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A kinetic study of the reactions of thiolate ions with three Fischer-type [aryloxy(phenyl)carbene]pentacarbonyl chromium(0) complexes in 50% MeCN-50% water (v/v) is reported. Brønsted plots of the second-order rate constants are biphasic with an initial steep rise for weakly basic thiolate ions (beta(nuc) approximately equal to 1.0) followed by a slightly descending leg with a negative slope (beta(nuc) approximately equal to -0.2) for strongly basic thiolate ions. This indicates a change from rate-limiting leaving group departure at low pK(RSH)(a) to rate-limiting nucleophilic attachment at high pK(RSH)(a). The negative beta(nuc) values result from a combination of minimal progress of C-S bond formation at the transition state and the requirement for partial desolvation of the nucleophile before it enters the transition state. Possible factors that may affect the degree of bond formation in reactions of Fischer carbene complexes as well as reactions of other unsaturated electrophiles with thiolate ions are discussed.

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 degrees C and in 50% DMSO/50% water (v/v) at 20 degrees C is reported. Intrinsic rate constants, k0, determined by extrapolation or interpolation of Brønsted plots have been determined. In comparison to proton transfers involving other nitroalkanes, the intrinsic rate constants for 4H are exceptionally high; for example, log k0 for the reaction of 4H with secondary alicyclic amines in water (1.22) is 1.81 log units higher than log k0 for nitromethane (-0.59), while in 50% DMSO/50% water, log k0 for 4H (2.44) is 1.71 log units higher than that for nitromethane (0.73). A general discussion of the factors affecting intrinsic rate constants of proton transfer from nitroalkanes is presented; it provides the context for an understanding as to why k0 is so high for the proton transfers from 4H. The correlation between intrinsic rate constants for the addition of nucleophiles to alkenes of the type R'R' 'C=CXY and the intrinsic rate constants of proton transfers from carbon acids of the type H2CXY is also discussed as a general proposition as well as with specific reference to the Ph(SMe)C=C(NO2)CO2Me/H2C(NO2)CO2Me pair.

A spectroscopic and kinetic study of the reaction of methyl beta-methylthio-alpha-nitrocinnamate (4-SMe) with morpholine, piperidine, and hydroxide ion in 50% DMSO/50% water (v/v) at 20 degrees 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 lambdamax (364 nm) of the substrate, but by only one process when monitored at lambdamax (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.

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 (T), and the subsequent steps are a fast acid–base equilibrium followed by a fast R‴CH2NH or H2O catalyzed expulsion of the leaving group. The Bronsted plot for k1 for the reaction of 2-SMe-H with four primary amines showed a good correlation between log k1 and pK 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.

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 OH-, primary aliphatic amines, secondary alicyclic amines, and carboxylate ions have been determined in water at 25 degrees C. As noted earlier by Kresge and Meng, 6H-S (pKa = 8.82) is considerably more acidic than 6H-O (pKa = 11.68), which mainly reflects the greater aromatic stabilization of the conjugate base of 6H-S (thiophene derivative) compared to that of 6H-O (furan derivative). The main focus of this paper is to assess how the difference in the aromaticity of the two enolate ions affects the intrinsic barrier to the proton transfer. These intrinsic barriers were determined from Brønsted plots for the reactions with the amines and carboxylate ions or calculated on the basis of the Marcus equation for the reactions with OH-. They are consistently somewhat higher for the reactions of 6H-S than for the reactions of 6H-O, implying that the aromaticity in the anion enhances the intrinsic barrier. This contrasts with a previous report on the deprotonation of some cyclic rhenium Fischer-type carbene complexes where the reaction that leads to the most aromatic conjugate base (thiophene derivative) has a lower intrinsic barrier than the reaction that leads to the less aromatic furan analogue. We are offering a detailed analysis of other potential factors that may affect the intrinsic barriers and which could explain these contradictory results.

A kinetic study of the reaction of hydroxide ion with (CO) 5 Mo C(XCH 2 CH 2 OH)(C 6 H 5 ) (X = O for Mo–OR , and X = S for Mo–SR ), and (CO) 5 W C(OCH 2 CH 2 OH)(C 6 H 4 –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 k 1 K OH 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 ( k 1 K OH ) 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.

