作者:Nizar El Guesmi、Taoufik Boubaker、Régis Goumont、François Terrier
DOI:10.1039/b810273b
日期:——
The kinetics of Ï-complexation of 2,6-bis(trifluoromethanesulfonyl)-4-nitroanisole (4) have been investigated over a large pH range of 2â15.68 in methanol. Two competitive processes have been identified with the initial addition of MeOâ 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 (pKa = 10.48) and 4b-Me (pKa = 12.23) relative to 5a-Me (pKa = 10.68) and 6a-Me (pKa = 12.56) or 5b-Me (pKa = 15.38) and 6b-Me (pKa = 16.46), is shown to derive from an especially high capacity of a para or an ortho SO2CF3group to stabilize a negative charge through FÏ-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 pKMeOHa value of â¼ 10, in agreement with our previous finding of a very effective solvent contribution to the Ï-complexation of 1,3,5-tris(trifluoromethanesulfonyl)benzene (13; pKMeOHa = 9.12) in methanol. Taking advantage of our observation that pKMeOHa and pKH2Oa values for Ï-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.
我们研究了 2,6-双(三氟甲烷磺酰)-4-硝基苯甲醚(4)在甲醇中 2â15.68 的较大 pH 值范围内的Ï-络合动力学。研究发现了两个竞争过程:最初在 4 的未取代 3 位加入 MeOâ,生成 1,3-二甲氧基加合物(4b-Me),随后该物种缓慢转化为 1,1-二甲氧基异构体(4a-Me)。4a-Me 和 4b-Me 比 2,6-二硝基-4-三氟甲磺酰基苯甲醚的相关加合物(即 5a-Me 和 5b-Me)和 2,4,6-三硝基苯甲醚的相关加合物(即 6a-Me 和 6b-Me)更稳定,后者是迈森海默络合物化学中传统的参考芳香亲电子体。与 5a-Me(pKa = 10.68)和 6a-Me(pKa = 12.56)或 5b-Me(pKa = 15.38)和 6b-Me(pKa = 16.46)相比,4a-Me(pKa = 10.48)和 4b-Me(pKa = 12.23)具有较高的热力学稳定性,这是因为对位或正位 SO2CF3 基团通过 FÏ 型极化效应稳定负电荷的能力特别强。从动力学数据来看,甲醇途径对 4a-Me 形成的贡献要比 2,6-二硝基-4-三氟甲基磺酰苯甲醚的 1,1 复合物 5a-Me 形成过程中的贡献弱得多,实验证据表明 4 和 5 的反应活性恰好位于甲醇中超亲电性和正常亲电性之间的边界区域之外。将我们的结果与现有的文献数据进行比较后发现,这一界限对应的 pKMeOHa 值为 § 10,这与我们之前发现的 1,3,5-三(三氟甲烷磺酰基)苯 (13; pKMeOHa = 9.12) 在甲醇中的Ï-络合反应中非常有效的溶剂贡献是一致的。根据我们的观察,未取代环位置上的Ï-络合的 pKMeOHa 值和 pKH2Oa 值呈良好的线性相关关系,可以根据 Mayr(Acc. Chem. Res.