Aldol condensation is an important reaction in organic chemistry and has garnered significant interest as a strategy in biomass conversion to increase carbon chain length and reduce oxygen content. While delicate approaches have been developed to control the regio- and stereoselectivity of aldol reactions in fine chemical synthesis, much less is known about the selectivity trends on various solid catalysts. Aldol reactions present a reasonably complex series of transformations consisting of ketone-enol tautomerization, the addition of a carbonyl compound to the enol, and the subsequent dehydration of the aldol addition product to make an α,β-unsaturated ketone. Both acids and bases catalyze this reaction and the choice of catalysts directly influences the nature of key intermediates and thus the reaction mechanisms. Further compounding the complexity are reports where the aldol reactants are shown to undergo a side reaction, first reported by Hardy and termed “fission”, which formally cleave the α,β-unsaturated ketone hydrolytically to make a carboxylic acid and an olefin and provides a potentially useful route to producing isobutene from acetone. To provide a plausible mechanistic picture of the competing pathways leading to aldol condensation versus fission products, we carried out a series of dispersion-corrected density-functional theory calculations to study the elementary steps leading to each of the transformations. Several fission mechanisms were examined, either through acid-catalyzed carbenium transition states or via the assistance of a solvent water molecule. We further studied how reactant structure, aromaticity, substitution, and the nature of acid sites (including Al-substituted and sulfonic acid functionalized silica slabs and zeolites) each affect the different elementary steps and allow the reactions to be driven towards the desired products.

Mechanisms and selectivity of solid acid catalyzed aldol condensation and fission reactions: A DFT study

ACS Spring Meeting 2021

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