Alkali Metal Ion Catalysis Research Articles

Alkali metal ion catalysis and inhibition in alkaline ethanolysis of O-Y-substituted-phenyl O-phenyl thionocarbonates: contrasting M+ ion effects upon changing electrophilic centre from C=O to C=S

Pseudo-first-order rate constants (kobsd) were measured for nucleophilic substitution reactions of O-Y-substituted-phenyl O-phenyl thionocarbonates (4a–4h) with alkali metal ethoxides (EtOM, M = Li, Na, or K) in anhydrous ethanol at 25.0 ± 0.1 °C. Plots of kobsd vs. [EtOM] exhibited upward curvature for the reaction of O-4-nitrophenyl O-phenyl thionocarbonate (4a) with EtOK in the presence of 18-crown-6-ether (18C6), but showed downward curvature for the reaction with EtOLi, indicating that the reaction is catalyzed by the 18C6-crowned K+ ion, but is inhibited by Li+ ion. The kobsd values were dissected into kEtO− and kEtOM, the second-order rate constant for the reaction with dissociated EtO− and ion-paired EtOM, respectively. The reactivity of EtOM toward 4a increases in the order EtOLi < EtONa < EtO− < EtOK < EtOK/18C6, which is in contrast to that reported previously for the corresponding reaction of 4-nitrophenyl phenyl carbonate (a C=O analogue of 4a), e.g., EtO− ≈ EtOK/18C6 < EtOLi < EtONa < EtOK. The reaction mechanism, including the transition-state model and the origin of the contrasting reactivity patterns found for the reactions of the C=O and C=S compounds, are discussed.

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.

Alkali-Metal Ion Catalysis and Inhibition in SNAr Reaction of 1-Halo-2,4-dinitrobenzenes with Alkali-Metal Ethoxides in Anhydrous Ethanol

A kinetic study is reported for SNAr reaction of 1-fluoro-2,4-dinitrobenzene (5a) and 1-chloro-2,4-dinitrobenzene (5b) with alkali-metal ethoxides (EtOM, M = Li, Na, K and 18-crown-6-ether complexed K) in anhydrous ethanol. The second-order rate constant increases in the order kEtOLi < kEtO− < kEtONa < kEtOK < kEtOK/18C6 for the reaction of 5a and kEtOLi < kEtONa < kEtO− < kEtOK < kEtOK/18C6 for that of 5b. This indicates that M ion behaves as a catalyst or an inhibitor depending on the size of M ion and the nature of the leaving group (F vs. Cl). Substrate 5a is more reactive than 5b, although the F in 5a is ca. 10 pKa units more basic than the Cl in 5b, indicating that the reaction proceeds through a Meisenheimer complex in which expulsion of the leaving group occurs after the rate-determining step (RDS). M ion would catalyze the reaction by increasing either the nucleofugality of the leaving group through a four-membered cyclic transition state or the electrophilicity of the reaction center through a π-complex. However, the enhanced nucleofugality would be ineffective for the current reaction, since expulsion of the leaving group occurs after the RDS. Thus, it has been concluded that M ion catalyzes the reaction by increasing the electrophilicity of the reaction center through a π-complex between M ion and the π-electrons in the benzene ring.

Alkali-Metal Ion Catalysis in Nucleophilic Substitution Reactions of 5-Nitro-8-quinolyl Picolinate with Alkali Metal Ethoxides: Effect of Modification of Nonleaving Group from Benzoyl to Picolinyl on Reactivity and Transition State Structure

