Role of Relative Transition Energy Level in Regioselective Chlorinations on Bithiophenes: Ab Initio Study

Abstract

The equilibrium and transition geometric structures from bithiophene to hexachlorobithiophene derivatives and relative energies of bithiophene derivatives in electrophilic chlorination were fully optimized using the ab initio Hartree-Fock and second-order Moller-Plesset perturbation method with a 6-311+G** basis set. The regioselectivity of the electrophilic chlorination of bithiophene derivatives was determined primarily by the gaps (Delta E-1, Delta E-2, AE(3), ...) of the relative transition energy levels in the transition pi-complexes. The gaps of the relative transition energy level were correlated with both the chlorinated positions and atomic charges of the carbon atoms on the bithiophene ring, the HOMO-LUMO gaps of the transition pi-complexes due to the (Cl+center dot center dot center dot C delta-) interaction, the geometric structures of the neutral and transition derivatives, and the pi-conjugation effect (delocalization energy) originating from the pi-orbitals. The order of relative stability in the transition pi-complexes was found to be 5,5'-dichlorinated > 3,3'-dichlorinated > 4,4'-dichlorinated derivatives. Particularly, with increasing the relative transition energy gaps (Delta E-2, Delta E-3, Delta E-8, Delta E-9, ...) in the transition pi-complexes, the regioselective chlorinations from BT to 6Cl-BT occurred at specific position of bithiophenes.