Electrophilic Bromination Of Substituted Stilbenes And Stilbazoles: A Quantum-Chemical Investigation

In order to study the effect of substituents on the electrophilic bromination of stilbenes and stilbazoles, the geometries of the reaction intermediates were optimised at the MP2/6-31G(d, p) and M06-2X/6-31G(d, p) levels of theory. Stilbenes with two electron-withdrawing substituents are shown to favour bromonium ion intermediates, whereas the presence of an electron-donating substituent leads to a carbocation intermediate. These observations are rationalised by means of Hammett and Taft parameters. Localised molecular orbitals indicate that the bonding between the bromine atom and the carbon atoms in the stilbene varies from a sigma-bond in a carbocation intermediate to a 3-centre-2-electron bond in a bromonium ion intermediate. Natural bond orbital (NBO) analyses reveal that carbocation intermediates are stabilised by resonance. The optimised geometries of charge-transfer complexes, the pre-reactive intermediates formed between molecular bromine and the stilbene, show that these species can be considered as halogen-bonded complexes with binding energies ranging from 14.5 to 20.1 kJ mol(-1).