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Predominant role of basicity of leaving group in α-effect for nucleophilic ester cleavage
It has been found that α-effects in nucleophilic reactions, unexpectedly large nucleophilicity due to adjacent unpaired electrons, are strongly dependent on the structure of substrate. The nucleophilic cleavages of 4-nitrobenzoate esters and 4-methylbenzo
Nomura, Yasuo,Kubozono, Takayasu,Hidaka, Makoto,Horibe, Mineko,Mizushima, Naoki,Yamamoto, Nobuyuki,Takahashi, Toshio,Komiyama, Makoto
p. 26 - 37
(2007/10/03)

The mechanism of the metal ion promoted cleavage of RNa phosphodiester bonds involves a general acid catalysis by the metal aquo ion on the departure of the leaving group
A series of uridine 3′-alkyl phosphates and 3′-aryl phosphates were synthesised and their cleavage was studied in the presence of Zn2+ aquo ions. A βlg value was determined for the Zn2+ promoted cleavage of both types of compounds. Comparison of the results obtained to those reported previously for the cleavage of the same substrates in the absence of metal ion catalysts suggests that the alkyl leaving group departs as an alcohol in the presence of metal ion catalysts. Furthermore, metal ion catalysts seem to enhance the departure. The aryl leaving group, in contrast, departs as an oxyanion.
Mikkola, Satu,Stenman, Eeva,Nurmi, Kirsi,Yousefi-Salakdeh, Esmail,Stroemberg, Roger,Loennberg, Harri
p. 1619 - 1625
(2007/10/03)

Acylphosphonate hemiketals - Formation rate and equilibrium. The electron-withdrawing effect of dimethoxyphosphinyl group
Examination of alcoholic solutions of dimethyl acetylphosphonate (1) and dimethyl benzoylphosphonate (2) by 31P NMR spectroscopy reveals the presence of considerable amounts of hemiketals. Because of the great difference between the 31P chemical shifts of acylphosphonates (ca. 0 ppm) and their hemiketals (17-21 ppm), 31P NMR spectroscopy is a uniquely suitable method for studying the rates and equilibria of hemiketal formation of acylphosphonates with different alcohols. The equilibrium constants Kf, K′f (K′f = Kf[ROH]), pseudo-first-order rate constants k′f, the second order rate constants, kf for hemiketal formation from dimethyl acetylphosphonate with various alcohols, as well as the reverse reaction rate constants, kr to starting materials, were determined. The kinetic isotope effect of 2.8 for the forward reaction kf (EtOH addition) and the backward reaction kr indicates a general catalysis pathway. On the other hand, the calculated values of the enthalpies of activation ΔH? = 10.37 kcal mol-1 (forward), ΔH? = 13.66 kcal mol-1 (backward) and the entropies of activation, ΔS? = -17.25 cal mol-1 K-1 (forward), ΔS? = -9.82 cal mol-1 K-1 (backward) are not in accord with high molecularity of the reaction (1 cal = 4.184 J). Our analysis led to the conclusion that this is probably due to the fact that the transition state is mainly reactant-like with the development of only limited extent of bond formation. Various plausible reaction pathways for hemiketal formation are discussed. In addition, we have calculated the value of 2.65 σ* for the P(O)(OMe)2 group based on proton affinity obtained from heats of formation (ΔHf) of applying the MNDO techniques. The following linear correlation between pKa values and PA values of hemiketals of the form (Me)(R)C(OH)(OCH2X) was developed: pKa = PA - 356.58 + 9.18 [σ*(Me) + σ*(R) + 0.2σ*(X)].
Katzhendler, Jehoshua,Ringel, Israel,Karaman, Rafik,Zaher, Hisham,Breuer, Eli
p. 341 - 349
(2007/10/03)

Intramolecular Nucleophilic Reactions of Dialkyl 1,1-Dichloro-2-hydroxyethylphosphonates
Methods are developed for preparing dialkyl (1,1-dichloro-2-hydroxyethyl)phosphonates. Under the action of sodium hydride these esters enter into intramolecular nucleophilic reactions via two routes yielding α-ketophosphonates and phosphates.
Guseinov,Klimentova,Kol'tsova,Egereva,Moskva
p. 441 - 443
(2007/10/03)

