New aspects of electrophylic aromatic substitution mechanism: Computational model of nitration reaction

摘要

Systematic studies of reaction of nitronium with substituted benzenes (Alk-, X-Ph-, CH3O-, Cl-, -COOCH3, -NO2, and other) were performed using semiempirical methods Austin model 1 and Solvation model v. 2.1 (SM2.1). Reaction path and maps of potential energy surface were investigated, including charge and thermodynamic properties of transition states and intermediate sigma-complexes. Main factors of reactivity-deformation energy and orbital and coulomb interaction-were considered by means of energy partitioning procedure, theory of Perturbation of Molecular Orbitals (PMO), etc. Firm correlation between calculated activation barriers and enthalpies of sigma-complex formation was found. It was found that the above-mentioned factors successfully joined in enthalpy of protonation of aromatic substrates; thus, this parameter appeared to be ideal index of reactivity of arenes in nitration reaction. Coulomb interaction of nitronium and substituent in transition states explained cases of abnormally high reactivity of ortho position (e.g., in substituted biphenyls). (C) 2004 Wiley Periodicals, Inc.