Metal ion catalysis in nucleophilic displacement reactions at carbon, phosphorus, and sulfur centers. 4. Mechanism of the reaction of aryl benzenesulfonates with alkali-metal ethoxides: catalysis and inhibition by alkali-metal ions

1991 ◽  
Vol 113 (9) ◽  
pp. 3545-3550 ◽  
Author(s):  
Marko J. Pregel ◽  
Edward J. Dunn ◽  
Erwin Buncel
Keyword(s):  
Alkali Metal ◽  
Metal Ions ◽  
Metal Ion ◽  
Alkali Metal Ions ◽  
Nucleophilic Displacement ◽  
Displacement Reactions ◽  
Metal Ion Catalysis ◽  
Mechanism Of The Reaction

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

2003 ◽  
Vol 81 (1) ◽  
pp. 53-63 ◽  
Author(s):  
Erwin Buncel ◽  
Ruby Nagelkerke ◽  
Gregory RJ Thatcher
Keyword(s):  
Alkali Metal ◽  
Transition State ◽  
Metal Ions ◽  
Metal Ion ◽  
Alkali Metal Ions ◽  
Phosphoryl Transfer ◽  
Alkali Metal Ion ◽  
Nucleophilic Displacement ◽  
Displacement Reactions ◽  
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.

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

1989 ◽  
Vol 67 (9) ◽  
pp. 1440-1448 ◽  
Author(s):  
Edward J. Dunn ◽  
Erwin Buncel
Keyword(s):  
Crown Ether ◽  
Alkali Metal ◽  
Transition State ◽  
Metal Ions ◽  
Ground State ◽  
Metal Ion ◽  
Complexing Agents ◽  
Transition State Stabilization ◽  
Nucleophilic Displacement ◽  
Metal Ion Catalysis

The effect of macrocyclic crown ether and cryptand complexing agents on the rate of the nucleophilic displacement reaction of p-nitrophenyl diphenylphosphinate by alkali metal ethoxides in ethanol at 25 °C has been studied by spectrophotometric techniques. For the reactions of potassium ethoxide, sodium ethoxide, and lithium ethoxide, the observed rate constant increased in the order KOEt < NaOEt < LiOEt. Crown ether and cryptand cation-complexing agents have a retarding effect on the rate. Increasing the ratio of complexing agent to base results in a decrease in kobs to a minimum value corresponding to the rate of reaction of free ethoxide ion. In complementary experiments, alkali metal ions were added to these reaction systems in the form of unreactive salts, causing an increase in reaction rate. The kinetic data were analysed in terms of ion-pairing treatments, which allowed evaluation of rate coefficients due to free ethoxide ions and metal ion – ethoxide ion pairs. Possible roles of the metal cations are discussed in terms of ground state and transition state stabilization. Evaluation of the equilibrium constants for association of the metal ion with ground state (Ka) and the transition state (K′a) shows that catalysis occurs as a result of enhanced association between the metal ion and the transition state, with (K′a) values increasing in the order K+ < Na+ < Li+. A model is proposed in which transition state stabilization arises largely from chelation of the solvated metal ion to two charged oxygen centers. This appears to be the first reported instance of catalysis by alkali metal cations in nucleophilic displacement at phosphoryl centers. Keywords: nucleophilic displacement at phosphorus, alkali-metal-ion catalysis.

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

1990 ◽  
Vol 68 (10) ◽  
pp. 1846-1858 ◽  
Author(s):  
Marko J. Pregel ◽  
Edward J. Dunn ◽  
Erwin Buncel
Keyword(s):  
Alkali Metal ◽  
Transition State ◽  
Metal Ions ◽  
Ground State ◽  
Rate Constants ◽  
Metal Ion ◽  
Ion Pairs ◽  
Alkali Metal Ions ◽  
Complexing Agent ◽  
Metal Ion Catalysis

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 < NaOEt < CsOEt < 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 < EtO−< NaOEt < CsOEt < 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.

