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The pH-rate profiles for the Veto-enol tautomerization of 17 β-Veto esters and amides (RCOCH 2 COX: R=methyl; phenyl; 2-, 3-, and 4-pyridyl; 3(and 4)-(N-methylpyridinio); X=OCH 3 , OC 2 H 5 , NH 2 , or N(CH 3 ) 2 ) have been measured by stopped-flow spectrophotometry in aqueous solution (ionic strength 0.1, 25 o C) over the range pH=2-12. Analysis of these profiles gives the microscopic rate constants for Vetonization and enolization of each of these species in these aqueous solutions. Analysis of the pH dependence of the buffer catalysis for the general-acid-catalyzed protonation of these enolate conjugate bases allowed the evaluation of pK a E for the deprotonation of each enol species

Second-order rate constants ( k 2 ) have been measured for the oxidation of 1-benzyl-1,4-dihydronicotinamide by the N -methyl pyrazinium, 3-aminocarbonylpyrazinium, quinoxalinium, and phenazinium cations in 20% acetonitrile-80% water at ionic strength 1.0 and 25°C. These rate constants are also shown to be independent of ionic strength, but to decrease by a factor of >10-fold as the solvent polarity is decreased in aqueous acetonitrile mixtures containing from 1 to 70% acetonitrile (v/v). The reactivities ( k N 2 ) of the above pyrazinium and benzopyrazinium cation oxidants are much greater than those ( k C 2 ) of the corresponding isoelectronic pyridinium and benzopyridinium cations. Both k N 2 and k C 2 increase dramatically in the order monocyclic k N 2 / k C 2 ) decrease systematically from 8000 to 200 with increasing reactivity in these series of cations. The current studies directly demonstrate that the reactivities of 5-deazaflavins toward reduction are indeed good chemical models for the reduction of the analogous isoelectronic flavin species.

The rates of the elimination reactions of N-(2-bromoethyl) pyridinium cations (1) and N,N′-ethylene bispyridinium dications (3) to give the corresponding N-vinyl pyridinium cations (2) have been me...

The rates of equilibration of N-(2-cyanoethyl) pyridinium cations (1) with the corresponding pyridines and acrylonitrile have been measured in aqueous solutions of ionic strength 0.1 at 25 o C. Second-order rate constants (k OH ) have been obtained for the hydroxide ion catalyzed elimination reactions of 16 ring-substituted 1 having pyridine leaving groups of pK BH in the range 1.5-9.7. Bronsted plots of log k OH vs pK BH are «concave down» with two distinct linear regions having β lg =−0.30 (for pK BH 5.8). This observation is consistent with a change in rate-determining step within an E1cb reaction mechanism from rate-determining deprotonation of 1 (i.e., (E1cb) irrev ) for pK BH 5.8

The pKa values for the deprotonation of a series of eight 1-(X-benzyl)-4-phenylacetylpyridinium cations (6) have been measured in aqueous solutions of ionic strength 0.1 at 25 °C: pKa = −0.18σ + 8.91. The pseudo-first-order rate constants for deprotonation of these carbon acids have been measured over the range pH = 11–13, and have been found to display kinetic saturation effects that are consistent with the addition of hydroxide ion to the carbonyl group (pKz) as the product of kinetic control upon basification of neutral aqueous solutions of these pyridinium cations, with the subsequent transformation of this anionic hydrate to the thermodynamically more stable enolate conjugate base. Analysis of the pH–rate profiles gives substituent effects upon pKz (ρ = −0.19) and upon the second-order rate constant (kOH (ρ = 0.09)) for deprotonation of 6 by hydroxide ion. Key words: carbon acids, deprotonation, pKa, kinetics, substituent effects.

The pK, values for the deprotonation of a series of 4-(X-phenylsulfonylrnethy1)pyridines (6) (pK, = 19.89 (X = H); p = 3.0) were determined in aqueous dirnethyl sulfoxide solutions at 25°C using the Hoq acidity function. The pK, values were also measured for the corresponding series of N-methyl 4-(X-phenylsulfonylmethy1)pyridiniurn cations (2) (pK, = 11.27 (X = H); p = 1.45) and also for a series of N-(X-benzyl) 4-phenylsulfonylrnethylpyridinium cations (7) (pK, = 10.70 (X = H); p = 0.65) in aqueous solution (ionic strength 0.1 at 25'C). Comparison of the substituent effects upon the pK, values of the sulfonyl-activated carbon acids 2, 6, and 7 with the substituent effects upon the pK, values of the corresponding three series of ketones gives insight into the electron-density distribution in the carbanionic conjugate bases of sulfone and ketone carbon acids. Extrapolation of a linear free energy relationship between the pK, values of neutral sulfones and ketones allows the estimation of pK, = 28.7 for the deprotonation of methyl phenyl sulfone in aqueous solution.

