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 sigma-complexation processes.
The kinetics of sigma 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 degrees 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 10(5)-10(6) 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 SO(2)CF(3) group(s). Analysis of the collected rate and equilibrium data for sigma complexation in the anisole series 2,4,6-tris(SO(2)CF(3))-, 2,6-bis(SO(2)CF(3))-4-nitro-, 4-SO(2)CF(3)-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 SO(2)CF(3) groups. A most significant result is the finding that the 1,1-dimethoxy adduct 8 d-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 approximately 10. This is consistent with the location of 7 d in the superelectrophilic region defined by pK(a)(MeOH) < or = 9.5-10.5. For comparison, the solvent contribution is negligible in the formation of the 1,3-isomer 9 d-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 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.
El Guesmi
Nizar
N
Unité de Recherche de Chimie Théorique et Réactivité, Faculté des Sciences de Monastir, Avenue de l'Environnement, 5019 Monastir, Tunisia.
Boubaker
Taoufik
T
Goumont
Regis
R
Terrier
François
F