Heterogeneous catalytic synthesis of flavanones: An investigation of the functional group substitution effects, kinetics, and reaction mechanism

The manufacturing practices that led to the development of a successful and profitable fine chemical and pharmaceutical industry have been challenged recently due to concerns about the continuous rising cost of the products and relatively high waste generation of processes in this sector. Replacing homogeneous catalysts typically used in such processes with heterogeneous ones is expected to mitigate these problems because of the inherent waste minimization characteristics of heterogeneous catalysis and the subsequent process and product cost minimization.; Although replacing one catalyst with another seems to be a simple solution, the details of the reactant-catalyst interactions that occur with a heterogeneous catalyst are quite complex and not well understood. Specific research into individual reaction systems is often needed to better define the surface phenomena involved.; This work focuses on flavanone synthesis, which occurs over acid or base catalysts in two steps. First, 2'-hydroxyacetophenone reacts with benzaldehydes through a Claisen-Schmidt condensation reaction step to form 2'-hydroxychalcones. These chalcones subsequently undergo isomerization to form the corresponding flavanones. The results indicate that MgO and alumina are active heterogeneous catalysts for this synthesis; however, the focus remained on the more active MgO system. Reactions were conducted with various para-, meta-, and ortho-substituted benzaldehydes (i.e., methoxy-, hydroxy-, nitro-, and halide-containing ones) in different solvent media (DMSO, nitrobenzene, benzonitrile, and tetralin).; Kinetic results indicate that dipolar aprotic solvents enhance the initial reaction rate. Furthermore, the observed initial reaction rates vary systematically with the nature of the functional group attached to the benzaldehyde molecule in the para-position to yield an excellent Hammett correlation with a tendency toward electron-accepting groups. These and supporting results from spectroscopic studies suggest that an adsorbed ketone anion intermediate is formed from 2'-hydroxyacetophenone during the reaction, which is similar to one previously proposed for the homogeneous base-catalyzed synthesis.; Increased initial reaction rates were observed with meta- and ortho-substituted benzaldehydes. Subsequent spectroscopic studies revealed that the benzaldehydes adsorb on the MgO surface through the carbonyl group and may form a benzoate-type surface species. This species appears to poison the catalyst, since reactants with the least tendency to form benzoates yielded highest initial reaction rates.