The pKa values for the deprotonation of a series of 4-(X-phenylsulfonylmethyl)pyridines (6) (pKa = 19.89 (X = H); ρ = 3.0) were determined in aqueous dimethyl sulfoxide solutions at 25 °C using the H0q acidity function. The pKa values were also measured for the corresponding series of N-mefhyl 4-(X-phenylsulfonylmethyl)pyridinium cations (2) (pKa = 11.27 (X = H); ρ = 1.45) and also for a series of N-(X-benzyl) 4-phenylsulfonylmethylpyridinium cations (7) (pKa = 10.70 (X = H); ρ = 0.65) in aqueous solution (ionic strength 0.1 at 25 °C). Comparison of the substituent effects upon the pKa values of the sulfonyl-activated carbon acids 2, 6, and 7 with the substituent effects upon the pKa values of the corresponding three series of ketones gives insight into the electron-density distribution in the carbanionic conjugate bases of sulfone and ketone carbon acids. Extrapolation of a linear free energy relationship between the pKa values of neutral sulfones and ketones allows the estimation of pKa = 28.7 for the deprotonation of methyl phenyl sulfone in aqueous solution.

Rates of equilibration of a series of 10 substituted pyridines and five Michael acceptors (CH 2 =CHZ, Z=CHO, COCH 3 , SO 2 CH 3 , CN, and CONH 2 ) with the corresponding N(ZCH 2 CH 2 ) pyridinium cations have been measured in aqueous solution at ionic strength 0.1 and 25 o C

The rates of the elimination reactions of N-(2-bromoethyl) pyridinium cations (1) and N,N′-ethylene bispyridinium dications (3) to give the corresponding N-vinyl pyridinium cations (2) have been measured spectrophotometrically in basic aqueous solutions (ionic strength 0.1, 25 °C) for a variety of substituents in the pyridine rings of each of these classes of pyridinium cation. The reaction kinetics are first order in 1 or 3 and first order in hydroxide ion. Brønsted-type plots of the second-order rate constants (kOH) as a function of the basicity (as pKBH) of the corresponding substituted pyridine are nonlinear for each of 1 and 3 and can be interpreted in terms of E1cb reaction mechanisms. For 1, the Brønsted-type plot displays two distinct "concave down" linear regions; rate-determining deprotonation for pKBH BH > 5.16 (slope −0.58). For 3, the Brønsted-type plot appears to be smoothly curved for symmetrically disubstituted bispyridinium dications, as a consequence of the multiple substituent effects upon each step of the E1cb reactions of these dications. However, log kOH for 3 is a smooth linear function of the previously reported log kOH for the E1cb reactions of N-(2-cyanoethyl) pyridinium cations over a range in which a change in rate-determining step has been directly demonstrated for these latter cations. Thus a change in rate-determining step as a function of pyridine basicity is also required within the E1cb mechanism for 3. The E1cb reactions of 1 are approximately 104-fold faster than the corresponding hydroxide ion catalyzed E2 eliminations from 2-phenylethyl bromides that are isoelectronic with 1.

Recent work on the reduction of heteroaromatic cations by 1,4-dihydronicotinamides and related reducing agents is reviewed. Extensive correlations are presented between the second-order rate constants (k2) for these reactions and the second-order rate constants (kOH) and equilibrium constants (pKR+) for hydroxide ion attack on these cations. Close correlations of log k2 with the electron affinities and one-electron reduction potentials of these cations are also presented. These relationships are considered in the context of a direct hydride transfer from donor to acceptor and also in terms of SET mechanisms which are also commonly discussed for such reactions. It is shown that the interpretation of these formal hydride transfer reactions in terms of an imbalanced development of electronic charge and CH bond fission within the transition state species leads to a rational merging of the single-step hydride transfer mechanism and the SET mechanisms. The structures of the transition state species are expected to be highly variable and quite dependent upon the nature of the hydride donor and acceptor species, with considerable contribution from charge-transfer interactions. Such imbalanced transition state species are analyzed in terms of two different types of reaction coordinate diagrams and also in terms of the valence bond configuration mixing theory.

The pKa values for the deprotonation of a series of eight 1-(X-benzyl)-4-phenylacetylpyridinium cations (6) have been measured in aqueous solutions of ionic strength 0.1 at 25 °C: pKa = −0.18σ + 8.91. The pseudo-first-order rate constants for deprotonation of these carbon acids have been measured over the range pH = 11–13, and have been found to display kinetic saturation effects that are consistent with the addition of hydroxide ion to the carbonyl group (pKz) as the product of kinetic control upon basification of neutral aqueous solutions of these pyridinium cations, with the subsequent transformation of this anionic hydrate to the thermodynamically more stable enolate conjugate base. Analysis of the pH–rate profiles gives substituent effects upon pKz (ρ = −0.19) and upon the second-order rate constant (kOH (ρ = 0.09)) for deprotonation of 6 by hydroxide ion. Key words: carbon acids, deprotonation, pKa, kinetics, substituent effects.