A kinetic study on nucleophilic substitution reaction of 5-nitro-8-quinolyl picolinate (6) with alkali-metal ethoxides (EtOM; M = K, Na, and Li) in anhydrous ethanol is reported. The plot of <TEX>$k_{obsd}$</TEX> vs. [EtOM] curves upward in the absence of crown ethers but is linear with significantly decreased reactivity in the presence of crown ethers. Dissection of <TEX>$k_{obsd}$</TEX> into <TEX>$k_{EtO}$</TEX>- and <TEX>$k_{EtOM}$</TEX> (i.e., the second-order rate constants for the reactions with the dissociated <TEX>$EtO^-$</TEX> and ion-paired EtOM, respectively) has revealed that the ion-paired EtOM is significantly more reactive than the dissociated <TEX>$EtO^-$</TEX> (e.g., <TEX>$k_{EtOM}/k_{EtO^-}$</TEX> = 33.4-141). This indicates that the reaction of 6 is catalyzed by <TEX>$M^+$</TEX> ions in the order <TEX>$Na^+$</TEX> > <TEX>$Li^+$</TEX> > <TEX>$K^+$</TEX> and the catalytic effect disappears in the presence of a proper crown ether. Picolinate ester 6 is much more reactive and is more strongly catalyzed by <TEX>$M^+$</TEX> ions than 5-nitro-8-quinolyl benzoate (5). It has been concluded that <TEX>$M^+$</TEX> ions catalyze the reaction of 6 by increasing electrophilicity of the reaction center through a cyclic transition state, which is structurally not possible for the reaction of 5.

Alkali-Metal Ion Catalysis and Inhibition in Nucleophilic Displacement Reaction of O-Ethyl O-4-Nitrophenyl Phenylphosphonothioate with Alkali-Metal Ethoxides

Abstract A kinetic study on nucleophilic displacement reactions of O-ethyl O-4-nitrophenyl phenylphosphonothioate (an insecticide called EPN) with alkali-metal ethoxides (EtOM; M = Li, Na, K, and K/18-crown-6-ether) is reported. Dissection of pseudo-first-order rate constant (kobsd) into the second-order rate constants for the reaction of EPN with the dissociated EtO− (kEtO-) and ion-paired EtOM (kEtOM) has revealed that the reactivity increases in the order kEtOLi &amp;lt; kEtO- &amp;lt; kEtONa &amp;lt; kEtOK &amp;lt; kEtOK/18-crown-6-ether, indicating that Li+ inhibits the reaction while the other M+ ions behave as a Lewis acid catalyst in the order Na+ &amp;lt; K+ &amp;lt; 18C6-complexed K+. The contrasting M+ ion effects have been explained in terms of the hard and soft acids and bases (HSAB) principle, since EPN, as a polarizable P=S centered electrophile, would exert a weak interaction with Li+ (a hard acid) but a strong interaction with the 18C6-complexed K+ ion (a soft acid). M+ ions catalyze the current reaction by increasing the electrophilicity of the reaction center. EPN is less reactive than O-4-nitrophenyl diphenylphosphinothioate but is more reactive than O-4-nitrophenyl O,O-diethyl thiophosphate. Factors governing the reactivity of these P=S centered electrophiles are discussed.

Alkali-Metal Ion Catalysis in Alkaline Ethanolysis of 2-Pyridyl Benzoate and Benzyl 2-Pyridyl Carbonate: Effect of Modification of Nonleaving Group from Benzoyl to Benzyloxycarbonyl

A kinetic study is reported on nucleophilic displacement reactions of benzyl 2-pyridyl carbonate 6 with alkalimetal ethoxides, EtOM (M = Li, Na, and K), in anhydrous ethanol at 25.0 ± 0.1 o C. The plots of pseudo-firstorder rate constant kobsd vs. [EtOM] curve upward, a typical phenomenon reported previously for alkaline ethanolysis of esters in which alkali-metal ions behave as a Lewis-acid catalyst. The kobsd value for the reaction of 6 with a fixed EtOK concentration decreases rapidly upon addition of 18-crown-6-ether (18C6), a complexing agent for K + ion up to [18C6]/[EtOK] = 1.0 and then remains constant thereafter, indicating that the catalytic effect exerted by K + ion disappears in the presence of excess 18C6. The reactivity of EtOM towards 6 increases in the order EtO – < EtOLi < EtONa < EtOK, which is contrasting to the reactivity order reported for the corresponding reactions of 2-pyridyl benzoate 4, i.e., EtO – < EtOK < EtONa < EtOLi. Besides, 6 is 1.7 and 3.5 times more reactive than 4 towards dissociated EtO – and ion-paired EtOK, respectively. The reactivity difference and the contrasting metal-ion selectivity are discussed in terms of electronic effects and transition-state structures.