The synthesis and characterization of [Cp(NO)(PPh3)Re(2,5-dimethyl-3-thienyl)carbene]+ (3H+(S)), a novel Fischer carbene complex, is described. 3H+(S) undergoes reversible deprotonation with a pKa of 2.64 in 50% MeCN−50% water (v/v). Rate constants for the deprotonation of 3H+(S) by carboxylate ions and by primary aliphatic and secondary alicyclic amines were determined. A major focus of this paper is a comparison of the kinetic and thermodynamic acidities of 3H+(S) with those of [Cp(NO)(PPh3)Re(2-thienyl)carbene]+, 1H+(S). The pKa of 1H+(S) (2.51) is virtually the same as that of 3H+(S). This appears to be the result of a fortuitous cancellation of acidity-reducing and acidity-enhancing factors, a conclusion supported by a computational study of gas phase acidities. An analysis of Bronsted plots indicates that the intrinsic rate constants for proton transfer are about 2 orders of magnitude lower for 3H+(S) than for 1H+(S). This reduction is the result of steric effects and of electronic effects connected...

A kinetic study of the reversible deprotonation of [Cp*(CO) 2 Fe C(OMe)Me] + ( 16H + ) by primary aliphatic amines, secondary alicyclic amines and carboxylate ions in 50% MeCN–50% water (v/v) at 25 °C is reported. A p K 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 Bronsted plots; they are log k 0 RNH 2 = 2.64 , log k 0 R 2 NH = 2.99 and log k 0 RCOO = 3.42 , 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.

Rate constants for the attachment of CH(CN)2- to Fischer carbene complexes of the type (CO)5MC(XR)C6H4Z with M = Cr, W and XR = OMe, OEt, SMe in 50% MeCN−50% water (v/v) at 25 °C are reported. The ...

The deprotonation of pentacarbonyl[(3-diethylamino-2,4-dimethyl)cyclobut-2-ene-1-ylidene]chromium (1d) and pentacarbonyl[(3-diethylamino-4-methyl-2-phenyl)cyclobut-2-ene-1-ylidene]chromium (1e) leads to antiaromatic conjugate anions by virtue of their being cyclobutadiene derivatives. Rate constants for the deprotonation of 1d and 1e by P2-Et and pKa values were determined in acetonitrile. Gas-phase B3LYP calculations of 1d, 1e, and their respective conjugate anions, using a generalized basis set, were also performed. Furthermore, for purposes of comparison with carbene complexes of similar structures, but having conjugate anions that are not antiaromatic, corresponding calculations were performed on pentacarbonyl[3-diethylamino-2,5-dimethyl)cyclopent-2-ene-1-ylidene]chromium (5), [dimethylamino(methyl)carbene]pentacarbonylchromium (3a), and [dimethylamino(iso-propyl)carbene]pentacarbonylchromium (3b) and their respective conjugate anions, and solution-phase pKa and kinetic measurements were carried out for 3a and 3b. Major points of interest include the effect of antiaromaticity on the kinetic and thermodynamic acidities of 1d and 1e, the large effect of the phenyl group on the gas-phase acidity of 1e, the strong attenuation of the acidities and the effect of the phenyl group in acetonitrile, and the position of the C=C double bonds in the cyclobutadiene ring of the conjugate anion of 1e.

A kinetic study of the aminolysis of (CO)5CrC(OEt)C6H5 (7-Cr) and (CO)5CrC(OEt)C6H5(Cr(CO)3) (8-Cr) by five primary aliphatic amines and of the hydrolysis of 8-Cr in 50% MeCN−50% water (v/v) at 25 °C is reported. The introduction of the Cr(CO)3 group has only a minor effect on the electrophilic reactivity of the carbene carbon; that is, the nucleophilic attack by OH- and the amines on 7-Cr and 8-Cr occurs at similar rates. However, the Cr(CO)3 group exerts a strong influence on the mechanisms of both types of reactions, which results in changes of what steps are rate limiting. In the hydrolysis reaction these changes lead to a more complex rate versus pH profile for 8-Cr compared to the previously reported one for 7-Cr. In the aminolysis the effect is to change the mechanism of general base catalysis from rate-limiting leaving group departure for the reactions of 7-Cr to rate-limiting proton transfer for the reactions of 8-Cr. We provide a detailed analysis of the reasons for these changes.

