Understanding the reaction mechanism of the regioselective piperidinolysis of aryl 1-(2,4-dinitronaphthyl) ethers in DMSO: Kinetic and DFT studies

Reactions of aryl 1-(2,4-dinitronaphthyl) ethers with piperidine in dimethyl sulfoxide at 25oC resulted in substitution of the aryloxy group at the ipso carbon atom. The reaction was measured spectrophotochemically and the kinetic studies suggested that the titled reaction is accurately third order. The mechanism is began by fast nucleophilic attack of piperidine on C1 to form zwitterion intermediate (I) followed by deprotonation of zwitterion intermediate (I) to the Meisenheimer ion (II) in a slow step, that is, SB catalysis. The regular variation of activation parameters suggested that the reaction proceeded through a common mechanism. The Hammett equation using reaction constant σo values and Brønsted coefficient value showed that the reaction is poorly dependent on aryloxy substituent and the reaction was significantly associative and Meisenheimer intermediate-like. The mechanism of piperidinolysis has been theoretically investigated using density functional theory method using B3LYP/6-311G(d,p) computational level. The combination between experimental and computational studies predicts what mechanism is followed either through uncatalyzed or catalyzed reaction pathways, that is, SB and SB-GA. The global parameters of the reactants, the proposed activated complexes, and the local Fukui function analysis explained that C1 carbon atom is the most electrophilic center of ether. Also, kinetics and theoretical calculation of activation energies indicated that the mechanism of the piperidinolysis passed through a two-step mechanism and the proton transfer process was the rate determining step.

Kinetics of the Nucleophilic Substitution Reactions of Methyl 2,4-Dichloro-3,5-Dinitrobenzoate with Piperidine, Piperazine, Morpholine and Thiomorpholine in Methanol and Benzene

The kinetic of the nucleophilic substitution of methyl 2,4-dichloro-3,5-dinitrobenzoate with piperidine, piperazine, morpholine and thiomorpholine in methanol and benzene were determined spectrophotometrically at different amine concentrations and at temperatures ranging from 25 to 45 °C. The second order rate constants and the thermodynamic parameters show that the reactions are not amine catalysed and are greatly dependent of the nature of solvent and amine. UV, IR, 1H NMR, and elemental analysis are used to prove the aminodechlorination at C–2.

Reaction of 4-Chloro-3,5-Dinitrobenzotrifluoride with Aniline Derivatives. Substituent Effects

N-(2,6-Dinitro-4-trifluoromethylphenyl)aniline derivatives were prepared by anilino-dechlorination of 4-chloro-3, 5-dinitrobenzotrifluoride. IR, UV and 1H NMR studies suggested an intramolecular hydrogen bond between the amino hydrogen and one o-nitro group. An addition-elimination mechanism was suggested based on the second-order kinetics and the dependence of rates on the nature and the position of the substituent in the aniline ring, as well as the high negative values of ρ(-3.14, −3.16, −3.01). Such values indicate a positive charge on the aniline nitrogen in the transition state and that the rate is affected by the polar effect of the substituent. The β value (0.85 at 30°C) indicates an appreciable degree of bond formation in the transition state.

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

Second-order rate constants ( k1) have been measured spectrophotometrically for reactions of 2,4,6-tris(trifluoromethanesulfonyl)anisole 1 with 4-X-substituted anilines (X = OH, OCH3, CH3, H, I and Cl) 2a–f in dimethyl sulfoxide (DMSO) at 25 °C. The Hammett plot is nonlinear, i.e. the anilines having an electron-donating substituent exhibit positive deviations from the log k1 versus correlation. The Hammett plot consists of two intersecting straight lines, i.e. ρ=– 1.40 for electron-withdrawing substituents (X = I and Cl) while ρ=– 3.26 for electron-donating substituents (X = OH, OCH3 and CH3). The Brønsted-type plot is also nonlinear with a change in slope from a small value (βnuc=0.50) to a large value (βnuc = 1.29 at p Ka=4.58). This change could be interpreted in terms of a change in mechanism from a polar SNAr process to a single electron transfer. On the other hand, the experimental data show that the rate constants ( k1) values exhibit good correlation with the oxidation potentials ( E°) of the aniline nucleophiles and are consistent with the suggested mechanism.

The Influence of Remote Substituents on the Amine-Catalysed Solvolysis Reaction of Some Para-Substituted Oxazolinones in Water–Dmso Mixtures

The kinetics of the morpholine-catalysed solvolysis of some p-substituted (Z)-4-benzylidene-2-phenyloxazolin-5-one in water- dimethylsulfoxide mixtures (20-90% V/V) at different temperatures in the range (40-60°C) have been studied. There is an electronic effect in the predicted direction for para-substitution in the benzylidene moiety in different percentage compositions of binary aqueous solvent mixtures. An electron-withdrawing substituent in the benzylidene ring leads to rate enhancement while an electron-donating substituent retarded the rate. A good linear relationship was obtained from plots of log kobs against the σ-Hammett parameter with a positive slope (ρ) at the various experimental temperatures and solvent compositions. Plots of the reaction rate constant ρ against the dielectric constant and the reciprocal temperature gave straight lines. The thermodynamic parameters Δ H≠, Δ S≠ and Δ G≠ have been determined; Δ G≠ increases gradually as the mole fraction of the cosolvent increase, due to a complex quasi-mirror image compensation of Δ H≠ and Δ S≠. A plot of Δ H≠ versus Δ S≠ for the reaction in water–solvent mixtures gave good straight lines with isokinetic temperatures which are higher than the experimental temperature. The higher negative values of the entropy suggest selective solvation by the more polar water molecules.