No abstract

The relative mobility of nitro and fluoro substituents in 1,3‐dinitro‐ and 1‐fluoro‐3‐nitrobenzenes, 3,5‐dinitro‐ and 3‐fluoro‐5‐nitrobenzotrifluorides under the action of the nucleophiles (2ArYH)·K2CO3 and ArY−K+ in solution and the nucleophiles ArYH·K2CO3 (Y = O, S) under heterogeneous conditions was studied by a competitive method in DMF at 40–140 °C. The unique dependences of ΔΔH≠ on ΔΔS≠ and ΔΔH≠ on ΔΔG≠ were determined for all the substrates and nucleophiles. The dependence of the mechanistic pathway on the nucleophile is discussed. Two results are relevant to the reactions studied: (i) substituent effects in the nucleophiles (2ArYH)·K2CO3 and ArYH·K2CO3 on the activation entropies suggest that the entropy favours the displacement of nitro group; (ii) the negative signs of ΔΔH≠ and ΔΔS≠ for the reactions of the nucleophiles ArY−K+ indicate that the enthalpy determines the displacement of nitro group. It is concluded that the selectivity of the reactions with aryloxide and arylthioxide ions cannot be explained by the hard–soft acid–base principle only. Copyright © 2007 John Wiley & Sons, Ltd.

Temperature dependences of the relative reactivity of potassium aryloxides XC6H4O−K+ toward 4‐nitrophenyl (1), 3‐nitrophenyl (2), 4‐chlorophenyl (3), and phenyl (4) benzoates in dimethylformamide (DMF) were studied using the competitive reactions technique. The rate constants kX for the reactions of 1 with potassium 4‐cyanophenoxide, 2 with potassium 3‐bromophenoxide, 3 with potassium 3‐bromo‐, 4‐bromo‐, and unsubstituted phenoxides, 4 with potassium 4‐methoxy‐ and 3‐methylphenoxides were measured at 25°C. Correlation analysis of the relative rate constants kX/kH(3‐Me) and differences in the activation parameters (∆∆Н≠ and ∆∆S≠) of competitive reactions revealed the existence of six isokinetic series. We investigated the substituent effect of X on the activation parameters for each isokinetic series and concluded that the reactions of aryl benzoates PhCO2C6H4Y with potassium aryloxides in DMF proceed via a four‐step mechanism. The large ρ0(Y) and ρXY values at 25°C obtained for the reactions of 1–3 with potassium aryloxides with an electron‐donating substituent refer to the rate‐determining formation of the spiro‐σ‐complex. The Hammett plots for the reactions of 1 and 2 exhibit a downward curvature, causing the motion of the transition state for the rate‐determining step according to a Hammond effect as the substituent in aryloxide changes from electron‐donating to electron‐withdrawing. Analysis of data in the terms of two‐dimensional reaction coordinate diagrams leads to the conclusion that significant anti‐Hammond effects arise in the cases of ortho‐substituted and unsubstituted substrates. It was shown that the isokinetic and compensation effects observed for the reactions of aryl benzoates with potassium aryloxides in DMF can be interpreted in the terms of the electrostatic bonding between the reaction centers.

No abstract

The relative mobilities of the nitro group and fluorine atom in 1,3-dinitrobenzene and 1-fluoro-3-nitrobenzene by the action of phenols in the presence of potassium carbonate in dimethylformamide at 95-125°C were studied by the competing reaction method. The rate constant ratios k(NO 2 )/k(F) were correlated with the differences between the corresponding activation parameters (ΔΔH ≠ and ΔΔS ≠ ). The greater mobility of the nitro group was found to be determined by the entropy control of the reactivity of arenes. The activation parameters (ΔH ≠ and ΔS ≠ ) were calculated, and the enthalpy-entropy compensation effect was revealed. The reaction mechanism is discussed.Nucleophilic aromatic substitution still remains an extensively developing field of organic chemistry [1, 2] due to diversity of mechanisms of these reactions [1][2][3][4] and their wide synthetic potential [5][6][7]. Nucleophilic substitution of hydrogen (S N H ) in arenes provides an effective tool for their functionalization [1,2,8,9], though replacement of a nucleofugal group in aromatic compounds is equally important. The latter process makes it possible to obtain not only ortho or para isomers but also meta-substituted derivatives with high regioselectivity [1]. meta-Substituted arenes are formed most readily by replacement of a halogen atom (Cl, Br, I) in copper-and palladium-catalyzed reactions [10] or of another readily departing group (such as nitro group or fluorine atom) by nucleophilic species generated in situ [11][12][13]. Systematic studies on such reactions were initiated relatively recently [11-13] due to wide prospects in further functionalization of the substrates [11,14].Studies on the kinetics of nucleophilic replacement in meta-substituted nitrobenzenes by the action of phenols in the presence of potassium carbonate have shown that the dependences of the rate constant on the temperature, reagent nature, and nucleofugality of the leaving group differ considerably [12,13,15,16] from those found for analogous reactions with potassium phenoxides [17][18][19][20]. With the goal of elucidating the general relations between the reactant structure and energy parameters of the reaction, which provide some information on the reaction mechanism, we studied the relative mobilities of the nitro group in 1,3-dinitrobenzene (I) and fluorine atom in 1-fluoro-3-nitrobenzene (II) in reactions with phenols III-VI in DMF in the presence of potassium carbonate (Scheme 1) at different temperatures. Scheme 1. X O 2 N I, II ArOH (III-VI), K 2 CO 3 DMF, 95-125°C OAr O 2 N VII-X I, X = NO 2 ; II, X = F; III, VII, Ar = 4-MeC 6 H 4 ; IV, VIII, Ar = Ph; V, IX, Ar = 4-ClC 6 H 4 ; VI, X, Ar = 3-O 2 NC 6 H 4 .We found that in the temperature range from 95 to 125°C the ratio of the rate constants for replacement of the nitro group and fluorine atom k(NO 2 )/k(F) is greater than unity and that it increases with rise in both temperature and acidity of phenols (Table 1). For each phenol III-VI, log[k(NO 2 )/k(F)] values are linearly related to the reciprocal tempera...