Second-order rate constants (k B ) have been measured in aqueous solution at ionic strength 0.1 and 25 o C for the general-base-catalyzed deprotonation of eight 1-eight 1-methyl-4-(X-phenacyl) pyridinium cations, eight 1-(X-benzyl)-4-phenacylpyridinium cations, and five 1-benzyl-4-(X-phenacyl) pyridinium cations by a variety of primary amines and also N-methyl- and N,N-dimethylbenzylamine

The rates of equilibration of N-(2-cyanoethyl) pyridinium cations (1) with the corresponding pyridines and acrylonitrile have been measured in aqueous solutions of ionic strength 0.1 at 25 o C. Second-order rate constants (k OH ) have been obtained for the hydroxide ion catalyzed elimination reactions of 16 ring-substituted 1 having pyridine leaving groups of pK BH in the range 1.5-9.7. Bronsted plots of log k OH vs pK BH are «concave down» with two distinct linear regions having β lg =−0.30 (for pK BH 5.8). This observation is consistent with a change in rate-determining step within an E1cb reaction mechanism from rate-determining deprotonation of 1 (i.e., (E1cb) irrev ) for pK BH 5.8

The reaction of acetone with four heteroaromatic cations (10-methylacridinium (1), 3-aminocarbonyl-1-methylquinolinium (2a), 3-cyano-1-methylquinolinium (2b), and 3-bromo-1-methylquinolinium (2c)) has been investigated in basic aqueous solutions (pH 9–12, ionic strength 0.1, 25 °C). For each of 2a and 2b, the kinetically controlled product is a 35:65 mixture of the C-2 and C-4 enolate ion adducts; the C-2 adduct subsequently isomerizes to give the C-4 adduct as the only observable species under thermodynamic control. For 2c, the C-2 enolate adduct appears to be favoured both kinetically and thermodynamically. Under kinetic control, the pH-dependence of adduct formation from each cation is consistent with rate-determining attack of the enolate ion upon the heterocyclic cation. Comparisons of regiochemical control of acetone enolate ion attack with hydroxide ion attack upon these same cations indicate that acetone enolate ion shows a more pronounced preference for C-4 attack over C-2 attack than does hydroxide ion. The thermodynamically controlled regiochemistry is similar for each of these two nucleophiles. Keywords: nucleophilic addition, regioselectivity, kinetic control, thermodynamic control, quinolinium cations.

The pH-dependences of the apparent second-order rate constants [Formula: see text] for the reduction of 2,4,6-cycloheptatrien-1-ol and 9-xanthydrol by each of 1-benzyl-1,4-dihydronicotinamide (BNH) and 10-methyl-9,10-dihydroacridine (MAH) have been measured in 20% acetonitrile – 80% water, at 25 °C and ionic strength 1.0. For each of these reactions, the pH-dependence of [Formula: see text] is only consistent with reduction occurring via the aromatic cation (either tropylium or xanthylium) that is present in equilibrium with these alcoholic species. The relative second-order rate constants [Formula: see text] for reductions by these two reducing agents (1700 for tropylium and 770 for xanthylium) are similar for these two cations. These ratios are also similar to those observed for a variety of nitrogen heteroaromatic hydride acceptors, even though the absolute magnitudes of these rate constants vary by 1010-fold. The second-order rate constants for the reductions of the tropylium and xanthylium cations are predicted reasonably well by their [Formula: see text] values, with the latter cation being (7 × 105)-fold more reactive than its π-isoelectronic N-methyl acridinium cation. The xanthylium cation has the greatest [Formula: see text] ratio yet observed for any heteroaromatic cation, and this value further extends the known range of this ratio as a function of reactivity. Keywords: hydride transfer, kinetics of reduction, 1,4-dihydropyridine derivatives, tropylium cation, xanthylium cation.

Effets des substituants sur l'equilibre et la cinetique de formation de l'ion enolate de huit (X-phenacyl-4) pyridines, huit (methyl-1 X-phenacyl-4) pyridiniums, cinq (benzyl-1 X-phenacyl-4) pyridiniums, et huit (X-benzyl-1 phenacyl-4) pyridiniums

The pH-dependences of the apparent second-order rate constants for the reduction of 2,4,6-cycloheptatrien-1-ol and 9-xanthydrol by each of 1-benzyl-1,4-dihydronicotinamide (BNH) and 10-methyl-9,10-...

Effets des substituants sur l'equilibre et la cinetique de formation de l'ion enolate de huit (X-phenacyl-4) pyridines, huit (methyl-1 X-phenacyl-4) pyridiniums, cinq (benzyl-1 X-phenacyl-4) pyridiniums, et huit (X-benzyl-1 phenacyl-4) pyridiniums

