Mechanism and transition state structure of aryl methylphosphonate esters doubly coordinated to a dinuclear cobalt(III) center

Mechanistic study of carboxylic acid and phosphate ester cleavage by oximate metal complexes surpassing the limiting reactivity of highly basic free oximate anions

Two tridentate and one tetradentate new ligands containing the terminal oxime group separated from secondary amino and pyridine groups as additional binding sites by two or three methylene groups have been prepared. Their acid-base properties, as well as the composition and stability of their complexes with Zn(ii) and Cd(ii) ions, were determined by potentiometric and spectrophotometric titrations. The X-ray structure of a Cd(ii) complex of a related tridentate oxime ligand previously studied in solution was determined. All oximate complexes show high reactivity in the cleavage of aryl acetates, paraoxon, parathion and 4-nitrophenyl diphenyl phosphate, with rate constants significantly surpassing the limiting rate constants observed for highly basic free oximate anions. The second-order rate constants for individual oximate complexes in solution are assigned to each ligand, metal cation and substrate. The results of the cleavage of 4-substituted phenyl acetates were analyzed in terms of Brønsted correlations with the leaving group pKa, which demonstrated a change in the rate determining step from the nucleophilic attack to the leaving group departure upon an increase in the leaving group basicity. The zero slope of the Brønsted correlation for the nucleophilic attack indicates transition state stabilization through electrophilic assistance by the metal ion. This interpretation is supported by metal selectivity in the relative efficiency of the cleavage of paraoxon and parathion. The existence of the alpha-effect in ester cleavage by coordinated oximates is confirmed by an analysis of the Brønsted correlations with the nucleophile basicity for metal bound oximate and alkoxo or hydroxo nucleophiles. The very high reactivity of the oximate complexes of the new ligands is attributed to transition state stabilization and to the removal of the solvational imbalance of oximate anions that impedes the expected increase in the reactivity of highly basic free anions.

Alkali-metal ion catalysis and inhibition in SNAr displacement: relative stabilization of ground state and transition state determines catalysis and inhibition in SNAr reactivity

We report here the first observation of alkali-metal ion catalysis and inhibition in SNAr reactions. The plot of kobsd versus [alkali-metal ethoxide] exhibits downward curvature for the reactions of 1-(4-nitrophenoxy)-2,4-dinitrobenzene with EtOLi, EtONa, and EtOK, but upward curvature for the corresponding reaction with EtOK in the presence of 18-crown-6-ether (18C6). Dissection of kobsd into the second-order rate constants for the reactions with the dissociated EtO(-) and the ion-paired EtOM (i.e., k EtO - and kEtOM , respectively) has revealed that the reactivity increases in the order EtOLi<EtONa<EtOK<EtO(-)<EtOK/18C6. This indicates that the reaction is inhibited by Li(+), Na(+), and K(+) ions but is catalyzed by 18C6 K(+) ion. The reactions of 1-(Y-substituted-phenoxy)-2,4-dinitrobenzenes have been proposed to proceed through a stepwise mechanism, in which expulsion of the leaving group occurs after the rate-determining step based on the kinetic result that σ(o) constants exhibit a much better Hammett correlation than σ(-) constants. Alkali-metal ion catalysis or inhibition has been discussed in terms of differential stabilization of ground-state and transition-state complexes through a qualitative energy profile. A π-complexed transition-state structure is proposed to account for the kinetic results.

