivity increases, on average, from ～60 to >90%. Experiments show that, in the same pressure range, a steep increase of the concentration profiles of reactant aldehydes takes place. By contrast, under solvent-free conditions, the reaction usually proceeds with a higher conversion, but nitroalkanols are the major products.