Alkali Metal Ion Catalysis and Inhibition in Nucleophilic Substitution Reactions of 3,4-Dinitrophenyl Diphenylphosphinothioate with Alkali Metal Ethoxides in Anhydrous Ethanol: Effect of Changing Electrophilic Center from P=O to P=S

Pseudo-first-order rate constants (<TEX>$k_{obsd}$</TEX>) have been measured spectrophotometrically for nucleophilic substitution reactions of 3,4-dinitrophenyl diphenylphosphinothioate 9 with alkali metal ethoxides (EtOM, M = Li, Na, K) in anhydrous ethanol at <TEX>$25.0{\pm}0.1^{\circ}C$</TEX>. The plot of <TEX>$k_{obsd}$</TEX> vs. [EtOM] is linear for the reaction of 9 with EtOK. However, the plot curves downwardly for those with EtOLi and EtONa while it curves upwardly for the one with EtOK in the presence of 18-crown-6-ether (18C6). Dissection of <TEX>$k_{obsd}$</TEX> into <TEX>$k_{EtO^-}$</TEX> and <TEX>$k_{EtOM}$</TEX> (i.e., the second-order rate constant for the reaction with dissociated <TEX>$EtO^-$</TEX> and ion-paired EtOM, respectively) has revealed that the reactivity increases in the order <TEX>$k_{EtOLi}$</TEX> < <TEX>$k_{EtONa}$</TEX> < <TEX>$k_{EtO^-}$</TEX> <TEX>${\approx}$</TEX> <TEX>$k_{EtOK}$</TEX> < <TEX>$k_{EtOK/18C6}$</TEX>, indicating that the reaction is inhibited by <TEX>$Li^+$</TEX> and <TEX>$Na^+$</TEX> ions but is catalyzed by 18C6-crowned <TEX>$K^+$</TEX> ion. The reactivity order found for the reactions of 9 contrasts to that reported previously for the corresponding reactions of 1, i.e., <TEX>$k_{EtOLi}$</TEX> > <TEX>$k_{EtONa}$</TEX> > <TEX>$E_{EtOK}$</TEX> > <TEX>$k_{EtO^-}$</TEX> <TEX>${\approx}$</TEX> <TEX>$k_{EtOK/18C6}$</TEX>, indicating that the effect of changing the electrophilic center from P=O to P=S on the role of <TEX>$M^+$</TEX> ions is significant. A four-membered cyclic transition-state has been proposed to account for the <TEX>$M^+$</TEX> ion effects found in this study, e.g., the polarizable sulfur atom of the P=S bond in 9 interacts strongly with the soft 18C6-crowned <TEX>$K^+$</TEX> ion while it interacts weakly with the hard <TEX>$Li^+$</TEX> and <TEX>$Na^+$</TEX> ions.

A kinetic study of the oxidation of formaldehyde by the octacyanomolybdate(V) ion in aqueous alkaline medium

The kinetics of the oxidation of formaldehyde by octacyanomolybdate(V) ions has been studied in aqueous alkaline medium. A formaldehyde-dependent and -independent reaction has been observed. The HCHO-dependent reaction displays first order kinetics in [Mo(CN)8 3−], [HCHO], [OH−] and alkali metal ion catalysis was observed. The rate law is consistent with: $$ \frac{{ - {\text{d}}[{\text{Mo}}\left( {\text{CN}} \right)_{ 8}^{ 3- } ]}}{\text{dt}} = { 2}K_{\text{h}} K_{ 1} K_{ 2} k_{ 3} \left[ {{\text{Mo}}\left( {\text{CN}} \right)_{ 8}^{ 3- } } \right] \, \left[ {\text{HCHO}} \right] \, \left[ {{\text{OH}}^{ - } } \right] \, \left[ {{\text{A}}^{ + } } \right]. $$ Activation parameters for both the formaldehyde-dependent and -independent reaction have been obtained. The activation parameters for the HCHO-independent reaction provide evidence for the identification of the reaction. A reaction mechanism is proposed.