Second-order rate constants (KNu) have been measured for the addition of 44 primary amines (including five α-effect amines), 28 secondary amines, 19 tertiary amines, ammonia and hydroxide ion to the vinyl group of the 1-methyl-4-vinylpyridinium cation (1) in aqueous solution at 25° C (ionic strength 0.1 mol dm–3). Nucleophilic attack is shown to be rate-determining for primary and secondary amines, with secondary amines being generally more reactive than primary amines of the same basicity. After classification of these species in terms of structure, they describe a number of Bronsted-type correlations having βnuc in the range 0.35–0.54 for six structural classes of primary amine, βnuc= 0.48 for α-effect amines, and βnuc in the range 0.23–0.34 for four structural classes of secondary amine. Substitution upon the α-carbon atom reduces amine nucleophilicity of both primary and secondary amines. The presence of an unsaturated carbon atom (either sp2- or sp-hybridized) as the β-carbon atom leads to an enhanced reactivity relative to the corresponding β-sp3 species in all cases. Tertiary amines are in general less reactive than other amines of the same basicity. Bronsted-type plots for tertiary amines present the appearance of random scatter which is not readily decipherable in terms of structure. β-Hydroxy and β-amino tertiary amines are unusually reactive relative to their basicity. All of these phenomena suggest that protonation of the carbanionic intermediate by a molecule of water is the rate-determining step for the addition of tertiary amines to 1.Rate constants for the attack of primary and secondary amines on 1 are shown to correlate with literature data for a variety of other reactions involving rate-determining nucleophilic attack of amines upon electrophilic carbon. These kNu for primary and secondary amines reacting with 1 are also shown to correlate with Ritchie's N+ parameters for nucleophilic attack at electrophilic sp2-carbon. N+ parameters for amine nucleophiles have not been widely available previously; the parameters that have been available for selected amines are known to be sensitive to the nature of the defining electrophile. The minimal steric hindrance at the electrophilic centre in nucleophilic attack upon 1 suggests that this species is an appropriate electrophile for the definition of N+ parameters for amine nucleophiles; these parameters are evaluated for 70 primary and secondary amines and ammonia and are suggested to provide an appropriate data base for future investigations of the reactivity and selectivity of amine attack upon sp2-carbon electrophiles in aqueous solution.

Second-order rate constants (kNu) have been measured in aqueous solution (I= 0.1 mol dm –3, 25 °C) for the SN2 reactions of methyl 4-nitrobenzenesulfonate with ammonia, 41 primary amines, 20 secondary amines. 29 tertiary amines and 7 anionic nucleophiles. For the aminolysis reactions, Bronsted-type correlations of nucleophilicity with basicity require the classification of all amines in terms of strictly defined structural classes with βnuc in the range 0.15–0.39. Swain–Scott plots indicate that simple amines, water and other light-atom nucleophiles (hydroxide, azide and cyanide, anions) are five times more reactive than heavy-atom nucleophiles (thiosulfate, thiocyanate, iodide and bromide ions). For amine nucleophiles there is a close linear correlation (of slope 0.44, and including both primary and secondary amines) between log kNu for the aminolysis of methyl 4-nitrobenzenesulfonate and log kNu for amine addition to the 1-methyl-4-vinylpyridinium cation. This correlation demonstrates a close linear relationship between the Swain–Scott n parameter and Ritchie's N+parameter for amine nucleophiles in aqueous solution (N+= 2.1n– 4.3).

The reversible inhibition of the hydrolysis of O-hippuryl-L-3-phenyllactic acid by bovine carboxypeptidase A, has been studied for a series of para-substituted benzoate ions (p-XC6H4-CO2−) at pH 7.5, 25°, ionic strength 0.2. For X = H, F, CN, NH2, CH3 competitive inhibition occurs, whereas non-competitive inhibition occurs for X = CF3, NO2, Cl, Br, (CH3)2N, CH3O, (CH3)2CH, (CH3)3C. For X = C2H5 mixed inhibition is observed and this can be separated into individual competitive and noncompetitive components. Uncompetitive inhibition occurs with X = I. The distinction between competitive and noncompetitive inhibition appears to depend on the size of X rather than on its chemical properties. The p-tolylacetate and 3-(p-tolyl)propanoate ions display partially competitive inhibition consistent with the formation of E.I2 species. The inhibition by the 3-(p-iodophenyl)propanoate ion is complex and depends on the binding of at least two inhibitor ions per enzyme molecule.

The kinetic and thermodynamic control of pseudobase formation from 3-W-1-methylquinolinium cations has been studied for a variety of substituents (W). Spectral data indicate that, in both aqueous and methanolic solution, the C-2 pseudobases predominate at equilibrium for W = H and Br, while the C-4 pseudobases are the thermodynamically preferred species for W = CONH2, CO2CH3, CN, and NO2. Stopped-flow studies indicate that in all cases the C-2 pseudobases are the kineticallycontrolled products upon basification of the aqueous solutions of these cations. Equilibrium constants (pKR+) have been measured for pseudobase formation at both C-2 and C-4 for each W in all cases where they are experimentally accessible. Substituent effects upon [Formula: see text] correlate with σm for W, while [Formula: see text] depends upon σp−. These substituent effects allow the prediction of [Formula: see text] and [Formula: see text] for the 1-methylquinolinium cation. Rates of C-2 to C-4 pseudobase equilibration have been measured in all cases where the latter species is thermodynamically more stable. These kinetic data allow the evaluation of rate constants for C-4 pseudobase equilibration with each cation. In all cases except W = CN, C-2 pseudobase formation is complete within the mixing time of the stopped-flow instrument.

