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The spectroscopic and kinetic evidence is presented for the reaction of methyl [beta]-methylthio-[alpha]-nitrocinnamate with piperidine, morpholine and hydroxide ion. It is found that a nucleophilic vinylic substitution by strongly basic amines leads to intermediate accumulation and that there is no accumulation in the reaction with hydroxide

A kinetic study of the reversible deprotonation of benzofuran-3(2H)-one (3H-O) and benzothiophene-3(2H)-one (3H-S) by amines and hydroxide ion in water is presented. The results have shown that aromatic stabilization of the transition state is ahead of proton transfer, which is in contrast with simple resonance effects.

A study was conducted to determine whether the nucleophilic substitution of the p-nitrophenoxy group in [(CO).sub.5]Cr=C(O[C.sub.6][H.sub.4]-N[O.sub.2])Ph by a series of substituted phenoxide ions is concerted or stepwise. The tetrahedral intermediates formed in the reactions of [(CO).sub.5]Cr=C(O[C.sub.6][H.sub.4]-N[O.sub.2])Ph are much more stable than those in acyl transfers.

An investigation of how steric effects affect the rates of hydrolysis of [alpha]-alkyl-[alpha]-(methylthio)methylene Meldrum's acids is reported. The kinetic results confirm that crowding at the transition state is an important effect but also show that for very large R groups destabilization of the substrate reverses the decreasing trend in the rate constants.

The replacement of the methylthio group of substituted methylthiobenzylidene Meldrum's acids (2-SMe-Z) by secondary alicyclic amines occurs by a three-step mechanism. A detailed structure-reactivity analysis indicates that for the reaction with piperazine, 1-(2-hydroxythyl)- piperazine, and morpholine it is deprotonation of T(sub A)(super plus or minus) to products is rate limiting.

The issue regarding whether the normal binding of AA is 'methyl-end first' or 'carboxylate-end first' is investigated by mutating the sLO-1 active site residues, Trp500 and Arg707, and determining their activation and catalytic kinetics. The results demonstrate that both Trp500 and Arg707 are critical to product and substrate binding in the active site, which indicates that the substrate binds 'carboxyl-end first'.

The results relating to the deprotonation of 1-benzyl-1-methoxy-2-nitroethylenes, by amines, which include structural features in the nitroalkanes are reported. The question addressed is whether the special structural features affect the actual size of the transition state imbalance and/or its manifestation in terms of alpha and beta values.

The deprotonation of substituted (methylthiophenylcarbene)pentacarbonyl tungsten(0) ((CO)5W=C(SC6H4Z)CH3) and (benzoxymethylcarbene)pentacarbonyl tungsten(0) ((CO)5W=C(OCH2C6H4Z)CH3) in aqueous acetonitrile is studied. This indicates a transition state imbalance in which the loss of the carbene complex lags behind the proton transfer.

Proton-transfer reactions of 3H-Z get characterized by high intrinsic barriers or low intrinsic rate constants. Manifestation of imbalance is marked by the pi-donor effect of methoxyl group.

An ab initio study of two series of carbon-to-carbon proton transfer reactions is conducted to examine the degree of aromaticity of [C.sub.4][H.sub.4]X (X = CH(-), NH, S, O, PH) and the antiaromaticity of [C.sub.4][H.sub.4]X (X = AlH, BH) is expressed at the transition state of the proton transfer and its effect on the respective intrinsic barriers. The results revealed a disproportionately large degree of aromaticity at the transition state for the aromatic systems and a disproportionately small degree of transition state antiaromaticity for the antiaromatic systems.

The density functional theory computation methods were used to study the role of negative hyperconjugation and anomeric and polar effects in stabilizing the [XZHC.sup.[beta]][C.sup.[alpha]]YY'-intermediates in [S.sup.N]V reactions. The stability of disubstituted ions were found to be larger compared to that of monosubstituted ions due to additional stabilization by negative hyperconjugation and an anomeric effect.

The reactions of thiolate ions with three Fischer-type [aryloxy(phenyl)carbene]pentacarbonyl chromium(0) complexes in MeCN-water are studied. The factors affecting the degree of bond formation in reactions of Fischer carbene complexes and reactions of other unsaturated electrophiles with thiolate ions are examined.

A kinetic study of the reversible deprotonation of methylnitroacetate (4H) by primary aliphatic amines, secondary alicyclic amines, hydroxide ion, and water in water at 25 degree C is reported. The factor affecting intrinsic rate constants of proton transfer from nitroalkanes is also presented.

