Oxidative aromatic substitutions: Hartree-Fock/density functional and ab initio molecular orbital studies of benzene and toluene nitrosation

Aromatic nitrosations are prototypes of a recently proposed reaction mechan ism-oxidative aromatic substitutions-incorporating ground-state electron tr ansfer prior to the substitution step. Ab initio MO and B3LYP hybrid Hartre e-Fock/density-functional (HF/DF) calculations confirm that nitrosation pro ceeds through initial formation of intermediate electron donor-acceptor (ED A) pi-complexes. Calculated pi-complex geometries and energies agree qualit atively with experimental data and indicate the applicability of HF/DF meth ods for modeling EDA complexes. Subsequent transformation of (benzene-NO)() and (toluene-NO)(+) pi-complexes into N-protonated nitroso-derivatives in B3LYP and MP2 calculations suggest an alternative to the currently propose d mechanism involving pi-complex transformation into Wheland type sigma-com plexes (supported by CISD calculations). Kinetic analysis suggests the alte rnative mechanism is plausible and indicates that proton transfer from the N-protonated nitroso-derivatives to the medium would not be rate limiting. Instead, low nitrosation rates would be assigned to significant potential e nergy barriers for pi-complex transformation into N-protonated nitroso-deri vatives by a novel migratory insertion of nitrogen into the aromatic C-H bo nd. The insertion step exhibits a large, primary kinetic isotope effect of 11.5, in qualitative agreement with the isotope effect of 8.5 +/- 2.4 previ ously measured for benzene nitrosation. The difference in B3LYP barrier hei ghts for direct pi-complex conversion to N-protonated nitroso-adducts would also account for the difference in substrate reactivities and regioselecti vity in nitrosation reactions. Thus, the conflict between B3LYP and MP2 cal culations vs the CISD results cannot be resolved by applying kinetic argume nts and must await more definitive work.