We report an ab initio study of the identity carbon-to-carbon proton-transfer NCCH(2)Y + NCCH=Y(-) right arrow over left arrow NCCH=Y(-) + NCCH(2)Y in the gas phase, where Y = H, CH=CH(2), CH=O, CH=S, CN, NO, and NO(2). The main focus is on a comparison with the previously reported systems CH(3)Y + CH(2)=Y(-) right arrow over left arrow CH(2)=Y(-) + CH(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(2)Y/NCCH=Y(-) systems is more imbalanced than that for the CH(3)Y/CH(2)=Y(-) 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 pi-acceptor strength of Y, i.e., the strongest pi-acceptors lead to the highest barriers. This is similar to proton transfers in solution but quite different from the CH(3)Y/CH(2)=Y(-) systems in the gas phase; in these latter systems pi-acceptor effects play a minor role while the barrier lowering field effect of Y is dominant.

A kinetic study of the reactions of (CO)5CrC(SMe)C6H4Z (Z = 4-CF3, 4-Cl, 4-F, 4-Me, 4-MeO, and 4-Me2N) with n-butylamine, 2-chloroethylamine, piperidine, and 1-(2-hydroxyethyl)piperazine in 50% MeCN−50% water (v/v) at 25 °C is reported. In all cases the nucleophilic attachment of the amine to the carbene complex is rate limiting, which contrasts with the previously reported aminolysis of the methoxy analogues ((CO)5CrC(OMe)C6H4Z), where leaving group departure is rate limiting in most cases. The substituent effect on the nucleophilic attachment step is much weaker (average ρ = 0.55) than for the methoxy analogue (ρ = 2.03). It is shown that the small ρ value for (CO)5CrC(SMe)C6H4Z is “normal” and reflects the near cancellation of the substituent effects on the partial positive and partial negative charges at the transition state. These findings provide evidence that the high ρ value for (CO)5CrC(OMe)C6H4Z is “abnormal” and the result of the strong π-donor effect of the methoxy group on the reactant carben...

The kinetics of the reactions of (CO)5CrC(SMe)Ph (4-Cr) with five primary aliphatic and four secondary alicyclic amines have been investigated in 50% MeCN−50% water (v/v) at 25 °C. With n-butylamine, 2-methoxyethylamine, 2-chloroethylamine, glycinamide, piperidine, piperazine, and 1-(2-hydroxyethyl)piperazine, the reaction is strictly second order, i.e., first order in 4-Cr and first order in the amine. With morpholine, base catalysis by the amine and by OH- was observed while with aminoacetonitrile, OH- catalysis but no amine catalysis was detected. These results are interpreted in terms of a three-step reaction mechanism (Scheme 1): the first step is nucleophilic attachment of the amine to the carbene complex, to form a zwitterionic intermediate, ; the second step is a fast equilibrium deprotonation of to generate the anionic intermediate ; the third step is MeS- expulsion by two concurrent pathways, one being spontaneous (solvent catalyzed) expulsion of MeS-, the other being RR‘NH2+ catalyzed loss of ...

Acidity constants of six substituted 1-benzyl-1-methoxy-2-nitroethylenes (2-Z with Z = m-NO(2), m-CF(3), m-Cl, H, p-Me, p-MeO) and their respective nitronic acids were determined in 50% DMSO-50% water (v/v) at 20 degrees C. Kinetic data were obtained on the reversible deprotonation of all six 2-Z by OH(-) and piperidine and on the reversible deprotonation of 2-NO(2)() by piperazine, 1-(2-hydroxyethyl)piperazine, and morpholine in the same solvent. These data allowed a determination of the Brønsted coefficients alpha (dependence on acidity of 2-Z) and beta (dependence on amine basicity). The fact that alphabeta indicates the presence of a transition state imbalance which, however, is much smaller than that for the deprotonation of arylnitromethanes. The reasons for this reduction in the imbalance and their relevance to a recent study of the deprotonation of Fischer carbene complexes are discussed.

A kinetic study of the reversible cyclization of (((2-hydroxyethyl)thio)arylcarbene)pentacarbonyltungsten(0) complexes, 2-SR(Z), in 50% MeCN−50% water (v/v) at 25 °C is reported. The results are co...