The rates of the reversible deprotonation of benzo[b]-2,3-dihydrofuran-2-one (6H-O) and benzo[b]-2,3-dihydrothiophene-2-one (6H-S) by O[H.sup.-], primary aliphatic amines, secondary alicyclic amines and carboxylate ions are determined in water at 25 degree Celsius. A detailed analysis of some potential factors that might affect the intrinsic barriers and that could explain the contradictory results is presented.

The experimental data on the kinetic and thermodynamic acidities of pentacarbonyl(cyclobutenylidene)chromium complexes in acetonitrile is presented. Results of the gas phase ab initio calculations indicate that in the gas phase, the effect of antiaromaticity on the thermodynamic acidities is large but strongly attenuated in acetonitrile.

The derivatives of aromatic heterocycles represented in the deprotonated forms of rhenium carbene complexes are examined and their intrinsic rate constants are studied. It is found that an increase in aromaticity leads to an increase in the intrinsic rate constant.

Research has been conducted on carbon-to-carbon proton-transfer. The authors describe the results of ab initio study of the proton-transfer identity in the gas phase, and discuss the effects of the cyano group on the proton-transfer barriers, transition state structure and acidities.

Enzyme inhibitor studies indicate that (Z)-9-octadecenyl sulfate binds to an allosteric site different from the catalytic iron site of soybean lipoxygenase-1, thereby not inhibiting the activation. Data indicate that (Z)-9-palmitoleyl sulfate acts as a competitive inhibitor. Furthermore, the allosteric site binding selectivity is dependent on inhibitor length.

A variety of factors that contribute to the differences in carbene complexes with the most important ones being that 5H(super+1) and 6H(super+1) are cationic which makes the C5H5(NO)(PPh3) Re moiety a stronger acceptor than the (CO)(sub5)M moieties are discussed, coupled with the fact that the deprotonated forms of 5H(super+1) and 6H(super+1) are aromatic molecules.

The question as to what extent aromaticity in a reactant or product is expressed in the transition state of a reaction has only recently received serious attention. Inasmuch as aromaticity is related to resonance, one might expect that, in a reaction that leads to aromatic products, its development at the transition state should lag behind bond changes as is invariably the case for the development of resonance in reactions that lead to delocalized products. However, recent experimental and computational studies on proton transfers from carbon acids suggest the opposite behavior, i.e., the development of aromaticity at the transition state is more advanced than the proton transfer. The evidence for this claim is based on the determination of intrinsic barriers that show a decrease with increasing aromaticity. According to the Principle of Nonperfect Synchronization (PNS), this decrease in the intrinsic barrier implies a disproportionately large amount of aromatic stabilization of the transition state. Additional evidence for the high degree of transition state aromaticity comes from the calculation of aromaticity indices such as HOMA, NICS, and the Bird Index. Possible reasons why the degree to which aromaticity and resonance are expressed at the transition state is different are discussed. [ABSTRACT FROM AUTHOR]

The reversible deprotoriation of 3(2H)-furanone (3H-O) and 3(2H)-thiophenone (3H-S) by a series of delocalized carbanions and by CN-, and the identity proton transfer of 3H-O to its conjugate base (3--O) and of 3H-S to 3--S have been studied at the MP2//6-31+G** level. The main objective has been to examine to what extent the aromaticity of 3--O and 3--S is expressed at the transition state of these reactions and how the intrinsic barriers are affected by the transition state aromaticity. Aromaticity parameters such as NICS values, HOMA and Bird Indices indicate a disproportionately high degree of aromatic stabilization of the transition state. This stabilization results in a reduction of the intrinsic barriers which is most clearly manifested in the identity reactions. However, these reductions are relatively modest compared to those reported previously for the identity proton transfers from the benzenium ion to benzene and of cyclopentadiene to its conjugate base, reflecting the smaller aromatic stabilization of 3--O and 3--S compared to those of benzene and cyclopentadienyl anion. [ABSTRACT FROM AUTHOR]

An ab initio study of six carbon to-carbon identity proton transfers is reported. They refer to the benzenium ion/benzene C6H7-/C6H6), the 2.4 cyclopentadiene/cyclopentadienyl anion (C5H6/C5H5-), and the cyclobutenyl cation/cyclobutadiene (C4H5+/C4H4) systems and their respective noncyclic reference systems that is, ... and .... For the aromatic C6H7-/C6H6 and C5H6/C5H5- systems, geometric parameters and aromaticity indices indicate that the transition states are highly aromatic. The proton-transfer barriers in these systems are quite low, which is consistent with a disproportionately high degree o f transition-state aromaticity. For the anti-aromatic C4H5+/C4H4 system, the geometric parameters and aromaticity indices indicate a rather small degree of antiaromaticity of the transition state. However, the proton-transfer barrier is higher than expected for a transition state with a low antiaromaticity. This implies that another factor contributes to the barrier: it is a suggested that this factor is angle and torsional strain in the transition state. The question whether charge delocalization at the transition state might correlate with the development of aromaticity was also examined. No such correlation was found, that is, charge delocalization lags behind proton transfer as is commonly observed in nonaromatic systems involving π-receptor groups. [ABSTRACT FROM AUTHOR]