Rates of cation–pseudobase equilibration have been measured for a series of C-9 substituted (CH3, CH3CH2, C6H5CH2, (CH3)2CH, C6H5, 4-(CH3)2NC6H4) 10-methylacridinium cations in aqueous solution at 25 °C over the pH range of approximately 9–13. Separation of the rate constants for formation (kOH) and decomposition (k2) of each of these pseudobases allowed the calculation of the [Formula: see text] value for each cation. The presence of a C-9 isopropyl substituent enhances the stability of the pseudobase relative to the cation, whereas all other C-9 substituted cations have [Formula: see text] values greater than the 10-methylacridinium cation. There is no simple quantitative relationship between [Formula: see text] and the size of the C-9 substituent. Rate constants (kOH) for hydroxide ion attack on these C-9 substituted cations are in the order: H > primary alkyl > secondary alkyl > aryl, while rate constants (k2) for pseudobase decomposition are less predictable but generally follow the order: H > primary alkyl > aryl > secondary alkyl. These phenomena are shown to be consistent with a competition between destabilization of the cation by peri interactions between the C-9 substituent and H(1) and H(8) and reduced pseudobase solvation for large C-9 substituents. Resonance interactions of 9-aryl substituents with the acridine moiety of these cations are shown to be quite small. 9-Aryl-10-methylacridinium cations are approximately 30-fold more reactive towards hydroxide ion attack than are their triphenylmethyl carbocation analogues.

The rates of oxidation of a series of N-(X-benzyl)-5-nitroisoquinolinium cations (8) to the corresponding 1-isoquinolinones have been investigated in 20% acetonitrile-water at 25/sup 0/C, ionic strength 1.0 over the range 0.02 to 1.0 M KOH. These reactions are strictly first order in both heterocycle and ferricyanide and are not inhibited by up to 12-fold excess of ferrocyanide ion over ferricyanide ion. The dependence of the rate of oxidation on (OH/sup -/) indicates that the rate-determining step involves ferricyanide attack on the alkoxide ion of the pseudobase derived from 8. The pH-independent second-order rate constant, k/sub 2//sup H/ is correlated with the Hammett sigma constants for the substituents X in the benzyl ring of 8: log k/sub 2//sup H/ = -1.29 sigma + 2.29. For the corresponding 1-deuterio cations the correlation line is log k/sub 2//sup D/ = -0.96 sigma + 1.93. The different rho values for k/sub 2//sup H/ and k/sub 2//sup D/ indicate a kinetic isotope effect (k/sub 2//sup H//k/sub 2//sup D/) which is dependent on the substituent X; this isotope effect varies from 1.41 for X = 4-CN to 3.25 for X = 4-CH/sub 3/O. The sign and magnitude of the rho values require close to a fullmore » unit positive charge on the ring nitrogen atom in the transition state, while the magnitude of the kinetic isotope effect is only consistent with C(1)-H bond breaking in this transition state. The only mechanism consistent with both of these requirements is rate-determining abstraction of hydride by ferricyanide ion to give the isoquinolinone and a species ''HFe(CN/sub 6//sup 4 -/'', which rapidly reacts with a second ferricyanide ion to give two ferrocyanide ions. The substituent-dependent isotope effect is readily rationalized in terms of a ''product-like'' transition state for hydride transfer. Possible structures for HFe(CN)/sub 6//sup 4 -/ are considered, and other reactions which probably also involve hydride transfer to ferricyanide are suggested. 5 tables, 6 figures.« less

The kinetics of hydrolysis of a series of 10 new hippurate esters (C6H5CONHCH2CO2CRR1CO2H (I)) by bovine pancreatic carboxypeptidase A have been investigated at pH 7.5, 25 °C, and ionic strength 0.5. Pronounced substrate inhibition was displayed by I: R = H, R1 = C6H5(CH2)2, 3-indolylmethyl, 4-HOC6H4CH2, and 4-FC6H4 whereas pronounced substrate activation was observed for I: R = H, R1 = 4-CH3C6H4, 4-C2H5C6H4, 4-C6H5C6H4, 1-naphthyl, 2-naphthyl, and R = R1 = C2H5. In all cases substrate activation and substrate inhibition were shown to be consistent with ES2 complex formation similar to that previously observed for other hippurate esters. Kinetic parameters were evaluated for each ester and it is noted that ail 13 hippurate esters now known to display substrate inhibition have kcat/Km > 106 M−1 min−1, whereas kcat/km 6 M−1 min−1 for all 9 hippurate esters known to display substrate activation. The enzymic specificity for the R1 unit of I suggests binding of R1 in a 'bent' hydrophobic pocket having a restricted entrance.

A series of para-substituted O-benzoyl-2-hydroxybutanoic acids (but not the unsubstituted ester) are hydrolyzed by bovine carboxypeptidase A (pH 7.5, ionic strength 0.5, 25°). For the CH3O, CH3, Cl, CN, and NO2 substituents, there exist linear correlations of kcat and Km with the Hammett σ constants for these substituents (log kcat = 1.1 7σ + 1.17; log Km = −0.53σ − 2.15), although the tert-butyl group shows significant deviations from both correlation lines. The above unsubstituted ester is a reversible inhibitor of the enzymic hydrolysis of O-hippuryl-L-3-phenyllactic acid and so the lack of observable hydrolysis of this ester is attributable to nonproductive binding. The pH–rate profiles for kcat/Km and kcat have been determined for the enzymic hydrolysis of O-(p-nitrobenzoyl)mandelic acid [Formula: see text] and O-hippuryl-L-3-phenyllactic acid [Formula: see text] For the latter ester kcat is pH independent in the range pH 5–10. The mechanism of ester hydrolysis catalyzed by carboxypeptidase A is discussed in the light of the above observations and the known crystal structure of the enzyme. A definition of specific and non-specific substrates for this enzyme based on the observed pH profiles for kcat/Km is proposed.

