THEORETICAL-STUDIES ON THE GAS-PHASE NUCLEOPHILIC AROMATIC-SUBSTITUTION REACTION

The gas-phase nucleophilic substitution reaction of pentafluoroanisole with OH- and NH2- nucleophiles have been studied theoretically using the AM1 method. Three reaction channels, S(N)2, IPSO and S(N)Ar (scheme 1), are all very exothermic so that all are accessible despite the varying central energy barriers which are much lower than the reactants level. In the IPSO and S(N)Ar channels, the reactants form directly a stable sigma-anion complex which proceeds to form a proton transfer complex via a transition barrier corresponding to a loose pi-type complex with the F-or OCH3-) leaving group. Due to a greater number of probable reaction sites available for S(N)Ar compared to the other two processes, the S(N)Ar channel is favored as experimentally observed.