Exploring the unexpected pyridine- and 4,4′-bipyridine-catalyzed isomerization of maleic acid: A DFT approach

DFT at B3LYP/6-31G(d,p) and B3LYP/6-311+G(d,p) levels calculations for the pyridine- and 4,4′-bipyridine-catalyzed isomerization of maleic acid demonstrated that the mechanism involves four steps: (1) a proton transfer from one of maleic acid carboxyl groups onto the amine nitrogen (pyridine or 4,4′-bipyridine) to yield an ion pair INT1. (2) Proton transfer from INT1 to the C–C double bond to give INT2. (3) Rotation about the central C–C single bond followed by proton abstraction by an amine molecule to yield INT3, and (4) proton transfer from the ammonium cation into the carboxylate anion of the fumarate thus formed to furnish the trans isomer, fumaric acid. Moreover, the calculations revealed that an abstraction of a proton from INT2 (step 3) was found to be the rate limiting step. However, the activation energy difference between steps 2 and 3 was not significant. Furthermore, it was found that the solvent dielectric constant has a profound impact on both the isomerization activation energy and the free energy difference between the cis and trans isomers (maleic and fumaric acids). While, polar solvents such as DMSO tend to lower the isomerization activation energy and to increase the free energy difference between the two isomers, apolar solvents such as chloroform behave in an opposite manner. In addition, linear correlation was found between the activation energy and the solvent dielectric constant.