A kinetic study of the reactions of (CO)5Cr C(OMe)C6H4X (X=4-Cl, 4-F, H, 4-Me, 4-OMe and 4-NMe2) and the respective tungsten carbene complexes with primary aliphatic amines (mainly n-butylamine) in 50% MeCN–50% water (v/v) at 25°C is reported. The rate law is complex and shows OH− and amine catalysis at low amine and OH− concentrations but not at high concentrations. The results are consistent with a three-step mechanism: the first step is nucleophilic addition of the amine to the carbene complex to form a zwitterionic steady state intermediate, TA±, which, in the second step, is followed by a rapid deprotonation of TA± to form the anionic steady state intermediate TA−, while the third step involves water and RNH3+ catalyzed methoxide ion expulsion, respectively, to form the substitution product, (CO)5M C(NHR)C6H4X (M=Cr or W). It is shown that the third step is rate limiting at low OH− and amine concentrations while the first step is rate limiting at high OH− and/or amine concentrations. The reasons why TA± and TA− cannot be observed directly are discussed and a detailed analysis of the substituent effects on various kinetic parameters is presented.

A kinetic study of the acid-catalyzed loss of alkoxide and thiolate ions from alkoxide and thiolate ion adducts, respectively, of benzylidene Meldrum's acid (1-H), methoxybenzylidene Meldrum's acid (1-OMe), and thiomethoxybenzylidene Meldrum's acid (1-SMe) is reported. The reactions appear to be subject to general acid catalysis, although the catalytic effect of buffers is weak and the bulk of the reported data refers to H(+)-catalysis. alpha-Carbon protonation and, in some cases, protonation of one of the carbonyl oxygens to form an enol compete with alkoxide or thiolate ion expulsion. This rendered the kinetic analysis more complex but allowed the determination of pK(a) values and of proton-transfer rate constants at the alpha-carbon. In conjunction with previously reported data on the nucleophilic addition of alkoxide and thiolate ions to the same Meldrum's acid derivatives, rate constants for nucleophilic addition by the respective neutral alcohols and thiols could also be calculated. Various structure-reactivity relationships are discussed that help define transition-state structures. Comparisons with similar reactions of alkoxide ion adducts of beta-alkoxy-alpha-nitrostilbenes provide additional insights.

The reaction of thiomethoxybenzylidene Meldrum's acid (5-SMe) with thiolate and alkoxide ion nucleophiles is shown to proceed by the two-step addition-elimination SNV mechanism in which the tetrahedral intermediate accumulates to detectable levels. For the reactions with thiolate ions, rate constants for nucleophilic addition (k1RX), its reverse (k-1RX), and for conversion of the intermediate to products (k2RX) were determined. For the reactions with alkoxide ions, only k1RX and k-1RX could be obtained; the intermediate in these reactions did not yield the expected substitution products, and hence no k2RX values could be determined. The reactions with OH- and water are believed to follow the same mechanism, but the respective intermediates remain at steady-state levels, and only k1OH and k1H²O for nucleophilic attack on 5-SMe were measurable. New insights regarding structure-reactivity behavior in SNV reactions are gained from comparisons of rate and equilibrium constants in the reactions of 5-SMe with the corresponding parameters in the reactions of methoxybenzylidene Meldrum's acid (5-OMe) and benzylidene Meldrum's acid (5-H). In particular, the relative importance of steric and pi-donor effects of the MeS vs. MeO group in 5-SMe and 5-OMe, respectively, and their role in affecting the intrinsic rate constants for nucleophilic addition, has been clarified by these comparisons. Our results also add support to a previous suggestion that soft-soft type interactions tend to increase intrinsic rate constants for thiolate ion addition to vinylic substrates, especially 5-SMe with the soft MeS group.Key words: nucleophilic vinylic substitution, intrinsic rate constants, transition state imbalances, steric/pi-donor/anomeric effects.

