The Ambident Reactivity of 2,4,6‐Tris(trifluoromethanesulfonyl)anisole in Methanol: Using the SO2CF3 Group as a Tool to Reach the Superelectrophilic Dimension in σ‐Complexation Processes

The kinetics of σ complexation of 2,4,6‐tris(trifluoromethanesulfonyl)anisole ( 7 d ) have been investigated over a large pH range of 2–13.70 at T=20 °C in methanol. Two competitive processes associated with the initial addition of MeO^? at the unsubstituted 3‐position of 7 d to give a 1,3‐dimethoxy adduct ( 9 d ‐Me) and a subsequent and slow conversion of this species into a 1,1‐dimethoxy isomer ( 8 d ‐Me) have been identified. Both adducts 8 d ‐Me and 9 d ‐Me are 10⁵–10⁶ times more stable than the related adducts 8 a ‐Me and 9 a ‐Me of 2,4,6‐trinitroanisole ( 7 a ), a conventional reference aromatic electrophile in Meisenheimer complex chemistry. The high stability of 8 d ‐Me and 9 d ‐Me is shown to derive from greater rates of formation and lower rates of decomposition than previously determined for 8 a ‐Me and 9 a ‐Me, thereby emphasising the especially high activation of a benzene ring by SO₂CF₃ group(s). Analysis of the collected rate and equilibrium data for σ complexation in the anisole series 2,4,6‐tris(SO₂CF₃)‐, 2,6‐bis(SO₂CF₃)‐4‐nitro‐, 4‐SO₂CF₃‐2,6‐dinitro‐ and 2,4,6‐trinitro‐ supports the idea that the especially high capacity of resonance stabilisation of the negative charge of the adducts through an F_π‐type (as defined in ref. 49 ) polarisation effect is a major factor that accounts for the strong activation provided by SO₂CF₃ groups. A most significant result is the finding that the 1,1‐dimethoxy adduct 8 d ‐Me is by far the most stable benzene σ adduct so far reported. With a p value of 7.32, this adduct is formed exclusively through methanol addition up to pH≈10. This is consistent with the location of 7 d in the superelectrophilic region defined by p ≤9.5–10.5. For comparison, the solvent contribution is negligible in the formation of the 1,3‐isomer 9 d ‐Me, the p (10.59) of which is situated on the upper limit of the boundary. Taking advantage of the simple relationship linking pK_a values for σ complexation in methanol and water, a ranking of the triflone 7 d on the general thermodynamic scale constructed for Meisenheimer electrophiles in water is informative. An approximate calibration on the electrophilicity scale kinetically derived by Mayr et al. has also been made.