Alkali Metal Ion Catalysis in Nucleophilic Substitution Reactions of 5-Nitro-8-quinolyl Benzoate with Alkali Metal Ethoxides in Anhydrous Ethanol: Unusually High Na+Ion Selectivity

from ion-pairing treatments. The kEtO– and kEtO–M+ values decrease in the order kEtO–Na+ > kEtO–K+ > kEtO–Li+ > kEtO– , indicating that ion-paired EtO – M + species are more reactive than the dissociated EtO – ion, and Na + ion exhibits the largest catalytic effect. The M + ions in this study form stronger complex with the transition state than with the ground state. Coordination of the M + ions with the O and N atoms in the leaving group of 5 has been suggested to be responsible for the catalytic effect shown by the alkali metal ions in this study.

Alkali Metal Ion Catalysis and Inhibition in Nucleophilic Displacement Reactions at Phosphorus Centers: Ethyl and Methyl Paraoxon and Ethyl and Methyl Parathion

We report on the ethanolysis of the P=O and P=S compounds ethyl and methyl paraoxon (1a and 1b) and ethyl and methyl parathion (2a and 2b). Plots of spectrophotometrically measured rate constants, kobsd versus [MOEt], the alkali ethoxide concentration, show distinct upward and downward curvatures, pointing to the importance of ion-pairing phenomena and a differential reactivity of free ions and ion pairs. Three types of reactivity and selectivity patterns have been discerned: (1) For the P=O compounds 1a and 1b, LiOEt > NaOEt > KOEt > EtO-; (2) for the P=S compound 2a, KOEt > EtO- > NaOEt > LiOEt; (3) for P=S, 2b, 18C6-crown-complexed KOEt > KOEt = EtO(-) > NaOEt > LiOEt. These selectivity patterns are characteristic of both catalysis and inhibition by alkali-metal cations depending on the nature of the electrophilic center, P=O vs P=S, and the metal cation. Ground-state (GS) vs transition-state (TS) stabilization energies shed light on the catalytic and inhibitory tendencies. The unprecedented catalytic behavior of crowned-K(+) for the reaction of 2b is noteworthy. Modeling reveals an extreme steric interaction for the reaction of 2a with crowned-K(+), which is responsible for the absence of catalysis in this system. Overall, P=O exhibits greater reactivity than P=S, increasing from 50- to 60-fold with free EtO(-) and up to 2000-fold with LiOEt, reflecting an intrinsic P=O vs P=S reactivity difference (thio effect). The origin of reactivity and selectivity differences in these systems is discussed on the basis of competing electrostatic effects and solvational requirements as function of anionic electric field strength and cation size (Eisenman's theory).

Solvent Effects and Alkali Metal Ion Catalysis in Phosphodiester Hydrolysis

The kinetics of the alkaline hydrolysis of bis(p-nitrophenyl) phosphate (BNPP) have been studied in aqueous DMSO, dioxane, and MeCN. In all solvent mixtures the reaction rate steadily decreases to half of its value in pure water in the range of 0-70 vol % of organic cosolvent and sharply increases in mixtures with lower water content. Correlations based on different scales of solvent empirical parameters failed to describe the solvent effect in this system, but it can be satisfactorily treated in terms of a simplified stepwise solvent-exchange model. Alkali metal ions catalyze the BNPP hydrolysis but do not affect the rate of hydrolysis of neutral phosphotriester p-nitrophenyl diphenyl phosphate in DMSO-rich mixtures. The catalytic activity decreases in the order Li+ > Na+ > K+ > Rb+ > Cs+. For all cations except Na+, the reaction rate is first-order in metal ion. With Na+, both first- and second-order kinetics in metal ions are observed. Binding constants of cations to the dianionic transition state of BNPP alkaline hydrolysis are of the same order of magnitude and show a similar trend as their binding constants to p-nitrophenyl phosphate dianion employed as a transition-state model. The appearance of alkali metal ion catalysis in a medium, which solvates metal ions stronger than water, is attributed to the increased affinity of cations to dianions, which undergo a strong destabilization in the presence of an aprotic dipolar cosolvent.

