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

Abstract

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.

Regioselectivity and High Electrophilicity in the σ Complexation of Aromatic Triflones

“…[18,19,21,32,33] A manifestation of this difference in the mode of electronic transmission of the two groups is that the replacement of the three NO 2 groups of 1d (TNA) by three SO 2 CF 3 groups goes along with a strong hypsochromic shift of the UV/Vis absorption. Thus, λ max = 480 nm is observed for the 1,1‐dimethoxy adduct 3d of TNA,32,34 but λ max = 355 nm is found for the corresponding 1,1‐dimethoxy adduct 3a of the triflone 1a 23a. As shown in Table 1, a similar hypsochromic shift is observed between the 1,3‐dimethoxy adducts 2a and 2e , respectively 23.…”

“…These variations agree with a number of reports in σ complex chemistry that changes in adduct stability induced by a modulation of the substitution pattern of the aromatic or heteroaromatic ring are, in most cases, the result of opposite changes in the rate constants for formation and decomposition. The reader interested in this behaviour may find more detailed discussions in general reviews [4–6,23,32] and in a previous paper 23a…”

Single electron transfer in SNAr process: Reactions of 2,4,6-tris(trifluoromethanesulfonyl)anisole with anilines in dimethyl sulfoxide

“…Moreover, plots of k obsd versus [2a -f] o were linear (R 2 > 0.9984) passing through the origin, confirming the first-order of the reactions with 2a -f. This suggested that general base catalysis by the second aniline molecule is negligible and the pseudo-first-order rate constants (k obsd ), for all the reactions can be readily derived from Eqn (2).…”

“…www.prkm.co.uk 131 2,6-bis(trifluoromethanesulfonyl)-4-nitro-anisoles [1,2], we have concluded that these aromatic triflones represent a class of neutral electron-deficient substrates which exhibit an extremely high electrophilic character. 2,4,6-tris(trifluoromethanesulfonyl)anisole 1 has an electrophilicity (E = -7.56) value which is 7 orders of magnitude more electrophilic than 1,3,5-trinitrobenzene (E = -13.19) [3], the common reference aromatic electrophile in nucleophilic addition or substitution processes [1,4 -15].…”

Substituent effects on the reactions of aromatic triflones with para‐X‐anilines in methanol: Low intrinsic reactivity and transition state imbalances

“…5 can be used for estimating the intrinsic rate constants, k o , of Marcus theory [22,23], defined as k o = k 1 when pK a (X) = pK a (Z), where K a (Z) is the thermodynamic constant for the reversible formation of the methoxy σ -adduct 4 (see Table III and Eq. (3)) [4,24]. The values of the intrinsic rate constants for 1a and 1b, as determined from the Brönsted plots, are presented in Table III.…”