Rate and equilibrium constants for the reaction between acetone and the 2-methyl-5-nitroisoquinolinium cation to give 1-acetonyl-1,2-dihydro-2-methyl-5-nitroisoquinoline (2) have been evaluated over the pH range 10.0–11.3 in aqueous solutions at 25 °C. This reaction is shown to occur under much milder conditions (temperature, pH) and in much shorter reaction times than previously used for the synthesis of this adduct. Analogous data for the reaction of hexadeuteroacetone with this heterocyclic cation are also presented. The formation of 2 is shown to be first order in each of acetone, isoquinolinium cation, and hydroxide ion, and is not catalyzed by carbonate buffer species. These data are consistent with the rate-determining attack of the acetone enolate anion upon the isoquinolinium cation. The microscopic reverse of this reaction is the uncatalyzed decomposition of 2, which is consistent with the observed pH independence of the first-order rate constant for this process. Quantitative comparisons of rates and equilibria for the addition of hydroxide ion and acetone enolate ion to this isoquinolinium cation and to substituted benzaldehydes are now available.

The reactions of the dipyrido[1,2-a:2′,1′-c]pyrazinium dication (1) in basic solutions have been investigated. In aqueous base, this cation is involved in two rapid pH-dependent equilibration reactions associated with pKa values of 9.9 ± 0.2 and 10.4 ± 0.2. Spectral studies indicate that pKa 9.9 is associated with pseudobase formation by the addition of hydroxide ion at C-2 or C-4 of one of the pyridinium rings. The pKa 10.4 is most probably attributable to the formation of a dipseudobase by addition of two hydroxide ions in the pyrazinium ring of 1. The kinetics of these pseudobase formation reactions have been investigated by stopped–flow spectrophotometry over the range pH 9–13. Similar methoxide addition reactions occur in basic maethanolic solutions.The dication 1 also undergoes a slow decomposition reaction in aqueous base to initially form the 1-(formylmethyl)-2,2′-dipyridyl cation (2). The rate of this reaction, which has been measured as a function of pH at 25 °C for pH 8–12, passes through a maximum in the vicinity of pH 10.2. The cation 2 undergoes a further decomposition reaction in these solutions to give an unidentified product.

The kinetics of disproportionation and ferricyanide ion oxidation of the 10-methylacridinium cation have been measured spectrophotometrically over the pH range 9–14 in.20% CH3CN – 80% H2O (v/v) and ionic strength 1.0 at 25 °C. Disproportionation is kinetically second-order in total acridine species. The pH–rate profile is consistent with the rate-determining reaction of one acridinium cation with the pseudobase alkoxide anion derived from a second acridinium cation. Ferricyanide ion oxidation is kinetically first-order in each of ferricyanide ion and total acridine species. The pH–rate profile requires three distinct pathways for the ferricyanide ion oxidation of the 10-methylacridinium cation. For pH 12.8 the predominant oxidation pathway involves reaction of ferricyanide ion with the pseudobase alkoxide ion. Between pH 9.7 and 12.8, the major oxidation pathway involves initial disproportionation of the acridinium cation followed by ferricyanide ion oxidation of the 9,10-dihydro-10-methylacridine product. This latter route accounts for a maximum of 69% of the total ferricyanide ion oxidation at pH 11.1.

Charge-transfer complexation of N,N-dimethyl-p-toluidine (DMT) with each of the following N-methyl cations has been investigated in acetonitrile solution at 25 °C: acridinium; phenanthridinium; 3-X-quinolinium and 4-X-isoquinolinium (X = H, Br, CONH2, CN); 5-nitroisoquinolinium; 3-X-pyridinium (X = CONH2, CN) (also the N-benzyl pyridinium cations); N,N′-dimethyl-4,4′-dipyridylium dication. Charge-transfer absorption maxima are reported, and extinction coefficients (εmax) at these maxima and association constants (K) for 1:1 complex formation have been evaluated from the dependence of absorbance upon [DMT]. In general, the longest wavelength absorption maximum increases with increasing association constant. There is a strictly linear relationship between εmax and 1/K. These observations are considered in the context of theories of charge-transfer spectra. Such charge-transfer species are considered as models for the association complexes that are believed to exist upon the reaction pathway for hydride transfer between heteroaromatic cations. Although some significant qualitative relationships are apparent, variations in susceptibility of heteroaromatic cations to charge-transfer complexation are much smaller than variations in the reactivity of these cations towards hydride donor species.