Nucleophilic displacement on 4-nitrophenyl dimethyl phosphinate by ethoxide ion: alkali metal ion catalysis and mechanism.

We report on a spectrophotometric kinetic study of the effect of Li(+) and K(+) cations on the ethanolysis of 4-nitrophenyl dimethylphosphinate () in ethanol at 25 [degree]C. The nucleophilic displacement reaction of with LiOEt and KOEt in the absence and presence of 18-crown-6 ether (18-C-6) furnished observed first-order rate constants which increase in the order EtO(-) < KOEt < LiOEt. The kinetic data are analyzed in terms of a scheme which assigns concurrent kinetic activity to free ethoxide and metal alkoxide, to obtain the second-order rate coefficients for reaction of the metal ion-ethoxide pairs, k(MOEt). Derived [small delta]G(ip), [small delta]G(ts) and [capital Delta]G(cat) values quantify ground state and transition state stabilization by the metal ions to give [small delta]G(ts) > [small delta]G(ip) for Li(+) and [small delta]G(ts)[similar][small delta]G(ip) for K(+). These results indicate moderate catalysis by Li(+), with manifesting lesser susceptibility to catalysis than other substrates previously studied. Second-order rate constants for the reaction of the aryl dimethylphosphinates with free EtO(-) were obtained from plots of log k(obs)vs. [KOEt], measured in the presence of excess 18-C-6. Hammett plots with [sigma] and [sigma][degree] substituent constants give significantly better correlation of rates than [sigma](-) and yield a moderately large [small rho]([small rho][degree]) value; this is interpreted in terms of a stepwise mechanism involving rate-limiting formation of a pentacoordinate intermediate. Comparison of the present results with those of Williams on the aqueous alkaline hydrolysis of Me(2)P(O)-OPhX and Ph(2)P(O)-OPhX esters, establishes the rationale for a change in mechanism in the more basic EtO(-)/EtOH nucleophile/solvent system by a stepwise mechanism instead of a concerted one in aqueous base. Structure-reactivity correlations following Jencks show that the change in mechanism is accounted for by cross interactions between the nucleophile and the leaving group in the transition state. The observed duality of mechanism is rationalized on the basis of the More O'Ferrall-Jencks diagram, as a spectrum of transition states covering a wide range of nucleophile and leaving group basicities.

Alkali metal ion catalysis in nucleophilic displacement by ethoxide ion on p-nitrophenyl phenylphosphonate: Evidence for multiple metal ion catalysis

In continuation of our studies of alkali metal ion catalysis and inhibition at carbon, phosphorus, and sulfur centers, the role of alkali metal ions in nucleophilic displacement reactions of p-nitrophenyl phenylphosphonate (PNPP) has been examined. All alkali metal ions studied acted as catalysts. Alkali metal ions added as inert salts increased the rate while decreased rate resulted on M+ complexation with 18-crown-6 ether. Kinetic analysis indicated the interaction of possibly three potassium ions, four sodium ions, and five lithium ions in the transition state of the reactions of ethoxide with PNPP. Pre-association of the anionic substrate with two metals ions in the ground state gave the best fit to the experimental data of the sodium system. Thus, the study gives evidence of the role of several metal ions in nucleophilic displacement reactions of ethoxide with anionic PNPP, both in the ground state and in the transition state. Molecular modeling of the anionic transition state implies that the size of the monovalent cation and the steric requirement of the pentacoordinate transition state are the primary limitations on the number of cations that can be brought to bear to stabilize the transition state and catalyze nucleophilic substitution at phosphorus. The bearing of the present work on metal ion catalysis in enzyme systems is discussed, in particular enzymes that catalyze phosphoryl transfer, which often employ multiple metal ions. Our results, both kinetic and modeling, reveal the importance of electrostatic stabilization of the transition state for phosphoryl transfer that may be effected by multiple cations, either monovalent metal ions or amino acid residues. The more such cations can be brought into contact with the anionic transition state, the greater the catalysis observed.Key words: alkali metal ion catalysis, nucleophilic displacement at phosphorus, multiple metal ion catalysis, phosphoryl transfer.