Why nature really chose phosphate

Phosphoryl transfer plays key roles in signaling, energy transduction, protein synthesis, and maintaining the integrity of the genetic material. On the surface, it would appear to be a simple nucleophile displacement reaction. However, this simplicity is deceptive, as, even in aqueous solution, the low-lying d-orbitals on the phosphorus atom allow for eight distinct mechanistic possibilities, before even introducing the complexities of the enzyme catalyzed reactions. To further complicate matters, while powerful, traditional experimental techniques such as the use of linear free-energy relationships (LFER) or measuring isotope effects cannot make unique distinctions between different potential mechanisms. A quarter of a century has passed since Westheimer wrote his seminal review, 'Why Nature Chose Phosphate' (Science 235 (1987), 1173), and a lot has changed in the field since then. The present review revisits this biologically crucial issue, exploring both relevant enzymatic systems as well as the corresponding chemistry in aqueous solution, and demonstrating that the only way key questions in this field are likely to be resolved is through careful theoretical studies (which of course should be able to reproduce all relevant experimental data). Finally, we demonstrate that the reason that nature really chose phosphate is due to interplay between two counteracting effects: on the one hand, phosphates are negatively charged and the resulting charge-charge repulsion with the attacking nucleophile contributes to the very high barrier for hydrolysis, making phosphate esters among the most inert compounds known. However, biology is not only about reducing the barrier to unfavorable chemical reactions. That is, the same charge-charge repulsion that makes phosphate ester hydrolysis so unfavorable also makes it possible to regulate, by exploiting the electrostatics. This means that phosphate ester hydrolysis can not only be turned on, but also be turned off, by fine tuning the electrostatic environment and the present review demonstrates numerous examples where this is the case. Without this capacity for regulation, it would be impossible to have for instance a signaling or metabolic cascade, where the action of each participant is determined by the fine-tuned activity of the previous piece in the production line. This makes phosphate esters the ideal compounds to facilitate life as we know it.

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.

Synthesis and reactivity of (benzoxazol-2-ylmethyl)phosphonic acid

An efficient three step synthesis of (benzoxazol-2-ylmethyl)phosphonic acid (6-H(2)) is described along with IR, mass spectrometry (MS), and (1)H, (13)C, and (31)P NMR spectroscopic characterization data, and a single crystal X-ray diffraction structure determination. 6-H(2) is unstable in acidic aqueous solutions (pH < 4) undergoing ring-opening to give [(2-hydroxyphenylcarbamoyl)methyl] phosphonic acid (7-H(2)) that is characterized by IR, MS, and NMR methods. The protonation constants (pK(a)) for 7-H(2) have been measured, and crystal structure determinations for (NH(4))(7-H) and K(7-H)·DMF are described. Reactions of NaOH and KOH with 6-H(2) in MeOH/H(2)O solutions led to isolation and crystal structure determinations of the salts [Na(6-H)·H(2)O](2), K(6-H), Na(3)(6)(6-H)·H(2)O, and [K(2)(6)](2)·3H(2)O. The complexation reactions of 7-H(2) with La(III), Nd(III), and Gd(III), as a function of pH, were also examined by titrametric methods, and a model for the 1:1 anion binding with Ln(III) cations is proposed.

Kinetic isotope effects for RNA cleavage by 2'-O- transphosphorylation: nucleophilic activation by specific base

To better understand the interactions between catalysts and transition states during RNA strand cleavage, primary (18)O kinetic isotope effects (KIEs) and solvent D(2)O isotope effects were measured to probe the mechanism of base-catalyzed 2'-O-transphosphorylation of the RNA dinucleotide 5'-UpG-3'. The observed (18)O KIEs for the nucleophilic 2'-O and in the 5'-O leaving group at pH 14 are both large relative to reactions of phosphodiesters with good leaving groups, indicating that the reaction catalyzed by hydroxide has a transition state (TS) with advanced phosphorus-oxygen bond fission to the leaving group ((18)k(LG) = 1.034 +/- 0.004) and phosphorus-nucleophile bond formation ((18)k(NUC) = 0.984 +/- 0.004). A breakpoint in the pH dependence of the 2'-O-transphosphorylation rate to a pH independent phase above pH 13 has been attributed to the pK(a) of the 2'-OH nucleophile. A smaller nucleophile KIE is observed at pH 12 ((18)k(NUC) = 0.995 +/- 0.004) that is interpreted as the combined effect of the equilibrium isotope effect (ca. 1.02) on deprotonation of the 2'-hydroxyl nucleophile and the intrinsic KIE on the nucleophilic addition step (ca. 0.981). An alternative mechanism in which the hydroxide ion acts as a general base is considered unlikely given the lack of a solvent deuterium isotope effect above the breakpoint in the pH versus rate profile. These results represent the first direct analysis of the transition state for RNA strand cleavage. The primary (18)O KIE results and the lack of a kinetic solvent deuterium isotope effect together provide strong evidence for a late transition state and 2'-O nucleophile activation by specific base catalysis.