The ambident electrophilic behavior of 5‐nitro‐3‐X‐thiophenes in σ‐complexation processes

Abstract

Kinetics of the reactions of 3,5‐dinitrothiophene 1 and 3‐cyano‐5‐nitrothiophene 2 with a series of parasubstituted phenoxide anions 3a–c have been investigated in aqueous solution at 20°C. Two unsubstituted electrophilic centers (C(2) and C(4)) of the two thiophenes have been identified. The Fukui functions correctly predict the C(2) and C(4) atoms as the most electrophilic centers of these electron‐deficient thiophenes 1 and 2. Analysis of the experimental data in terms of Brønsted relationships reveals that the reaction mechanism likely involves a single‐electron transfer (SET) process. The excellent correlations upon plotting the rate constants versus the oxidation potentials Eo values is an additional evidence that reactions between thiophenes and phenoxide anions are proceeding through an initial electron transfer. It is of particular interest to note that the systems studied in this paper provide a rare example of a SET mechanism in σ‐complexation reactions. According to the free energy relationship log k = s(N + E) (Angew. Chem., Int. Ed. Engl., 1994, 33, 938–957), the electrophilicity parameters E of the C‐4 and C‐2 positions of the thiophenes have been determined and compared with the reactivities of other ambident electrophiles. On the other hand, the second‐order rate constants for the reactions of these thiophenes with the hydroxide ion has been measured in water and 50% water–50% acetonitrile and found to agree with those calculated theoretically using Mayr's equation from the E values determined in this work and from the previously published N and s parameters of OH−.

Ambident electrophilicity of 4‐nitrobenzochalcogenadiazoles: Kinetic studies and structure‐reactivity relationships

“…The ρ values are comparable to that reported for reactions, which have been suggested to proceed through a polar σ‐complexation process, for example, ρ = −2.29 for reaction of 3‐cyano‐5‐nitrothiophene with parasubstituted phenoxide anions in aqueous solution at 20°C (Ref. 1) and ρ = −1.84 to −1.75 for reactions of 2,6‐dimethoxy‐3,5‐dinitropyridine with the same series of para‐substituted phenoxide anions in aqueous solution at various temperatures 14 …”

“…Another interesting result is that electrophiles 1a , 1b , and 1c exhibits at C−5 position 1.89 ± 0.29, 2.03 ± 0.61, and 2.04 ± 0.30 E units higher than C−7 position, respectively. Similarly, comparison of the reactivity of 2,4‐dinitrothiophene 4a and 4‐cyano‐2‐nitrothiophene 4b shows that the C−3 position is approximately 2 orders of magnitude more electrophilic than the C−5 position 1 …”

“…Figure 6 summarizes the electrophilicity parameters E and allows to compare these values with other neutral electron‐deficient nitroaromatics and heteroaromatics, namely thiophenes 4 , 1 4,6‐dinitobenzofuroxan 11 , 15 1,3,5‐trinitrobenzene 12 , 15 and various cations such as 13 , 16 14 , 17 and 15 18 …”

“…In a previous work, we have reported on the first example of ambident electrophilic reactivity in Meisenheimer σ‐complex formation 1 . Thus, 2,4‐dinitrothiophene 4a and 4‐cyano‐2‐nitrothiophene 4b were found to react with para‐Y‐substituted phenoxide anions (Y = OMe, Me, H, Cl) to yield the σ−adducts 5 which are kinetically favored while the isomers 6 have a higher thermodynamic stability.…”

Predicting experimental electrophilicities from quantum and topological descriptors: A machine learning approach

“…For instance, in 2,5-dichloroquinone, there is one electrophilicity parameter associated to the C Cl carbon and one for the C H atom (−12.28). [15] Similar investigations were led by the Boubaker's group on 5-nitro-3-X-thiophenes, [16] demonstrating that kinetic measurements are instrumental in determining which site is the most reactive.…”