A PM3 study on intrinsic decarboxylation process of methyl–ethyl‐α–pyridylacetic acid

To probe the decarboxylation process of methyl–ethyl–α pyridylacetic acid (MEPA), molecular orbital calculations on the optimized geometry, transition‐state geometry, and intrinsic reaction coordinate were performed by the MNDO–PM3 method. The salient features of the optimized structure of MEPA are that the carboxyl anion is nearly on the plane of the pyridine ring (the dihedral angle of C8C7C2N1 is 14.7°) and that the interatomic distance of O⁻9 … H1′ is 1.6 Å (exchange of electrons exists between their atoms). The transition‐state geometry of the decarboxylation process has the following features: (1) the activation enthalpy is 6.0 kcal/mol, (2) the dihedral angle of C8C7C2N1 is −50.2°, and (3) the interatomic distance of O⁻9H1′ and C7C8 increase by 111 and 124%, respectively, as compared with the optimized geometry. From the extreme beginning of the intrinsic decarboxylation process, the exchange of electrons between O⁻9 … H1′ begins to decrease. This decrease, which is considered to be induced by the rotation of C2C7, seems to initiate the dissociation of C7C8. © 1995 John Wiley & Sons, Inc.