Ab Initio, NBO and NICS Studies of the Decomposition ((2+2) elimination) Mechanisms of 2-Halo-2, 3-dihydropyridines

The decomposition of 2-Fluoro-2,3-Dihydropyridine (1) 2-Chloro-2,3-Dihydropyridine(3) 2-Bromo-2,3Dihydropyridine(5) to pyridine and HX(X=F, Cl, Br) was investigated using AB initio Molecular Orbital (MO) and Density Functional Theory (DFT). This is possible for all HX elimination reactions examined here, the two pass ways as well as three or four centered transition state structures. Calculation results for the B3LYP/6-311+G**, HF/6-311+G**// B3LYP /6-311+G** and MP2/6-311+G**// B3LYP /6-311+G** levels of theory for comparison of these two pass ways, showed that passing from four centered-transition state is more likely than three centered-transition state. Study on the B3LYP/6-311+G** level of theory revealed that the required energy for decomposition of compound 1, 3 and 5 to pyridine is 32.483165, 23.329822 and 19.987130 kcal mol −1 , respectively. In a similar vein, these calculations were performed on HF/6-311+G**// B3LYP /6-311+G** and MP2/6-311+G**// B3LYP /6-311+G** levels of theory. Natural Bond Orbitals (NBO) population analysis and Nuclear Independent Chemical Shift (NICS) results showed that in transition state (four-membered planar structure) C-H and C-X bonds break and H-X bonds appear; besides, the results showed that reactants are non-aromatic, but elimination reaction products are aromatic.