Abstract
The kinetics of sigma complexation of 2,4,6-tris(trifluoromethanesulfonyl)anisole (7d) have been investigated over a large pH range of 2-13.70 at T=20 degrees C in methanol. Two competitive processes associated with the initial addition of MeO- at the unsubstituted 3-position of 7d to give a 1,3-dimethoxy adduct (9d-Me) and a subsequent and slow conversion of this species into a 1,1-dimethoxy isomer (8d-Me) have been identified. Both adducts 8d-Me and 9d-Me are 10(5)-10(6) times more stable than the related adducts 8a-Me and 9a-Me of 2,4,6-trinitroanisole (7a), a conventional reference aromatic electrophile in Meisenheimer complex chemistry. The high stability of 8d-Me and 9d-Me is shown to derive from greater rates of formation and lower rates of decomposition than previously determined for 8a-Me and 9a-Me, thereby emphasising the especially high activation of a benzene ring by SO2CF3 group(s). Analysis of the collected rate and equilibrium data for sigma 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-pi-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 8d-Me is by far the most stable benzene sigma adduct so far reported. With a pK(a)(MeOH) value of 7.32, this adduct is formed exclusively through methanol addition up to pH approximate to 10. This is consistent with the location of 7d in the superelectrophilic region defined by pK(a)(MeOH) <= 9.5-10.5. For comparison, the solvent contribution is negligible in the formation of the 1,3-isomer 9d-Me, the pK(a)(MeOH) (10.59) of which is situated on the upper limit of the boundary. Taking advantage of the simple relationship linking pK(a) values for sigma complexation in methanol and water, a ranking of the triflone 7d 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.