Cyclization-Activated Prodrugs: N-(Substituted 2-hydroxyphenyl and 2-hydroxypropyl)carbamates Based on Ring-Opened Derivatives of Active Benzoxazolones and Oxazolidinones as Mutual Prodrugs of Acetaminophen
N-(Substituted 2-hydroxyphenyl)- and N-(substituted 2-hydroxypropyl)carbamates based on masked active benzoxazolones (model A) and oxazolidinones (model B), respectively, were synthesized and evaluated as potential drug delivery systems.A series of alkyl and aryl N-(5-chloro-2-hydroxyphenyl)carbamates 1 related to model A was prepared.These are open drugs of the skeletal muscle relaxant chlorzoxazone.The corresponding 4-acetamidophenyl ester named chloracetamol is a mutual prodrug of chloroxazone and acetaminophen.Chlorzacetamol and two other mutual prodrugs of active bezoxazolones and acetaminophen were obtained in a two-step process via condensation of 4-acetamidophenyl 1,2,2,2-tetrachloroethyl carbonate with the appropiate anilines.Based on model B, two mutual prodrugs of acetaminophen and active oxazolidinones (metaxalone and mephenoxalone) were similarly obtained using the appropiate amines.All the carbamate prodrugs prepared were found to release the parent drugs in aqueous (pH 6-11) and plasma (pH 7.4) media.The detailed mechanistic study of prodrugs 1 carried out in aqueous medium at 37 deg C shows a change in the Broensted-type relationship log t1/2 vs pKa of the leaving groups ROH: log t1/2 = 0.46pKa - 3.55 for aryl and trihalogenoethyl esters and log t1/2 = 1.46pKa - 16.03 for alkyl esters.This change is consistent with a cyclization mechanism involving a change in the rate-limiting step from formation of a cyclic tetrahedral intermediate (step k1) to departure of the leaving group ROH (step k2) when the leaving group ability decreases.This mechanism occurs for all the prodrugs related to model A.Regeneration of the parent drugs from mutual prodrugs related to model B takes place by means of a rate-limiting elimination-addition reaction (E1cB mechanism).This affords acetaminophen and the corresponding 2-hydroxypropyl isocyanate intermediates which cyclize at any pH to the corresponding oxazolidinone drugs.As opposed to model A, the rates of hydrolysis of mutual prodrugs of model B clearly exhibit a catalytic role of the plasma.It is concluded from the plasma studies that the carbamate substrates can be enzymatically transformed into potent electrophiles, i.e., isocyanates.In the case of the present study, the prodrugs are 2-hydroxycarbamates for which the propinquity of the hydroxyl residue and the isocyanate group enforces a cyclization reaction.This mechanistic particularity precludes their potential toxicity in terms of potent electrophiles capable of modifying critical macromolecules.
Vigroux, Alain,Bergon, Michel,Zedde, Chantal
p. 3983 - 3994
(2007/10/03)

Hydrolysis of Nitrite Esters: Putative Intermediates in the Biotransformation of Organic Nitrates
Study and comparison of the pH-independent hydrolysis of eight alkyl nitrites shows 3-nitroso-1,2-glyceryl dinitrate, a putative intermediate in the biotransformation of glyceryl trinitrate, to be unexpectedly reactive and too labile to be detected as a biotransformation intermediate in aqueous solution, suggesting a role for neighbouring group participation by the β-nitrate group.
Buckell, Felicity,Hartry, Jeffrey D.,Rajalingam, Umarani,Bennett, Brian M.,Whitney, Ralph A.,Thatcher, Gregory R. J.
p. 401 - 404
(2007/10/02)

Evidence for a concerted mechanism in the solvolysis of phenyldimethylsilyl ethers
The trifluoroethoxide-catalyzed trifluoroethanolysis and the hydroxide-catalyzed hydrolysis of a series of phenyldimethylsilyl ethers were examined. A Bronsted plot of the logarithm of the second-order rate constant KTFE for reaction with trifluoroethanol against the pKLG is not linear. The nonlinear plot might be taken as evidence for a change in rate-determining step of a reaction that proceeds through a pentavalent intermediate. However, the Bronsted plot for the hydroxide-catalyzed hydrolysis, where all the leaving groups are of lower pKa than hydroxide, has an identical shape as the Bronsted plot for the trifluoroethanolysis reaction. Therefore, the unusual shape of the Bronsted plots is not due to a change in rate-determining step. It is suggested that the results are most consistent with a one-step concerted mechanism and not with a mechanism involving a pentavalent intermediate.
Xu, Yiying,Dietze, Paul E.
p. 10722 - 10727
(2007/10/02)