Alkali-metal ion catalysis and inhibition in nucleophilic displacement reactions at carbon, phosphorus and sulfur centres. IX.For Part VIII see ref. 1. p-Nitrophenyl diphenyl phosphateElectronic supplementary information (ESI) available: kinetic data for the reaction of p-nitrophenyl diphenyl phosphate with various reagents in differing media. See http://www.rsc.org/suppdata/ob/b2/b208408b/

We report on the catalytic effects by alkali-metal ions in the ethanolysis of p-nitrophenyl diphenyl phosphate, in continuation of our studies on alkali-metal ion catalysis and inhibition in nucleophilic displacement reactions at carbon, phosphorus and sulfur centres. The following selectivity order of catalytic reactivity was observed for nucleophilic displacement at the phosphorus center with p-nitrophenoxide as leaving group: Li+ > Na+ > K+ > Cs+. A minor reaction pathway with phenoxide leaving was also found. The kobs data have been dissected into reaction pathways by free ions (kEtO) and by ion pairs (kMOEt), with the latter being dominant, in a 4-membered transition-state. Further analysis is given in terms of initial-state and transition-state stabilization by the alkali-metal ions in terms of the Eisenman model (electrostatic interaction vs. desolvation). Results of ab-initio MO calculations are presented based on interaction between M+ and a model bipyramidal phosphorane intermediate and compared with the sulfurane analogue.

Alkali-metal ion catalysis and inhibition of acetyl transfer from p-nitroaryl acetates to hexanoate ion

Deacetylation of 2-acetoxy-5-nitro-1,3-xylylene-18-crown-5 (21 -acetoxy-18-nitro-2,5,8,11,14-pentaoxa[18]metacyclophane) by hexanoate ion in DMF is strongly inhibited by Li+ ion, slightly retarded by Na+ ion, and significantly promoted by the larger alkali-metal ions. In contrast, the analogous reaction of the simple model p-nitrophenyl acetate is inhibited by all of the alkali-metal ions. These effects are discussed in terms of differential binding of metal ions to reactants and transition states. In line with conclusions from previous works, it is confirmed that coordinative binding of metal ions with the oxygen donors of the polyether bridge of the crown-ether substrate is an effective force for catalysis.

Solvation and metal ion effects on structure and reactivity of phosphoryl compounds. Part 2. Alkali metal ion catalysis in the demethylation of methyl di(3-nitrophenyl)phosphinate by iodide ion

Second-order rate constants for the demethylation of methyl di(3-nitrophenyl)phosphinate by iodide in acetone-d6 at 25 °C have been measured. For a series of iodide salts the rate increased in the order: NBu4I &lt; KI ≤ NaI &lt; LiI. The addition of crown ethers or D2O has a retarding effect on the reaction rate. The results are discussed in terms of the increase of the nucleofugality of the phosphinate group via the complexation of metal ions with the phosphoryl function.

Alkali-metal ion catalysis of the oxidation of L-ascorbic acid by hexacyanoferrate(III) in strongly acidic media

The kinetics of the oxidation of L-ascorbic acid by hexacyanoferrate(III) ions have been investigated in strongly acidic media over a wide range of acidity. Two different mechanisms involving the monoascorbate anion, AH–, the neutral molecule of ascorbic acid AH2, and the protonated species AH+3, as well as the hexacyanoferrate(III) ion Fe(CN)3–6, and the monoprotonated form HFe(CN)2–6 are suggested. Kinetic evidence for a specific catalytic effct by binding of alkali-metal ions to oxidant is reported. The derived rate equations are in good agreement with the experimental observations.

