Aldol condensation is an important C-C bond formation reaction. It occurs in acidic media by the addition of a carbonyl compound to an enol, forming a Î²-hydroxy carbonyl molecule (the aldol step), and the subsequent dehydration of the aldol intermediate to form an Î±,Î²-unsaturated condensation product. Recent reports1,2
have shown that the same aldol reactants can also undergo a fission reaction that produces a carboxylic acid and an olefin, a potentially useful route to producing isobutene from acetone. It has been suggested that the fission products in the self aldol reaction of acetone are formed through the cleavage of the final dehydration product,1
while our studies using the cross aldol reactions of benzaldehyde and various aliphatic ketones showed that the fission reaction can still proceed even in the absence of dehydration. In order to obtain a detailed mechanistic picture of the aldol condensation and fission reactions, we carried out a series of dispersion-corrected, periodic density-functional theory calculations to map out all the elementary steps leading to each of the reaction pathways. We examined several plausible fission mechanisms, either through acid-catalyzed carbenium transition states or via the assistance of a neighboring water molecule. The intrinsic and apparent barriers obtained from the calculations are consistent with the activation energies determined experimentally, and the nature of the transition states in each reaction step explains the observed selectivity trends.
(1) Herrmann, S.; Iglesia, E. Selective Conversion of Acetone to Isobutene and Acetic Acid on Aluminosilicates: Kinetic Coupling between Acid-Catalyzed and Radical-Mediated Pathways. J. Catal. 2018, 360, 66â80.
(2) Ponnuru, K.; Manayil, J. C.; Cho, H. J.; Fan, W.; Wilson, K.; Jentoft, F. C. Intraparticle Diffusional versus Site Effects on Reaction Pathways in Liquid-Phase Cross Aldol Reactions. ChemPhysChem 2018, 19, 386â401.