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

Abstract

The kinetics of the reactions of 4‐nitrobenzofurazane 1a, 4‐nitrobenzothiadiazole 1b, and 4‐nitrobenzoselenadiazole 1c with a series of 4‐Y‐substituted phenoxide anions 2a‐e (Y = OMe, Me, H, Cl, and CN) in aqueous solution at 20°C were investigated photometrically. The derived second‐order rate constants (k2) have been combined with the nucleophilicity parameters values of these series of anions 2a‐e to determine the electrophilicity parameters E of electrophiles 1a‐c according to the linear free‐energy relationship (log k2)/s versus N. General reactivity of these electrophiles 1a‐c is found to be fairly similar with E values ranging in −10.77 ± 0.61 < E < −7.53 ± 0.29. The comparison with structurally related neutral electron‐deficient heteroaromatic and aromatic compounds revealed that 1a‐c are more reactive than 1,3,5‐trinitrobenzene, as benchmark aromatic electrophile used in nucleophilic addition or substitution processes. The rate constants for the reactions of 4‐nitrobenzochalcogenadiazoles 1a‐c with some other nucleophiles were measured and found to agree with those calculated from Mayr's equation. Finally, analysis of the rate data in terms of the Brønsted approach reveals that 1a‐c exhibits especially low intrinsic reactivity in σ‐adducts 3 forming reactions.

Electrophilic reactivities of 7‐L‐4‐nitrobenzofurazans in σ‐complexation processes: Kinetic studies and structure–reactivity relationships

“…Figure 5 also reveals that the nitrobenzofurazans 1 and 2 exhibits of C‐5 position approximately 7 units of E higher than C−7 position, while the difference in reactivity between C‐5 and C‐7 positions of 4‐nitrobenzofurazan 6 , 20,21 as well as of 4‐nitrobenzoselenadiazole 7 21 and 4‐nitrobenzothiadiazole 8 21 is approximately 2 unit of E . The reason for this apparent disparity is not clear at this stage.…”

Quantification of the Electrophilic Reactivities of Benzotriazoles and Structure‐Reactivity Relationships

“…Details of the materials, synthetic procedures, kinetic measurements and pK a measurements. Table S1 containing the characteristics 1…”

“…We have previously reported that the 4-nitrobenzochalcogenadiazoles 4, [1][2][3] the 7-L-4-nitrobenzofurazans 5, [4][5][6] and the 2methoxy-3-X-5-nitrothiophene 6 [7][8][9] (Scheme 1) represent a class of neutral electron-deficient substrates which can be described by the experimental model based on the following three-parameter equation (1):…”

Nucleophilicity Parameters for Nitroalkyl Anions in Methanol and Structure‐Reactivity Analysis

“…Chalcogen–nitrogen heterocycles have been the subject of intensive experimental and theoretical studies due to their importance in biological processes as well as synthetic applications [1–5] . In this regard, we and others have been interested in the reactivity of a large variety of nucleophiles towards 4‐nitrobenzofurazan 1 a , [6–12] 4‐nitrobenzothiadiazole 1 b , [13,14] and 4‐nitrobenzoselenadiazole 1 c [14,15] . Recently, we have determined the second‐order rate constants (k) for the σ‐complexation reactions of 4‐nitrobenzochalcogenadiazoles 1 a – c , [7–9] and these values were used to estimate an electrophilicity parameter E for these electrophiles according to equation (1), [16–21] in which N and s N characterize the nucleophiles reactivity. $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr {\rm log}\ {\rm k}\ (20\mskip2mu {^\circ}{\rm C})=s{_{{\rm N}}}\ (E\ +\ N)\hfill\cr}}$$$ …”