Reactions of phenyl cations with methanol and methyl fluoride

Benzenium ions formed by the interaction of the phenyl cation with methanol and methyl fluoride as well as the transition states for their rearrangeme nt were optimized at the B3LYP/6-31G(d,p) level of theory. Among the reacta nt complexes [C6H5. CH3X](+) corresponding to the different sites of the ph enyl cation attack on the CH3X molecules (X = OH,F) the addition complex (O -protonated anisole) was the most stable one for X = OH, followed by the CH insertion complex (ipso-protonated benzyl alcohol), while for X = F the CF insertion complex had the lowest energy. Addition and insertion complexes may transform into the ring protonated isomers. Barrier heights for subsequ ent proton migrations in the arenium ions studied were in the 1-23 kcal/mol range and those for methyl migrations were in the 5-28 kcal/mol range. All these rearrangements were allowed, since their barriers were substantially lower than the complexation energy (67 kcal/mol for X = OH and 42 kcal/mol for X = Fl. The global minimum was para-protonated anisole for X = OH and meta-protonated ortho-fluorotoluene for X = F. These theoretical prediction s were compared with experimental results in studies of nucleogenic phenyl cation reactions with methanol and methyl fluoride. Low barriers for methyl migrations, especially that of CH3 shift from ipso- to ortho-positions rel ative to F in the phenyl cation-methyl fluoride encounter complex, relieved objections to proton shifts in arenium ions as a mechanism of the observed H/T scrambling.