Aminolysis of Y-Substituted Phenyl Diphenylphosphinates and Benzoates: Effect of Modification of Electrophilic Center from CO to PO

Solvent and solvation effects on reactivities and mechanisms of phospho group transfers from phosphate and phosphinate esters to nucleophiles

Organophosphorus esters fulfil many industrial, agricultural, and household roles. Nature has deployed phosphates and their related anhydrides as energy carriers and reservoirs, as constituents of genetic materials in the form of DNA and RNA, and as intermediates in key biochemical conversions. The transfer of the phosphoryl (PO₃) group is thus a ubiquitous biological process that is involved in a variety of transformations at the cellular level such as bioenergy and signals transductions. Significant attention has been paid in the last seven decades to understanding the mechanisms of uncatalyzed (solution) chemistry of the phospho group transfer because of the notion that enzymes convert the dissociative transition state structures in the uncatalyzed reactions into associative ones in the biological processes. In this regard, it has also been proposed that the rate enhancements enacted by enzymes result from the desolvation of the ground state in the hydrophobic active site environments, although theoretical calculations seem to disagree with this position. As a result, some attention has been paid to the study of the effects of solvent change, from water to less polar solvents, in uncatalyzed phospho transfer reactions. Such changes have consequences on the stabilities of the ground and the transition states of reactions which affect reactivities and, sometimes, the mechanisms of reactions. This review seeks to collate and evaluate what is known about solvent effects in this domain, especially their effects on rates of reactions of different classes of organophosphorus esters. The outcome of this exercise shows that a systematized study of solvent effects needs to be undertaken to fully understand the physical organic chemistry of the transfer of phosphates and related molecules from aqueous to substantially hydrophobic environments, since significant knowledge gaps exist.

Kinetic Study on Pyridinolysis of Aryl Benzenesulfonates: Factors Governing Regioselectivity and Reaction Mechanism

A kinetic study is reported for reactions of 2,4-dinitrophenyl X-substituted-benzenesulfonates (1a-1f) and Y-substituted-phenyl benzenesulfonates (2a-2f) with Z-substituted-pyridines. The reactions proceed through S‒O and C‒O bond scissions competitively. The Yukawa-Tsuno plot for the reactions of 1a-1f with 4-oxypyridine (S‒O bond fission) exhibits an excellent linearity. The Brønsted-type plot for the reactions of 2,4-dinitrophenyl benzenesulfonate (1d) with pyridines (S‒O bond fission) is also linear with βnuc = 0.62. The Brønsted-type plot for the reactions of 2a-2f with 4-oxypyridines (S‒O bond fission) is linear with βlg= ‒1.17. Thus, the reactions have been concluded to proceed through a concerted mechanism, in which leaving-group expulsion is significantly more advanced than bond formation between the electrophilic center and nucleophile at the transition state. The Hammett plot for the reactions of 1a-1f with 4-oxypyridine (C‒O bond fission) exhibits scattered points with ρX= 0.98. The Brønsted-type plot for the reactions of 1d with Z-substituted-pyridines (C‒O bond fission) results in an excellent linear correlation with βnuc = 0.38. Thus, the reactions (C‒O bond fission) have been concluded to proceed through a stepwise mechanism with a Meisenheimer complex, in which expulsion of the leaving group occurs after the rate-determining step. Factors governing regioselectivity and reaction mechanism are discussed.

Kinetic study on aminolysis of aryl X-substituted-cinnamates in acetonitrile: differential medium effect determines reactivity and reaction mechanism