General base catalysis of ester hydrolysis
The hydrolysis of alkyl formates with leaving groups in the range pKa = 12-16 is catalyzed by substituted acetate anions. There is an increase in the Br?nsted β value for general base catalysis with decreasing pKa of the leaving alcohol and a complementary increase in -β1g with decreasing pKa of the catalyzing base, both of which are consistent with a value of pxy = ?β/-pK1g = ?β1g/-?pKBH ? 0.11. This result supports a class n mechanism of general base catalysis, in which a proton is abstracted from the nucleophilic water molecule by the base catalyst in the transition state; it is not consistent with the kinetically equivalent class e mechanism of electrophilic catalysis by general acids of a reaction with hydroxide ion, by proton donation to the leaving alcohol. Solvent deuterium isotope effects in the range kH2O/kD2O = 3.6-5.3 for the buffer-independent reaction and 2.5-2.8 for catalysis by CHaCOO- support concerted proton transfer and O-C bond formation. The secondary isotope effect for catalysis of the hydrolysis of LCOOMe by acetate ion is kD/kH = 1.05. Both nucleophilic and general base mechanisms of catalysis by acetate anions are observed for the hydrolysis of substituted phenyl formates with leaving groups of pKa = 7.1-10.1. A small value of β = 0.12 for general base catalysis of the hydrolysis of phenyl formate and p-methylphenyl formate represents catalysis of the addition of water by hydrogen bonding of water to the base catalyst. On the other hand, a larger value of β = 0.35 and a decrease in kH2O/kD2O to 1.2 were observed for general base catalysis of the hydrolysis of p-nitrophenyl formate. It is suggested that the increase in β with decreasing pK1g (an apparent "anti-Hammond effect") may be accounted for by a change in mechanism, from catalysis of a stepwise reaction of phenyl and p-methylphenyl formates to concerted general base catalysis of formyl transfer to water for the reaction of p-nitrophenyl formate.
Stefanidis, Dimitrios,Jencks, William P.
p. 6045 - 6050
(2007/10/02)

Reactivity of Nucleophilic Nitrogen Compounds towards the Nitroso Group
We discuss the reactivity of 43 nucleophilic nitrogen compounds towards the nitroso group of N-methyl-N-nitrosotoluene-p-sulfonamide (MNTS), and in some cases with alkyl nitrites.The series of nucleophiles considered is structurally very varied, includes members exhibiting the alpha effect, and covers 8 pKa units and a range of reactivities of almost five orders of magnitude.The values of solvent isotope effects and activation parameters have been measured and throw light on the structure of the transition states involved.Reactivities do not correlate well with thebasicity of the nucleophile, largely owing to the behaviour of primary amines, ammonia and nucleophiles with an alpha effect.Application of the curve crossing model suggests a relationship with vertical ionization potentials.The relationship with Ritchie's N+ scale is discussed, and interesting correlations with the reactivities of the same nucleophiles in various other chemical processes are noted.
Garcia-Rio, Luis,Iglesias, Emilia,Leis, J. Ramon,Pena, M. Elena,Rios, Ana
p. 29 - 37
(2007/10/02)

Laser Photolysis/Laser-Induced Fluorescence Studies of the Reaction of OH with 1,1-Dichloroethane over an Extended Temperature Range
Absolute rate coefficients are determined for the gas-phase reaction of OH radicals with 1,1-dichloroethane over an extended temperature range using a laser photolysis/laser-induced fluorescence technique.Experiments were performed in a flow system at a total pressure of 740 +/- 10 Torr using He as diluent and carrier gas.The rate coefficients, obtained over the temperature range 294-800 K, exhibited pronounced non-Arrhenius behavior and were best described by the modified Arrhenius equation k(T)=(8.29 +/- 0.36) x 10-14(T/300)2.67exp cm3 molecule-1 s-1.Comparison of the data with one previous room-temperature measurement is presented.The temperature dependence of the data is compared with empirical and transition-state model calculations.The influence of C-H bond energy and Cl substitution is discussed.
Jiang, Zhen,Taylor, Philip H.,Dellinger, Barry
p. 8964 - 8966
(2007/10/02)

