Abstract
The kinetics of sigma complexation of unsymmetrical 2-nitro-4,6-bis(trifluoromethylsulfonyl)anisole (1e) has been investigated over a large pH range of 2.7-15.68 in methanol. Two competitive sigma complexation processes have been identified that lead, respectively, to the formation of the 1,3- and 1,1-adducts 2e and 3e of 1e. No evidence for nucleophilic addition at the unsubstituted 5-position para to the nitro group of 1e to afford the 1,5-dimethoxy adduct 4e could be obtained. The observed regioselectivity supports the view derived from previous investigations that a SO2CF3 group exerts a much stronger electron-withdrawing effect than a NO2 group, both in the ortho and para positions of a reactive centre in an aromatic ring. In accord with this conclusion, the 1,1-dimethoxy adduct 3e is found to be 10(2)-fold less stable than the tris-SO2CF3 analogue 3a but ten times more stable than the 1,1-dimethoxy adduct 3b of 4-nitro-2,6-bis(trifluoromethylsulfonyl)anisole (1b). However, a most significant result is the finding that the solvent contribution to the formation of 3e is not negligible in the pH range of 9-10. With a pK(a)(MeOH) value of 9.42, the electrophilic reactivity of the parent triflone 1e is clearly located in the region of extremely high electrophilicity defined for sigma complexation and SNAr substitutions in methanol (pK(a)(MeOH) 9-10). This work adds to the recognition that the activating effect of a SO2CF3 group is in general much greater than that of a NO2 group when bonded to an aromatic or heteroaromatic ring. A return to the origin of the exceptional electron-transmission capability of the SO2CF3 group is helpful to predict reactivity.