A kinetic study on nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted-cinnamates (1a–1f) and Y-substituted-phenyl cinnamates (2a–2g) with a series of alicyclic secondary amines in MeCN at 25.0 ± 0.1 °C is reported. The Brønsted-type plots for the reactions of 1a–1f are linear with βnuc = 0.47∼0.50, indicating that the bond formation between the amine nucleophile and the electrophilic center is advanced slightly in the transition state. The Brønsted-type plot for the reactions of 2a–2g with piperidine is also linear with βlg = –0.66, which is a typical βlg value for reactions reported previously to proceed through a concerted mechanism. Furthermore, the Hammett plot correlated with σ– constants results in much better linearity than that correlated with σo constants, implying that expulsion of the leaving group is advanced in the rate-determining step (RDS). Thus, the reactions are concluded to proceed through a concerted mechanism. The Hammett plots for the reactions of 1a–1f consist of two intersecting straight lines, whereas the corresponding Yukawa–Tsuno plots exhibit excellent linear correlations with ρX = 0.62∼0.71 and r = 0.65∼0.68. Apparently, the nonlinear Hammett plots are not due to a change in the reaction mechanism (or the RDS) but are caused by stabilization of the substrate possessing an electron-donating group (EDG) in the cinnamoyl moiety through resonance interactions between the EDG and the C=O bond of the substrate. Medium effects on reactivity and reaction mechanism are also discussed.

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.

A kinetic study on nucleophilic displacement reactions of aryl benzenesulfonates with potassium ethoxide: role of K+ ion and reaction mechanism deduced from analyses of LFERs and activation parameters

Pseudofirst-order rate constants (k(obsd)) have been measured spectrophotometrically for the nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted benzenesulfonates 4a-f and Y-substituted phenyl benzenesulfonates 5a-k with EtOK in anhydrous ethanol. Dissection of k(obsd) into k(EtO(-)) and k(EtOK) (i.e., the second-order rate constants for the reactions with the dissociated EtO(-) and ion-paired EtOK, respectively) shows that the ion-paired EtOK is more reactive than the dissociated EtO(-), indicating that K(+) ion catalyzes the reaction. The catalytic effect exerted by K(+) ion (e.g., the k(EtOK)/k(EtO(-)) ratio) decreases linearly as the substituent X in the benzenesulfonyl moiety changes from an electron-donating group (EDG) to an electron-withdrawing group (EWG), but it is independent of the electronic nature of the substituent Y in the leaving group. The reactions have been concluded to proceed through a concerted mechanism from analyses of the kinetic data through linear free energy relationships (e.g., the Brønsted-type, Hammett, and Yukawa-Tsuno plots). K(+) ion catalyzes the reactions by increasing the electrophilicity of the reaction center through a cyclic transition state (TS) rather than by increasing the nucleofugality of the leaving group. Activation parameters (e.g., ΔH(‡) and ΔS(‡)) determined from the reactions performed at five different temperatures further support the proposed mechanism and TS structures.

Hydrolysis of 1-(X-substituted-benzoyl)-4-aminopyridinium ions: effect of substituent X on reactivity and reaction mechanism

A kinetic study is reported for hydrolysis of 1-(X-substituted-benzoyl)-4-aminopyridinium ions 2a-i, which were generated in situ from the nucleophilic substitution reaction of 2,4-dinitrophenyl X-substituted-benzoates 1a-i with 4-aminopyridine in 80 mol% H(2)O/20 mol% DMSO at 25.0 ± 0.1 °C. The plots of pseudo-first-order rate constants k(obsd) vs. pyridine concentration are linear with a large positive intercept, indicating that the hydrolysis of 2a-i proceeds through pyridine-catalyzed and uncatalyzed pathways with the rate constant k(cat) and k(o), respectively. The Hammett plots for k(cat) and k(o) consist of two intersecting straight lines, which might be taken as evidence for a change in the rate-determining step (RDS). However, it has been proposed that the nonlinear Hammett plots are not due to a change in the RDS but are caused by stabilization of 2a-i in the ground state through a resonance interaction between the π-electron-donor substituent X and the carbonyl functionality. This is because the corresponding Yukawa-Tsuno plots exhibit excellent linear correlations with ρ(X) = 1.45 and r = 0.76 for k(cat) while ρ(X) = 1.39 and r = 0.72 for k(o). A possibility that the hydrolysis of 2a-i proceeds through a concerted mechanism has been ruled out on the basis of the large ρ(X) values. Thus, the reaction has been concluded to proceed through a stepwise mechanism in which the leaving group departs after the RDS since OH(-) is more basic and a poorer nucleofuge than 4-aminopyridine.