How the Nature of an Alpha-Nucleophile Determines a Brønsted Type-Plot and Its Reaction Pathways. An Experimental Study

The reactions between 2-chloro-5-nitro pyrimidine with a serie of α-nucleophile derivatives were kinetically evaluated. The kinetic study was carried out in aqueous media and the data shown an unusual split on the Brønsted type-plot, opening a controversial discussion based on reactivities and possible reaction pathways. These split Brønsted type-plots are discussed over the hypothetical transition state (TS) structures associated to concerted or stepwise mechanisms with emphasis on hydrogen bond interactions between electrophile/nucleophile pair able to determine the reactivities and the plausible reaction routes.

Cross-Coupling between Hydrazine and Aryl Halides with Hydroxide Base at Low Loadings of Palladium by Rate-Determining Deprotonation of Bound Hydrazine

Reported here is the Pd-catalyzed C-N coupling of hydrazine with (hetero)aryl chlorides and bromides to form aryl hydrazines with catalyst loadings as low as 100 ppm of Pd and KOH as base. Mechanistic studies revealed two catalyst resting states: an arylpalladium(II) hydroxide and arylpalladium(II) chloride. These compounds are present in two interconnected catalytic cycles and react with hydrazine and base or hydrazine alone to give the product. The selectivity of the hydroxide complex with hydrazine to form aryl over diaryl hydrazine was lower than that of the chloride complex, as well as the catalytic reaction. In contrast, the selectivity of the chloride complex closely matched that of the catalytic reaction, indicating that the aryl hydrazine is derived from this complex. Kinetic studies showed that the coupling process occurs by rate-limiting deprotonation of a hydrazine-bound arylpalladium(II) chloride complex to give an arylpalladium(II) hydrazido complex.

Hydrazinolysis of aryl cinnamates and related esters: the α-effect arises from stabilization of five-membered cyclic transition state

A kinetic study is reported for nucleophilic substitution reactions of Y-substituted-phenyl cinnamates (1a–1h) with a series of primary amines including hydrazine in H2O containing 20 mol % DMSO at 25.0 °C. The Brønsted-type plot for the reaction of 2,4-dinitrophenyl cinnamate (1a) is linear with βnuc = 0.57 except hydrazine, which exhibits positive deviation from the linear correlation (i.e., the α-effect). The Brønsted-type plots for the reactions of 1a–1h with hydrazine and glycylglycine (glygly) are also linear with βlg = –0.71 and –0.87, respectively, when 1a is excluded from the linear correlation. Thus, the reactions have been concluded to proceed through a concerted mechanism on the basis of the linear Brønsted-type plots and magnitudes of the βnuc and βlg values. The α-effect shown by hydrazine is dependent on electronic nature of the substituent Y in the leaving group, e.g., it increases as the substituent Y becomes a weaker electron-withdrawing group (or as basicity of the leaving aryloxide increases), indicating that the α-effect is not due to destabilization of the ground state but mainly due to stabilization of the transition state. A five-membered cyclic TS structure, which could increase nucleofugality of the leaving aryloxide through H-bonding interaction, has been proposed to account for the leaving-group dependent α-effect found in this study. The theories suggested previously to rationalize the α-effect found for the related systems are also discussed.

Medium effect (water versus MeCN) on reactivity and reaction pathways for the SNAr reaction of 1-aryloxy-2,4-dinitrobenzenes with cyclic secondary amines

A kinetic study on SNAr reactions of 1-aryloxy-2,4-dinitrobenzenes (1a–1h) with a series of cyclic secondary amines in 80 mol% water – 20 mol% DMSO at 25.0 ± 0.1 °C is reported. The plots of kobsd versus amine concentration curve upward except for the reactions of substrates possessing a strong electron-withdrawing group in the leaving aryloxide with strongly basic piperidine. The curved plots indicate that the reactions proceed through both uncatalytic and catalytic routes. Linear Brønsted-type plots have been obtained for the uncatalyzed and catalyzed reactions of 1-(4-nitrophenoxy)-2,4-dinitrobenzene (1a) with βnuc = 0.84 and 0.78, respectively. The Yukawa–Tsuno plot for the uncatalyzed reactions of 1a–1h with piperidine results in an excellent linear correlation with ρ = 1.66 and r = 0.31. In contrast, rate constants for catalyzed reactions are independent of the electronic nature of the substituent in the leaving group. The current SNAr reactions have been proposed to proceed via a zwitterionic intermediate (MC±) that partitions to products through uncatalytic and catalytic routes. The catalyzed reaction from MC± has been concluded to proceed through a concerted mechanism with a six-membered cyclic transition state (TScycl) rather than via a stepwise pathway with a discrete anionic intermediate (MC−), the traditionally accepted mechanism. Medium effects on the reactivity and reaction mechanism are discussed. Particularly, hydrogen bonding of the amines to water precludes formation of kinetically significant dimers found in some aprotic solvents; no explicit role for water in the catalytic transition state is required or proposed. The specific stabilization of the leaving aryloxides substituted with strong electron-withdrawing groups accounts for the lack of the catalytic pathway in these systems (1a–1c) with piperidine nucleophile.

Theoretical estimation of kinetic parameters for nucleophilic substitution reactions in solution: an application of a solution translational entropy model

The kinetic parameters, such as activation entropy, activation enthalpy, activation free-energy, and reaction rate constant, for a series of nucleophilic substitution (SN) reactions in solution, are investigated using both a solution-phase translational entropy model and an ideal gas-phase translational entropy model. The results obtained from the solution translational entropy model are in excellent agreement with the experimental values, while the overestimation of activation free-energy from the ideal gas-phase translational entropy model is as large as 6.9 kcal mol(-1). For some of the reactions studied, such as and in methanol, and and in aqueous solution, the explicit + implicit model, namely, a cluster-continuum type model, should be employed to account for the strong solvent-solute interactions. In addition, the explicit + implicit models have also been applied to the DMSO-H2O mixtures, which would open up a door to investigate the reactions in a mixed solvent using density functional theory (DFT) methods.