A PM3 STUDY ON INTRINSIC DECARBOXYLATION PROCESS OF METHYL-ETHYL-ALPHA-PYRIDYLACETIC ACID

To probe the decarboxylation process of methyl-ethyl-alpha pyridylacet ic acid (MEPA), molecular orbital calculations on the optimized geomet ry, transition-state geometry, and intrinsic reaction coordinate were performed by the MNDO-PM3 method. The salient features of the optimize d 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-degre es) and that the interatomic distance of O-9...H1' is 1.6 angstrom (e xchange 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-degrees, 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 d ecarboxylation process, the exchange of electrons between O-9...H1' be gins to decrease. This decrease, which is considered to be induced by the rotation of C2-C7, seems to initiate the dissociation of C7-C8. (C ) 1995 John Wiley & Sons, Inc.