Mechanistic Insights into the Kinetic and Regiochemical Control of the Thiol-Promoted Catalytic Synthesis of Diphenolic Acid

The mechanism of the acid-catalyzed condensation between levulinic acid and phenol to form diphenolic acid (DPA) was investigated using a combination of sulfonated hyperbranched poly(arylene oxindole)s and thiols. Taft-type steric and electronic parameters were applied to study the correlation between the nature of the thiols and the condensation rate. The kinetic effect of thiols to tune the regiochemical control was shown to be dependent on the substituent size of the thiols. Although thiols cause an increase in the regioselectivity to the desired p,p ′-DPA, the isomer distribution is found to converge to a thermodynamic equilibrium at high conversions of levulinic acid. The hitherto unconsidered acid-catalyzed isomerization of p,p ′-DPA to o,p ′-DPA was demonstrated by condensation reactions with both m -cresol and 13 C-labeled phenol supported by density functional theory (DFT) calculations.