Metal-ion catalysis in nucleophilic displacement reactions at carbon, phosphorus, and sulfur centers. 5. Alkali-metal ion catalysis and inhibition in the reaction of p-(trifluoromethyl)phenyl methanesulfonate with ethoxide ion

Metal-ion catalysis in nucleophilic displacement reactions at carbon, phosphorus, and sulfur centers. 5. Alkali-metal ion catalysis and inhibition in the reaction of p-(trifluoromethyl)phenyl methanesulfonate with ethoxide ion

Kinetics and mechanism of the reduction of dodecatungstocobaltate(III) by ?-hydroxyacids in aqueous acid solution. Evidence for alkali metal ion catalysis

The kinetics of the redox reaction of dodecatungstocobaltate(III) [abbreviated as CoIIIW] with three α-hydroxy-carboxylic acids:e.g., mandelic, glycolic and lactic acid, were investigated in the 0–5.4 pH range where the complex remains stable and does not decompose. All reactions are first-order both in oxidant and reductant over the experimental pH range. A specific metal ion effect has been observed. Oxidations of the molecular form of the three acids and the anionic form of mandelic acid are catalysed by alkali metal ions and follow a general rate law: $$ - d[Co^{III} W]/dt = 2(k_s + k_c [M^ + ])[R][Co^{III} W]$$ where ks and kc account for the spontaneous and catalysed paths respectively, M+ is either an alkali metal ion (Li+, Na+ or K+) and [R], the reductant concentration. The oxidation of lactate and glycolate ions occurs only in the presence of alkali cations. The cation catalytic order: K+>Na+>Li+ observed in this study is in accord with the polarizability of the cations. An outer-sphere alkali cation-bridged mechanism is suggested for the electron transfer step.

Metal ion catalysis in nucleophilic displacement reactions at carbon, phosphorus, and sulfur centers. III. Catalysis vs. inhibition by metal ions in the reaction of p-nitrophenyl benzenesulfonate with ethoxide

The rate of the nucleophilic displacement reaction of p-nitrophenyl benzenesulfonate (1) with alkali metal ethoxides in ethanol at 25 °C has been studied by spectrophotometric techniques. For lithium ethoxide, sodium ethoxide, potassium ethoxide, and cesium ethoxide, the observed rate constants increase in the order LiOEt &lt; NaOEt &lt; CsOEt &lt; KOEt. The effect of added crown ether and cryptand complexing agents was also investigated. Addition of complexing agent to the reaction of KOEt results in the rate decreasing to a minimum value corresponding to the reaction of free ethoxide. Conversely, addition of complexing agent to the reaction of LiOEt results in the rate increasing to a maximum value that is identical to the minimum value seen in the reaction of KOEt in the presence of excess complexing agent. In complementary experiments, alkali metal ions were added in the form of unreactive salts. Addition of a K+ salt to the reaction of KOEt increases the reaction rate, while addition of a Li+ salt to the reaction of LiOEt decreases the rate. The involvement of metal ions in the reaction of 1 is proposed to occur via reactive alkali metal – ethoxide ion pairs. The kinetic data are analyzed in terms of an ion pairing treatment that allows the calculation of second-order rate constants for free ethoxide and metal–ethoxide ion pairs; the rate constants increase in the order LiOEt &lt; EtO−&lt; NaOEt &lt; CsOEt &lt; KOEt. Thus, Li+isaninhibitorofthereactionofethoxidewith1, whiletheothermetalsionsstudiedareallcatalysts. Equilibrium constants for the association of the various metal ions with the transition state are calculated using a thermodynamic cycle, and are compared to association constants in the ground state. Consistent with the observed kinetic results, Li+ is found to stabilize the ground state more than the transition state, while Na+, K+, and Cs+ all stabilize the transition state more than the ground state. The trend in the magnitude of the transition state stabilization is interpreted in terms of interactions of the transition state with bare or solvated metal ions. It is concluded that the transition state for the reaction of 1 with ethoxide forms solvent separated ion pairs with alkali metal ions. Analogous data were available for the reaction of p-nitrophenyl diphenylphosphinate (2) with ethoxides, where Li+, Na+, K+, and Cs+ all function as catalysts, and the results are analyzed as above. In contrast to the sulfonate system, it is proposed that the phosphinate transition state forms contact ion pairs with alkali metal ions. The difference is attributed to a greater localization of negative charge in the phosphinate transition state, leading to stronger interactions with metal ions, which overcome metal ion – solvent interactions. Keywords: nucleophilic substitution at sulfur, alkali metal ion catalysis.