Electrophilicity of aromatic triflones in σ-complexation processes

Abstract

The kinetics of σ-complexation of 2,6-bis(trifluoromethanesulfonyl)-4-nitroanisole (4) have been investigated over a large pH range of 2–15.68 in methanol. Two competitive processes have been identified with the initial addition of MeO⁻ at the unsubstituted 3-position of 4 to give a 1,3-dimethoxy adduct (4b-Me) and a subsequent and slow conversion of this species into the 1,1-dimethoxy isomer (4a-Me). Both 4a-Me and 4b-Me are more stable than the related adducts of 2,6-dinitro-4-trifluromethanesulfonylanisole, i.e.5a-Me and 5b-Me, and 2,4,6-trinitroanisole, i.e.6a-Me and 6b-Me, the latter compound being a conventional reference aromatic electrophile in Meisenheimer complex chemistry. The high thermodynamic stability of 4a-Me (pK_a = 10.48) and 4b-Me (pK_a = 12.23) relative to 5a-Me (pK_a = 10.68) and 6a-Me (pK_a = 12.56) or 5b-Me (pK_a = 15.38) and 6b-Me (pK_a = 16.46), is shown to derive from an especially high capacity of a para or an ortho SO₂CF₃group to stabilize a negative charge through Fπ-type polarization effects. From the kinetic data, it appears that the contribution of a methanol pathway to the formation of 4a-Me is much weaker than that found to operate in the formation of the 1,1-complex 5a-Me of 2,6-dinitro-4-trifluromethanesulfonylanisole, the experimental evidence suggesting that the reactivity of 4 and 5 is located just beyond the region defining the boundary between super- and normal-electrophilicity in methanol. Comparison of our results with available literature data show that this boundary corresponds to a pK^MeOH_a value of ∼ 10, in agreement with our previous finding of a very effective solvent contribution to the σ-complexation of 1,3,5-tris(trifluoromethanesulfonyl)benzene (13; pK^MeOH_a = 9.12) in methanol. Taking advantage of our observation that pK^MeOH_a and pK^H₂O_a values for σ-complexation at unsubstituted ring positions are related by a nice linear correlation, an approximate ranking of the electrophilicity of our aromatic triflones on the E scale developed by Mayr (Acc. Chem. Res. 2003, 36, 66) can be made.