Improving Selectivity of Toluic Acid from Biomass-Derived Coumalic Acid

Selective formation of bioaromatics from renewable sources to access greener alternatives to petroleum-derived aromatics is of great interest in the development of sustainable chemicals. An innovative and efficient biobased route toward the formation of toluic acid using coumalic acid (or methyl coumalate) cycloaddition with bioavailable propylene is demonstrated with selectivities of >99 mol % utilizing a Diels-Alder/decarboxylation/dehydrogenation domino sequence in conjunction with a heterogeneous catalyst. An inexpensive as well as easy to separate dienophile such as propylene offers an attractive alternative to access bioaromatics with high atom efficiency. Herein, we report an in-depth kinetic investigation to evaluate the activation energies of individual reaction steps and identify the rate limiting step to guide the development of an improved overall process. The kinetic analysis is complemented by first principle density functional theory calculations, which corroborate the experimental results. We further explore the influence of solvents on the production of aromatics from both coumalic acid as well as methyl coumalate. We show that toluic acid formation is highly susceptible to the solvent used to mediate the reaction as well as the chemical moiety (ester vs acid) of the staring substrate. Kinetic analyses of methyl coumalate cycloaddition with propylene and the subsequent decarboxylation reactions indicate that decarboxylation is likely the rate limiting step. Finally, we combine our results with propylene and different dienophiles reported previously to show that activation barriers as well as regioselectivity of the cycloaddition depends on the nature of the dienophile. Electron-rich dienophiles appear to have lower activation barriers and higher para to meta ratio, while electron deficient dienophiles result in high activation barriers and lower para to meta ratio.