scholarly journals Metal ion catalysis in nucleophilic displacement reactions at carbon, phosphorus, and sulfur centers. II. Metal ion catalysis in the reaction of p-nitrophenyl diphenylphosphinate with alkali metal phenoxides in ethanol

1990 ◽  
Vol 68 (10) ◽  
pp. 1837-1845 ◽  
Author(s):  
Edward J. Dunn ◽  
Robert Y. Moir ◽  
Erwin Buncel ◽  
J. Garfield Purdon ◽  
Robert A. B. Bannard
Keyword(s):  
Alkali Metal ◽  
Transition State ◽  
Metal Ions ◽  
Metal Ion ◽  
Ion Pairs ◽  
Rate Coefficients ◽  
Complexing Agents ◽  
Specific Rate ◽  
Nucleophilic Displacement ◽  
Metal Ion Catalysis

The reactions of p-nitrophenyl diphenylphosphinate (1) with lithium, sodium, potassium, and benzyltrimethylammonium phenoxides (BTMAOPh) have been studied by spectrophotometric techniques in anhydrous ethanol at 25 °C. The reactivity (kobs) of the alkali metal phenoxides increases in the order BTMAOPh < KOPh < NaOPh < LiOPh. The rate of reaction of 1 with LiOPh is enhanced when lithium salts (LiSCN, LiNO3, LiClO4, LiOAc) are added to the reaction media. The addition of the alkali metal complexing agents dicyclohexyl-18-crown-6 ether or [2.2.2]cryptand for Na+, and [2.1.1]cryptand for Li+, to each of the alkali metal phenoxide reactions resulted in a decrease in rate, indicating catalysis by the alkali metal ions. The kinetic data are analyzed to obtain specific rate coefficients of reactions of phenoxide and ethoxide as the dissociated ions and as alkali metal – phenoxide ion pairs. Reactivities follow the order [Formula: see text]; [Formula: see text]; [Formula: see text]; [Formula: see text]. A mechanism is proposed in which the ion-paired metal phenoxide is more reactive towards the substrate than is the dissociated phenoxide. Analysis of the data in terms of initial state and transition state interactions with metal ions indicates that the increased reactivity of the ion-paired species results from greater stabilization of the negatively charged transition state relative to stabilization of the ion-paired nucleophile. Keywords: nucleophilic displacement at phosphorus by phenoxide, alkali-metal-ion catalysis.

Membrane transport systems. II. Transport of alkali metal ions against their concentration gradients

1981 ◽  
Vol 59 (12) ◽  
pp. 1734-1744 ◽  
Author(s):  
Thomas M. Fyles ◽  
Virginia A. Malik-Diemer ◽  
Dennis M. Whitfield
Keyword(s):  
Alkali Metal ◽  
Metal Ions ◽  
Metal Ion ◽  
Complex Function ◽  
Membrane System ◽  
Ion Concentration ◽  
Transport Systems ◽  
Alkali Metal Ions ◽  
Concentration Gradients ◽  
Membrane Transport Systems

An artificial membrane system based on a series of macrocyclic polyether carriers (crown ethers) is described. Under the influence of a proton gradient the carriers move alkali metal ions from basic to acidic solution through a chloroform membrane phase. Transport occurs against the concentration gradient of the transported ion as a result of a coupled counterflow of protons. Different transport behaviors are observed depending upon the metal ion concentration. At high metal ion concentration the amount transported is a linear function of time; at lower metal ion concentration the amount transported is a complex function of time which may be described as the result of a pair of consecutive first order processes. Effects of metal ion, carrier, and proton concentration on transport rate are considered. The rate increases with increasing metal ion or carrier concentration but is essentially independent of the pH of either aqueous phase. Increased lipophilicity of the carrier also results in a rate increase. Carriers derived from 18-crown-6 transport potassium selectively and all ions more rapidly than 15-crown-5 derivatives which are, however, selective for sodium. The overall efficiency of the system is discussed in terms of competing "leak" reactions, either of cations from the basic phase or of anions from the acidic phase.

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