The rates of reduction of a series of 1-(Z-benzyl)nicotinonitrile cations by a series of 1-(X-benzyl)-1,4-dihydronicotinamides have been studied at 25 °C in 20% CH3CN – 80% H2O (pH 7.0 (5 mM phosphate), ionic strength 1.0 (KCl)). Spectral studies indicate the formation of 1,4-dihydronicotinonitrile products, without the formation of the isomeric 1,2-dihydro- or 1,6-dihydro-nicotinamide intermediates. Second-order rate constants (k2) for these reductions are closely correlated with the Hammett σ constants for X and Z. Thus, for X = H, log k2 = 0.63σz − 1.05, while for Z = 4-CN, log k2 = −0.64σx − 0.65. The close correspondence between these ρx and ρz values indicates that charge neutralization on the nicotinonitrile cation exactly balances charge generation on the nicotinamide cation product in the rate-determining transition state. Thus the migrating hydrogen species is electrically neutral in the rate-determining transition state, which contrasts with the hydridic transition states previously reported in the reduction of isoquinolinium cations by 1,4-dihydronicotinamides. When 1-benzyl-4,4-dideuterio-1,4-dihydronicotinamide is used as the reductant, primary kinetic isotope effects of 3.0 and 2.7 are observed for the reduction of the 1-methylnicotinonitrile and 1-(4-cyanobenzyl)-nicotinonitrile cations, respectively. These data are evaluated in terms of the various mechanistic possibilities for hydride transfer.

The reduction of a series of 3-W-1-methylquinolinium cations (1: W = H, Br, CONH2, CO2CH3, CN, NO2) by 1-benzyl-1,4-dihydronicotinamide has been investigated. In all cases the kinetically controlled product from these reactions is the appropriate 3-W-1,4-dihydro-1-methylquinoline. Only for W = Br is any significant amount of the 1,2-dihydro isomer obtained (15% in this case). This kinetic preference for C-4 attack over C-2 attack in dihydronicotinamide reductions contrasts with the kinetically preferred attack at C-2 by hydroxide ion and in borohydride reductions. Rates of reduction were measured for each 1 and also 1,2-dimethyl- and 1,4-dimethylquinolinium cations in 20% CH3CN – 80% H2O, ionic strength 1.0 at 25 °C, under pseudo-first-order conditions. Kinetic saturation due to nonproductive 1:1 complex formation was observed for several cations at high concentrations (> 0.1 M). Second-order rate constants [Formula: see text] were evaluated for each W, and also kinetic isotope effects from second-order rate constants [Formula: see text] for reduction by 1-benzyl-4,4-dideuterio-1,4-dihydronicotinamide. Second-order rate constants are correlated with σp− for W with ρ = 4.5, and are also closely correlated with [Formula: see text] for pseudobase formation at C-4 of these quinolinium cations by: [Formula: see text]. Values of [Formula: see text] vs. [Formula: see text] describe a Westheimer curve reaching a maximum of 5.8 for W = Br and falling to 1.5 for W = NO2 and 4.2 for W = H. These data are consistent with an intrinsic barrier of 2.9 ± 0.5 kcal/mol for hydride transfer between this 1,4-dihydronicotinamide and quinolinium cations. However, quinolinium cations display a dramatically enhanced rate of dihydronicotinamide reduction relative to hydroxide ion attack when compared with isoquinolinium cations. This observation, and the predominance of C-4 rather than C-2 reduction, suggests that these reactions may not be simple one-step hydride transfer processes.

Deprotonation de phenylacetal-3 et -4 pyridines et pyridiniums substitues sur le groupe phenyle

The cyclohexylacetate ion is a partially competitive inhibitor for the hdyrolysis of l -2-aceturoxybutanoic acid by bovine carboxypeptidase A (peptidyl- l -amino acid hydrolase, EC 3.4.12.2). The kinetics of hydrolysis in the presence of this inhibitor are similar for this ester, which does not display substrate inhibition, and for O- hippuryl- l -3-phenyllactic acid, for which pronounced substrate inhibition is apparent. This suggests that partially competitive inhibition kinetics are not related to the tendency of this latter substrate to form 2:1 substrate-enzyme complexes. It is shown that for this enzyme, partially competitive inhibition is not consistent with the formation of E—I—S complexes. Rather, all inhibition data can be rationalized in terms of a scheme involving the formation of an E—I2 complex.

Pseudobase formation and methoxide ion addition have been investigated spectroscopically for some N-methylacridinium, -phenanthridinium, and -benzoquinolinium cations. Susceptibility to nucleophilic attack decreases in the order 10-methylacridinium (pKROH = 9.86) > 5-methylphenanthridinium (pKROH = 11.94) > 1-methyl-5,6-benzoquinolinium ≈ 1-methyl-7,8-benzoquinolinium (pKROH &([a-z]+); 14). A qualitative correlation of pKROH with loss of resonance energy upon pseudobase formation is shown to exist. For the 9,10-dimethylacridinium cation, the C-9 pseudobase (or methoxide adduct) is the kinetically preferred product in basic solutions but this is subsequently converted to the thermodynamically more stable anhydrobase. With the corresponding 9-ethyl- and 9-benzyl-10-methylacridinium cations, the pseudobase, rather than the anhydrobase, seems to predominate at equilibrium.

The hydrolyses of the O-hippuryl derivatives of glycolic acid (1a), 2-methyllactic acid (1b), and p-chloromandelic acid (1c) by bovine carboxypeptidase A display substrate activation. The hydrolyses of the latter two esters also display substrate inhibition at high substrate concentrations (>0.03 and >0.05 M respectively). Partial kinetic analyses are presented, and these phenomena are discussed in terms of reaction schemes which involve substrate binding at both activating and inhibiting regulatory sites.The hydrolysis of 1b by this enzyme is the first indication that the presence of a hydrogen atom on the α-carbon atom of the alcohol moiety is not obligatory for ester substrates of carboxypeptidase A. The binding of 1c at the catalytic site is approximately 1000 times weaker than for O-hippurylmandelic acid and indicates a dramatic influence for the p-chloro substituent on the binding of the phenyl ring of the mandelic acid unit.