A spectroscopic and kinetic study of the reaction of methyl β-methylthio-α-nitrocinnamate (4-SMe) with morpholine, piperidine, and hydroxide ion in 50% DMSO/50% water (v/v) at 20 °C is reported. The reactions of 4-SMe with piperidine in a pH range from 10.12 to 11.66 and those with morpholine at pH 12.0 are characterized by two kinetic processes when monitored at λmax (364 nm) of the substrate, but by only one process when monitored at λmax (388) nm of the product. The rate constants obtained at 388 nm were the same as those determined for the slower of the two processes at 364 nm. These rate constants refer to product formation, whereas the faster process observed at 364 nm is associated with the loss of reactant to form an intermediate. In contrast, for the reaction of 4-SMe with morpholine at pH 8.62 the rates of product formation and disappearance of the substrate were the same, i.e., there is no accumulation of an intermediate. Likewise, the reaction of 4-SMe with OH- did not yield a detectable intermediate. The factors that allow the accumulation of intermediates in certain SNV reactions but not in others are discussed in detail, and structure-reactivity comparisons are made with reactions of piperidine and morpholine with other highly activated vinylic substrates. [ABSTRACT FROM AUTHOR]

A kinetic study of the aminolysis of substituted (methylthio)benzylidene Meldrum's acids, 2-SMe-Z with the aliphatic primary amines, n-butylamine, glycinamide, methoxyethylamine, and aminoacetonitrile, in aqueous DMSO at 20 °C is reported. With all amines the reaction is strictly second-order, that is, first-order in 2-SMe-Z and first-order in the amine. A three steps mechanism has been proposed. The first step is a rate-limiting attachment of the amine to form the tetrahedral intermediate (TA±), and the subsequent steps are a fast acid–base equilibrium followed by a fast R‴CH2NH3+ or H2O catalyzed expulsion of the leaving group. The Brönsted plot for k1 for the reaction of 2-SMe-H with four primary amines showed a good correlation between log k1 and pKaAH with βnuc = 0.32 ± 0.02, a slightly lower value than that for the reactions of secondary amines (βnuc = 0.41 ± 0.01). The Hammett plot for n-butylamine yields ρ(k1) = 0.40 ± 0.05 which is smaller than that for the reaction of piperidine with the same substrates (ρ(k1) = 0.72 ± 0.07) and also for the reactions of HOCH2CH2S- (ρ(k1) = 1.18), CF3CH2O- (ρ(k1) = 1.14) and OH- (ρ(k1) = 1.11) addition to 2-H-H (ρ(k1) = 1.11). All these observations are consistent with the suggested mechanism. Copyright © 2006 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

A kinetic study of the reaction of hydroxide ion with (CO)5Mo=C(XCH2CH2OH)(C6H5) (X=O for Mo–OR, and X=S for Mo–SR), and (CO)5W=C(OCH2CH2OH)(C6H4–Z) (W–OR(Z)) is reported. The results are consistent with a pathway in basic solution that involves rapid deprotonation of the OH group followed by rate-limiting cyclization. The parameter k1KOH for the reaction of W–OR(Z) was determined as a function of the phenyl substituents. They were found to correlate well with the Hammett equation. The dependence of the reactivity on the metal atoms in the complexes M–OR (M=Cr, Mo and W) shows that the reactivity decreases slightly down the group of the Periodic Table, while for M–SR the reactivity increases slightly down the group. A plausible explanation of these results is offered based on electronegativity values of the metal atoms. The much higher ρ(k1KOH) value for W–OR(Z) over W–SR(Z) arises mainly due to the stabilization of the reactant carbene complex by the stronger π-donor effect of oxygen over sulfur. [Copyright &y& Elsevier]

