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

1993 ◽  
Vol 71 (4) ◽  
pp. 469-472 ◽  
Author(s):  
Agnes M. Modro ◽  
Tomasz A. Modro
Keyword(s):  
Alkali Metal ◽  
Metal Ions ◽  
Crown Ethers ◽  
Rate Constants ◽  
Metal Ion ◽  
Reaction Rate ◽  
Retarding Effect ◽  
Metal Ion Catalysis ◽  
Ion Effects ◽  
Metal Ion Effects

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 < KI ≤ NaI < 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.

Solvation and metal ion effects on structure and reactivity of phosphoryl compounds. Part 4. Dealkylation of phosphate esters by thiophenoxide ion in methanol

1994 ◽  
Vol 72 (9) ◽  
pp. 1933-1936 ◽  
Author(s):  
Marian Mentz ◽  
Agnes M. Modro ◽  
Tomasz A. Modro
Keyword(s):  
Alkali Metal ◽  
Transition State ◽  
Rate Constants ◽  
Metal Ion ◽  
Ion Pairs ◽  
Phosphate Esters ◽  
Phosphoryl Group ◽  
Ion Effects ◽  
Metal Ion Effects ◽  
Catalytic Effects

Second-order rate constants for the demethylation of three phosphate esters by thiophenoxide salts, PhS−M+ (M+ = Me4N+, K+, Na+, Li+) in methanol-d4 at 25 °C have been measured. In contrast to the demethylation by iodide salts, metallic counterions do not exhibit any catalytic effects on the demethylation rate. The absence of the catalysis indicates that the salts react exclusively as ion pairs, in which an alkali metal ion is not available for the interactions with the phosphoryl group in the transition state.

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.

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 effects on ion channel gating

2003 ◽  
Vol 36 (4) ◽  
pp. 373-427 ◽  
Author(s):  
Fredrik Elinder ◽  
Peter Århem
Keyword(s):  
Ion Channels ◽  
Metal Ions ◽  
Metal Ion ◽  
Classical Model ◽  
Voltage Sensor ◽  
Surface Charges ◽  
Ion Effects ◽  
Group 2 ◽  
Metal Ion Effects ◽  
Special Case

1. Introduction 3742. Metals in biology 3783. The targets: structure and function of ion channels 3804. General effects of metal ions on channels 3824.1 Three types of general effect 3824.2 The main regulators 3835. Effects on gating: mechanisms and models 3845.1 Screening surface charges (Mechanism A) 3875.1.1 The classical approach 3875.1.1.1 Applying the Grahame equation 3885.1.2 A one-site approach 3915.2 Binding and electrostatically modifying the voltage sensor (Mechanism B) 3915.2.1 The classical model 3915.2.1.1 The classical model as state diagram – introducing basic channel kinetics 3925.2.2 A one-site approach 3955.2.2.1 Explaining state-dependent binding – a simple electrostatic mechanism 3955.2.2.2 The relation between models assuming binding to smeared and to discrete charges 3965.2.2.3 The special case of Zn2+ – no binding in the open state 3965.2.2.4 Opposing effects of Cd2+ on hyperpolarization-activated channels 3985.3 Binding and interacting non-electrostatically with the voltage sensor (Mechanism C) 3985.3.1 Combining mechanical slowing of opening and closing with electrostatic modification of voltage sensor 4005.4 Binding to the pore – a special case of one-site binding models (Mechanism D) 4005.4.1 Voltage-dependent pore-block – adding extra gating 4015.4.2 Coupling pore block to gating 4025.4.2.1 The basic model again 4025.4.2.2 A special case – Ca2+ as necessary cofactor for closing 4035.4.2.3 Expanding the basic model – Ca2+ affecting a voltage-independent step 4045.5 Summing up 4056. Quantifying the action: comparing the metal ions 4076.1 Steady-state parameters are equally shifted 4076.2 Different metal ions cause different shifts 4086.3 Different metal ions slow gating differently 4106.4 Block of ion channels 4127. Locating the sites of action 4127.1 Fixed surface charges involved in screening 4137.2 Binding sites 4137.2.1 Group 2 ions 4147.2.2 Group 12 ions 4148. Conclusions and perspectives 4159. Appendix 41610. Acknowledgements 41811. References 418Metal ions affect ion channels either by blocking the current or by modifying the gating. In the present review we analyse the effects on the gating of voltage-gated channels. We show that the effects can be understood in terms of three main mechanisms. Mechanism A assumes screening of fixed surface charges. Mechanism B assumes binding to fixed charges and an associated electrostatic modification of the voltage sensor. Mechanism C assumes binding and an associated non-electrostatic modification of the gating. To quantify the non-electrostatic effect we introduced a slowing factor, A. A fourth mechanism (D) is binding to the pore with a consequent pore block, and could be a special case of Mechanisms B or C. A further classification considers whether the metal ion affects a single site or multiple sites. Analysing the properties of these mechanisms and the vast number of studies of metal ion effects on different voltage-gated ion channels we conclude that group 2 ions mainly affect channels by classical screening (a version of Mechanism A). The transition metals and the Zn group ions mainly bind to the channel and electrostatically modify the gating (Mechanism B), causing larger shifts of the steady-state parameters than the group 2 ions, but also different shifts of activation and deactivation curves. The lanthanides mainly bind to the channel and both electrostatically and non-electrostatically modify the gating (Mechanisms B and C). With the exception of the ether-à-go-go-like channels, most channel types show remarkably similar ion-specific sensitivities.

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.

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

2017 ◽  
Vol 95 (1) ◽  
pp. 45-50 ◽  
Author(s):  
Ik-Hwan Um ◽  
Ji-Sun Kang ◽  
Julian M. Dust
Keyword(s):  
Alkali Metal ◽  
Metal Ion ◽  
Order Rate Constant ◽  
Substitution Reactions ◽  
First Order ◽  
Order Rate ◽  
Metal Ion Catalysis ◽  
Upward Curvature ◽  
Pseudo First Order ◽  
Ion Effects

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.

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