Acid- and Base-Catalysed Decomposition of Acetaldehyde Hydrate and Hemiacetals in Aqueous Solution
Rate constants and structure-reactivity parameters for general acid and general base catalysis of the reactions of acetaldehyde with water and alcohols at 25 deg C and ionic strength 1.0 (KCl) were determined by trapping the carbonyl group formed upon cleavage of acetaldehyde hydrate or hemiacetal with semicarbazide or hyrazine.The Broensted coefficients of α=0.50-0.59 for general acid catalysis by a class e mechanism are larger than those for the corresponding reactions with formaldehyde.The difference is described by a normalized interaction coefficient pxy'=0.067=δα/-δ?=δρ/δpKHB.The increase in α with decreasing pKa of the alcohol is described by the coefficient pxy=0.020=δα/-δpKlg=δβlg/-δpKHB.A small increase in βlg for the acetaldehyde compared with formaldehyde reactions corresponds to a normalized coefficient pyy'=0.014=Δβlg/-δ?=δr/-δpKlg.These coefficient are consistent with a concerted reaction mechanism and a largely diagonal reaction coordinate on an energy contour diagram that is defined by the structure-reactivity parameters; there may be a larger vertical than horizontal component to the reaction coordinate.A stepwise mechanism is excluded by a requirement for rate constants that would have to be in the range 1E13-1E18 s-1 for proton transfer and for decomposition of the dipolar addition species R.Extrapolated rate constants for the cleavage of R in the range 1E20-1E23 s-1 suggest that the concerted mechanism is enforced by the absence of a significant lifetime for R.The general base catalyzed reactions show a decrease in βlg and an increase in β for acetaldehyde compared with formaldehyde that correspond to normalized values of pyy'=-0.052=δβlg/-δ?=δρ/-δpKlg and pxy=0.042=δβ/-δρ=δ?/-δpKBH.The increase in Broensted β values with decreasing pKa of the leaving group and the increase in βlg with decreasing catalyst pKa are described by an interaction coefficient pxy'=0.07=δβ/-δpKlg=δβlg/-δpKBH, and upward curvature in the dependence of log k on the pKa of the leaving group (an "anti-Hammond effect") is described by a value of py'=-0.20=δβlg/-δpKlg.These interaction coefficients and a value of px=0=δβ/-δpKBH from the linear Broensted plots correspond to a reaction coordinate that is rotated 57 deg clockwise from the vertical on an energy contour diagram that is defined by the structure-reactivity parameters, with β for proton transfer on the x axis.The results support a fully concerted reaction mechanism with an important component of proton transfer in the transition state.Rate constants for "water-catalyzed" reaction are consistent with those for buffer bases; there is no evidence for a cyclic mechanism involving proton transfer through water.Hydroxyde ion catalysis of the reaction represents specific base catalysis.The equilibrium constant for acetaldehyde hydration was found to be Kb=1.2+/-0.1 at 25 deg C, ionic strength 1.0.
Soerensen, Poul E.,Jencks, William P.
p. 4675 - 4690
(2007/10/02)

Catalysis by Undissociated H3PO4 in Aqueous H2PO4-/HPO42- Buffer Solutions: Dependence on The Magnitude of the Broensted Exponent
An expression is derived which relates catalysis by undissociated H3PO4 in aqueous H2PO4-/HPO42- buffer solutions of pH ca. 7 to buffer ratio and the Broensted exponent α for the system.This expression predicts that H3PO4 catalysis will be significant when α is near unity but will have reached negligible proportions by the time α has dropped to 0.6-0.7.These predictions are borne out by experimental data for the hydrolysis of several 2- alkoxy-2-phenyl-1,3-dioxolanes.
Chiang, Y.,Kresge, A. J.,Do, S. Van,Weeks, D. P.
p. 4035 - 4037
(2007/10/02)