The rates of hydrolysis of α-R-α-(methylthio)methylene Meldrum's acids (8-R with R = H, Me, Et, s-Bu, and t-Bu) were determined in basic and acidic solution in 50% DMSO-50% water (v/v) at 20 °C. In basic solution (KOH), nucleophilic attack to form a tetrahedral intermediate (T-OH) is rate limiting for all substrates (k1OH). In acidic solution (HCI) and at intermediate pH values (acetate buffers), water attack (k1H2O) is rate limiting for 8-Me, 8-Et, and 8-s-Bu; the same is presumably the case for 8-t-Bu, but rates were too slow for accurate measurements at low pH. For 8-H, water attack is rate limiting at intermediate pH but at pH < 4.5 MeS- departure from the tetrahedral intermediate becomes rate limiting. Our interpretation of these results is based on a reaction scheme that involves three pathways for the conversion of T-OH to products, two of which being unique to hydrolysis reactions and taking advantage of the acidic nature of the OH group in T-OH. This scheme provides an explanation why even at high [KOH] T-OH does not accumulate to detectable levels even though the equilibrium for OH- addition to 8-R is expected to favor T-OH, and why at low pH water attack is rate limiting for R = Me, Et, s-Bu, and t-Bu but leaving group departure becomes rate limiting with the sterically small R = H. The trend in the k1OH and k1H2O indicates increasing steric crowding at the transition state with increasing size of R, but this effect is partially offset by a sterically induced twisting of the C=C double bond in 8-R which leads to its elongation and makes the substrate less stable and hence more reactive. Our computational results suggest that this effect becomes particularly pronounced for R = t-Bu and explains why k1OH for 8-t-Bu is somewhat higher than for the less crowded 8-s-Bu. [ABSTRACT FROM AUTHOR]

Abstract: A kinetic study of the reversible deprotonation of [Cp*(CO)2Fe+ (16H +) by primary aliphatic amines, secondary alicyclic amines and carboxylate ions in 50% MeCN–50% water (v/v) at 25°C is reported. A pK a value of 8.25 for 16H + was determined kinetically. Intrinsic rate constants, k 0 for each buffer family were obtained by suitable extrapolation or interpolation of the corresponding Brønsted plots; they are and , respectively. A major focus of this paper is a discussion of how structural variations of Fischer carbenes affect their acidities and intrinsic rate constants for proton transfer and how the results obtained for 16H + add new insights into the interplay of these structural factors. These factors include the charge type of the carbene complex, the metal/ligand combination, the π-donor group attached to the carbene carbon, the push–pull resonance effect in conjunction with the metal moiety, the π-acceptor groups attached to the carbene carbon, and the potential for aromaticity/antiaromaticity of the conjugate base. [Copyright &y& Elsevier]

Acidity constants and rates of reversible deprotonation of acetonyltriphenylphosphonium ion (1H+), phenacyltriphenylphosphonium ion (2H+), N-methyl-4-phenacylpyridinium ion (3H+), and N-methyl-4-(phenylsulfonylmethyl)pyridinium ion (4H+) by amines in water, 50% DMSO—50% water (v/v), and 90% DMSO—10% water (v/v) have been determined. From the respective Brønsted plots, log k0 values for the intrinsic rate constants of the various proton transfers were obtained. Solvent transfer activity coefficients of the carbon acids and their respective conjugate bases were also determined which helped in understanding how the pKa values and intrinsic rate constants depend on the solvent. Some of the main conclusions are as follows: (1) The pKa values of 1H+, 2H+, and 3H+ are significantly higher than that of 4H+ because of a stronger resonance stabilization of the corresponding conjugate bases 1, 2 and 3, respectively. (2) The electronic effects of the PPh3+ and the N-methyl-4-pyridylium group are similar but the mix between inductive and resonance effect is different. (3) All four acids become more acidic upon addition of DMSO to the solvent. In all cases, the main factor is the stronger solvation of H3O+ in DMSO; for 1H+, 2H+, and 3H+ but not 4H+ this factor is significantly attenuated by stronger solvation of the carbon acid in DMSO. (4) The intrinsic rate constants for proton transfer are relatively high for all four carbon acids and show little solvent dependence; this contrasts with nitroalkanes which have much lower intrinsic rate constants and show a strong solvent dependence. These results can be understood by a detailed analysis of the interplay between inductive, resonance, and solvation effects. [ABSTRACT FROM AUTHOR]

The replacement of the methylthio group of substituted methylthiobenzylidene Meldrum's acids (2-SMe-Z) by secondary alicyclic amine s occurs by a three-step mechanism. The first step is a nucleophilic attachment of the amine to 2-SMe-Z to form a zwitteriOnic intermediate TA† the second step involves deprotonation of TA- to form TA&Daggger; while the third step represents general acid-catalyzed conversion of TA- to products. At high amine and/or high KOH concentration nucleophilic attachment is rate limiting. At low amine and low KOH concentration the reaction follows a rate law that is characteristic for general base catalysis which, in principle, is consistent with either rate-limiting deprotonation of TA‡ or rate-limiting conversion of TA‡ to products. A detailed structure reactivity analysis indicates that for the reactions with piperazine, 1-(2-hydroxyethyl)- piperazine, and morpholine it is deprotonation of TADagger; that is rate limiting, while for the reaction with pip eridine, conversion of TA‡ to products is rate limiting. [ABSTRACT FROM AUTHOR]

