THEORETICAL-STUDY OF THE ELECTROPHILIC SUBSTITUTION REACTIVITY IN BENZOCYCLOBUTADIENE AND BIPHENYLENE

The electronic and geometric structures of benzocyclobutadiene (1), bi phenylene (2) and their protonated forms are studied by the ab initio HF/6-31G and;MP2(fc)/G-31G*//HF/6-31G* methods. It is shown that beta -protonated species are more stable in both cases indicating greater s usceptibility of the beta-positions towards electrophilic substitution s. This finding is rationalized by the lower pi-electronic energy esti mated by the valence bond resonance structures and a decreased antagon ism between sigma- and pi-electrons in beta-Wheland intermediates. The inherent generalized strain energies in the parent compounds (1) and (2) are estimated by homodesmic reactions. It is found that the intrin sic destabilization energy is approximate to 8 kcal mol(-1) higher in 1, which is interpreted as the inability of the peripheral double bond to escape the cyclobutadiene antiboding pattern in an efficient way. The semiempirical AM1 calculations give correct qualitative results, b ut fail at the quantitative level.