Equilibrium Constants for the Interconversion of Substituted 1-Phenylethyl Alcohols and Ethers. A Measurement of Intramolecular Electrostatic Interactions
Equilibrium constants for the reactions of ring-substituted 1-phenylethyl alcohols with a series of aliphatic alcohols of pKa 12.4-16 to form the corresponding ethers, and for interconversion of the ethers, have been determined in 50:45:5 HOH/CF3CH2OH/ROH (v/v/v), μ = 0.5 (NaCIO4), at 25 deg C.Formation of ethers from the alcohols is favorable, with values of K = 3-74; replacement of water by methanol is favored by factors of 50-74.Equilibrium constants increase with increasing pKa of the alcohol with values of βeq = δlog K/δpKROH in the range 0.17-0.27.This is attributed to hydrogen bonding of the alcohol to the solvent and to an electrostatic interaction between substituents on the alcohol and the aryl group.The contribution from hydrogen bonding to the solvent is estimated to be β = 0.17; for 90percent HOH it is 0.25.An increase in βeq with electron-withdrawing substituents on the benzene ring and a complementary increase in ρeq with electron-donating substituents on ROH are described by an electrostatic interaction coefficient τ = δβeq/δ? = δρeq/δpKROH = 0.10 +/- 0.01.No change in τ for dipole-dipole interactions was observed with increasing water concentration in the range 50-90percent (v/v).The electrostatic interactions that are described by τ can cause changes in structure-reactivity parameters, such as ρ or β, in the absence of changes in transition-state structure.
Rothenberg, Marc E.,Richard, John P.,Jencks, William P.
p. 1340 - 1346
(2007/10/02)

Reaction Mechanism from Structure-Energy Relations. 1. Base-Catalyzed Addition of Alcohols to Formaldehyde
There is a long-standing evidence that in the general base catalyzed addition of alcohols to formaldehyde, C...O bond formation and proton transfer occur simultaneously.Structure-energy relations for this reaction are complicated.The slopes of Bronsted plots vary greatly, even for structurally similar alcohols, and plots of log k vs. pKa of the alcohols go through minima.The data thus provide good material for testing the author's recent theory of structure-energy relations for concerted reactions.The theoretical equations are nonlinear and depend specifically on the reaction mechanism.Of four mechanisms, only that favored by previous independent work fits the data well, even onto reproducing the rate-constant minima.The rate-determining step of this mechanism is R'CH2COO(-) + RCH2OH + H2C=O -> R'CH2COOH + RCH2OCH2O(-).R ranges in electronegativity from CH3 to CF3, R' from H to CN.Assuming this mechanism to be correct, progress of C...O bond formation (u) and proton transfer (v) were deduced from the theoretical equations.The results show that mean progress at the transition state, (u + v)/2, varies only sightly with substitution, but that disparity of progress of the two reaction events, (v-u)/2, varies markedly both with R and R' and changes sign within the series.Rate-constant minima occur near points where u = v, thus proving the effectiveness of disparity at lowering the free energy of activation.
Grunwald, Ernest
p. 4710 - 4715
(2007/10/02)

Reaction Mechanism from Structure-Energy Relations. 2. Acid-Catalyzed Addition of Alcohols to Formaldhyde
Previous theory of structure-energy relations is extended to mechanisms with three concerted reaction events.Data for 25 reactions (five alcohols, five acid catalysts) are examined on the basis of four different mechanisms.Only one of the mechanisms fits well.It involves concerted C...O bond formation, proton donation by the acid catalyst, and proton acceptance by a water molecule, according to H2O + HOCH2R + H2C=O + HOOCCH2R' -> H2OH(+) + RCH2OCH2OH + (-)OOCCH2R'.The fit for this mechanism is decisively good in three practically independent tests.The contrast between the present mechanism and that which fits the base-catalyzed reaction (which does not involve HOH as a reagent) can be explained as due to peculiarities of relative acid-base properties of RCH2OCH2OH and H2O.No inconsistency appears with other data.Transition-state coordinates for the acid-catalyzed reaction are tabulated.Progress of C...O bond formation, though variable within the reaction series, is well ahead of that of the proton transfers.Further analysis of the theoretical free-energy surfaces indicates that disparity of progress of the concerted reaction events reaches a maximum at the transition state.
Grunwald, Ernest
p. 4715 - 4720
(2007/10/02)