We report an ab initio study of the identity carbon-to-carbon proton-transfer NCCH[sub 2]Y + NCCH=Y[sup -] ... NCCH=Y[sup -] + NCCH[sub 2]Y in the gas phase, where Y = H, CH=CH[sub 2], CH=O, CH=S, CN, NO, and NO[sub 2]. The main focus is on a comparison with the previously reported systems CH[sub 3]Y + CH[sub 2]=Y[sup -] ... CH[sub 2]=Y[sup -] + CH[sub 3]Y, i.e., on the effect of the cyano group on acidities, proton-transfer barriers, and transition state structures. The conclusions of this study are as follows: (1) The transition state for the NCCH[sub 2]YfNCCH=Y systems is more imbalanced than that for the CH[sub 3]Y/CH[sub 2]=Y[sup -] systems. (2) The cyano group leads to an increase in the acidities but to a decrease in the proton transfer barriers. This barrier reduction results from the fact that the stabilizing effect of the cyano group on the transition state is greater than that on the anion. (3) Within a reaction series, the barriers are largely dominated by the π-acceptor strength of Y, i.e., the strongest π-acceptors lead to the highest barriers. This is similar to proton transfers in solution but quite different from the CH[sub 3]Y/ CH[sub 2]=Y[sup -] systems in the gas phase; in these latter systems π-acceptor effects play a minor role while the barrier lowering field effect of Y is dominant. [ABSTRACT FROM AUTHOR]

Examines the physical organic chemistry of transition metal carbene complexes. Thermodynamic and kinetic acidities of phenyl-substituted complexes; Delocalization of negative charges in the carbonyl ligands; Characteristics of Fischer carbene complexes.

Examines the carbon-to-carbon bond identity proton transfer from allene, ketene, ketenimine and thioketene to their respective conjugate anions in the gas phase by ab initio methods. Effect of unsaturation and s-character on intrinsic barriers; Comparison of transition-state imbalances among different reactions; Rate constant of protonation.

Investigates the mechanism of nucleophilic vinylic substitution reaction. Kinetics of the reactions of methoxybenzylidenemalononitrile; Use of MeO[sub 2]CCH[sub 2]S as thiolate ions; Effect of activating substituents on rate and equilibrium constants.

Determines gas-phase acidities and charge distribution of proton transfers. Application of carbon-to-carbon identity proton transfer; Occurrence of charge delocalization in propyne anion; Factors attributing to intrinsic barriers in proton transfer.

Investigates the kinetics and mechanism of hydrolysis of transition metal carbene complexes in aqueous acetonitrile. Stages of the reactions; Reactivity differences between metal carbene complexes and complexes with alkoxy leaving groups; Factors affecting the reactivity ratios.

Part XIX. Investigates the effect of the nucleophilic addition of MeO- to Fischer carbene complexes. Formation of tetrahedral adducts; Observance of imbalance in the transition state of alkoxide ion in the addition of carbene complexes; Description of proton transfer to the leaving group in the carbon-oxygen bond cleavage.

Examines the kinetic analysis of the acid-catalyzed loss of alkoxide and thiolate ions of Meldrum's acid derivatives. Determination of the pKa values; Identification of proton-transfer rate constants at the alpha-carbon; Calculations of rate constants of nucleophilic; Relationships between structure and reactivity.

Examines the kinetics of the reaction of beta-methoxy-alpha-nitrostilbene (1-OMe) with cyanamide over a pH range from 8.5 t 12.4. Nucleophilic reactivity of cyanamide; Reactions of 1-OMe with thiolate ions, alkoxide ions and some amines; Combined field effect/pi-acceptor properties of the cyano group; Reactivity of the neutral cyanamide.

Examines the physical organic chemistry of transition metal carbene complexes. Kinetics of the reactions of (arylthioalkoxycarbene)pentacarbonyl complexes of chromium and tungsten with thiolate ions in aqueous acetonitrile; Rate and equilibrium constants for the nucleophilic attachment of various thiolate ions.

Presents information on a study which investigated the physical organic chemistry of transition metal carbene complexes. Difference between the Fischer carbene complexes and the carboxylic esters; Results; Discussion.