Kinetic data are reported for the equilibration of the 1-methyl-3-nitropyridinium cation with its pseudobase (hydroxide adduct) and for the reduction of this cation by 1-benzyl-1, 4-dihydronicotinamide. The C-2 hydroxide adduct is the kinetically controlled product (pKR+ = 11.6) when this pyridinium cation is mixed with aqueous base, however, this species rearranges to the C-4 adduct as the themodynamically more stable product (pKR+ = 9.42). The pH-dependence of this equilibration may be analysed to give kOH = 1600 M-1s-1 for hydroxide ion attack at C-4 of this cation. Reduction of this pyridinium cation by 1-benzyl-1, 4-dihydronicotinamide appears to occur exclusively at C-4 with second-order rate constant k2 = 0.72 M-1s-1 and k2 H/k2D = 2.0 in 20% CH3CN - 80% H2O, ionic strength 1.0, 25°C. The reactivities of pyridinium, quinolinium, isoquinolinium, acridinium and phenanthridinium cations of pKR+ = 10.0 towards both hydroxide ion and 1-benzyl-1, 4-dihydronicotinamide are evaluated. Relative reactivities (K2/KOH) for these two processes are shown to be acridinium : quinolinium (C-4) : pyridinium (C-4) : quinolinium (C-2) : isoquinolinium : phenanthridinium = 1.6×105 : 3400 : 80 : (4 : 1.0 : 0.7 for predominantly aqueous reaction media. These data support the hypothesis that formation of 1, 2-dihydropyridine systems upon reduction of heteroaromatic cations by 1, 4-dihydronicotinamides occurs via direct one step hydride transfer, while formation of 1,4-dihydro-pyridines in such processes occurs preferentially by a mechanistica11y more complex process involving electron transfer.

A kinetic study of the formation and decomposition of the pseudobase of the 1-methyl-3-nitroquinolinium cation has revealed that although 1,4-dihydro-4-hydroxy-1-methyl-3-nitroquinoline is the predominant product at equilibrium, the kinetically controlled product is the isomeric pseudobase, 1,2-dihydro-2-hydroxy-1-methyl-3-nitroquinoline. This less stable isomer has been identified from its u.v-visible spectrum, and the pKROH value for its formation (9.16) has been measured by the stopped-flow technique.

Second-order rate constants ( k 2 ) are reported for the reduction of 9-R-10-methylacridinium cations ( 5 :R = H, CH 3 , CH 3 CH 2 , C 6 H 5 CH 2 , (CH 3 ) 2 CH, C 6 H 5 , 4-(CH 3 ) 2 NC 6 H 4 ) by 1-benzyl-1,4-dihydronicotinamide ( 2 :R = C 6 H 5 CH 2 ) in 20% CH 3 CN-80% H 2 O at 25°C. All 5 :R ≠ H are reduced in the range 20- to 140-fold more slowly than 5 :R = H. However, there is no simple relationship between k 2 and the nature of R, nor between k 2 and the second-order rate constant for hydroxide ion attack at C-9 of these cations in pseudobase formation. Rates of reduction of 5 by 1-benzyl-4,4-dideuterio-1,4-dihydronicotinamide allow the calculation of the following kinetic isotope effects in this solvent medium: 5 :R, k H k D : H , 1.56; C 6 H 5 CH 2 , 2.7; C 6 H 5 , 5.4. Substituent effects upon k 2 were evaluated for the reduction of 5 by 1-(X-benzyl)-1,4-dihydronicotinamides, and lead to the following Hammett ϱ parameters: 5 :R, ϱ: H, −0.68; C 6 H 5 CH 2 , −0.92; C 6 H 5 , −0.96. The latter two values require essentially complete unit positive charge generation on the nicotinamide moiety in the rate-determining transition state. It is shown that these Hammett ϱ values and the above isotope effects can only be rationalized by a two-step e − + H • mechanism for hydride transfer from 2 to 5 in this solvent system. This result contrasts with our earlier conclusion of direct, one-step hydride transfer in the reduction of isoquinolinium cations by 2 , but is consistent with our observation that acridinium cations are reduced 37500-fold faster by 2 than predicted on the basis of the relative rates of nucleophilic attack (hydroxide ion) on acridinium and isoquinolinium cations. It is suggested that the availability of both Hammett ϱ values and primary kinetic isotope effects will generally allow the establishment of the mechanism of hydride transfer in these systems. Application of these ideas to literature data suggests that 5 :R = H is reduced by direct hydride transfer in acetonitrile solution, in contrast to the above result in predominantly aqueous solution. The ready formation of acridanyl radicals by electron transfer to acridinium cations is demonstrated by the formation of Wurster's Blue radical cation upon mixing solutions of acridinium cations with N,N,N′,N′ -tetramethyl- p -phenylenediamine.