A kinetic study of the reaction of beta-methoxy-alpha-nitrostilbene (1-OMe) with cyanamide (CNA) over a pH range from 8.5 to 12.4 shows that it is the anion (CNA(-), pK(a) = 11.38) rather than the neutral amine that is the reactive species. Attempts at monitoring the reaction with the neutral CNA at low pH were unsuccessful because of competing hydrolysis. It is shown that the nucleophilic reactivity of CNA is abnormally low, probably because of a resonance effect, and that the reactivity of CNA(-) is high, higher than that of strongly basic oxyanion because of relatively weak solvation. The high reactivity of both 1-OMe and CNA(-) appeared to constitute favorable conditions conducive to the detection of the S(N)V intermediate, as has been the case in the reactions of 1-OMe with thiolate ions, alkoxide ions, and some amines. However, no intermediate was observed. Reasons for this failure are discussed.

Despite the central importance of monoketones as carbon acids, no intrinsic barriers or intrinsic rate constants for proton transfer to a standard family of bases such as amines have been reported. This paper presents the results of such determinations for the reaction of 2-acetyl-1-methylpyridinium ion (1) with secondary alicyclic amines. They yield log ko = 0.92 in water and log ko = 1.70 in 50% DMSO−50% water (v/v) at 20 °C. These intrinsic rate constants are lower than for any sterically unhindered carbon acids except for nitroalkanes, reflecting the strong π-acceptor character of the CO group as well as the strong solvation of the oxygen in the enolate ion. Rate constants, , for deprotonation of 1 by OH- have also been determined in water and 50% DMSO−50% water, with somewhat different results than those reported by Tobin and Frey (J. Am. Chem. Soc. 1996, 118, 12253). The value in water shows a small positive deviation from a correlation of such rate constants with the corresponding pKa of 17 simple ...

The pKa = 13.41 of the title compound (5), determined by a kinetic method, is about 1.3 units lower than the pKa of (2-oxacyclopentylidene)pentacarbonylchromium(0) (1) in 50% MeCN−50% water at 25 °C. The acidifying effect of the methyl group is attributed to its stabilizing effect on the CC double-bond resonance structure of the anion (5-). The rate constant for deprotonation of 5 by OH- is about the same as for deprotonation of 1, despite the higher acidity of 5. This means that the intrinsic rate constant for proton transfer from 5 is significantly lower than that from 1. This reduction in the intrinsic rate constant is mainly the result of the imbalanced nature of the transition state which deprives the transition state from the stabilizing effect of the methyl group. Based on precedents, the most likely mechanism for the hydrolysis of 5 involves rate-limiting protonation of 5- concerted with metal−carbon bond cleavage. Even though the relatively small kinetic solvent isotope effect cannot rigorously e...

(Phenylthio)nitromethane, PhSCH2NO2, is about as acidic as PhCH2NO2 and about 4 pKa-units more acidic than CH3NO2 in water or aqueous DMSO, showing the well-known acidifying effect of thio substituents in the α-position of carbon acids. Over the years various interpretations have been offered for the acidifying effect of sulfur groups: d−p π-resonance, polarizability, and negative hyperconjugation. Assuming that the nature of the factors that potentially stabilize the transition state of the proton transfer from the carbon acid are the same as those that potentially stabilize the carbanion, we show that a distinction between these interpretations can be based on the effect of the phenylthio group on the intrinsic rate constants (ko) of proton transfer. Such intrinsic rate constants were determined for the deprotonation of PhSCH2NO2 and PhCH2CH2NO2 by amines in water and 90% DMSO−10% water; in both solvents ko for PhSCH2NO2 was found to be substantially higher than for PhCH2CH2NO2 as well as for other nit...

A study of the acidity of various Fischer-type (alkoxyalkylcarbene)pentacarbonylmetal complexes and of their rates of deprotonation by OH- and amines in 50% acetonitrile−50% water at 25 °C is reported. The pKa values of (CO)5MC(OMe)CH3 for M = W (12.36) and M = Mo (12.81) are quite comparable to that for M = Cr (12.50) determined previously, and the pKa of (CO)5WC(OMe)CH2Ph (10.18) is similar to that of the chromium analog (10.40) reported earlier. These results indicate that the stabilization of the negative charge of the anion by the (CO)5M moiety depends little on the metal. The pKa of (CO)5CrC(OEt)CH3 (12.98) is 0.48 units higher than that of the methoxy analog; this increase probably reflects an enhanced stabilization of the zwitterionic resonance structure of the carbene complex, (CO)5Cr-C(O+Et)CH3, by the stronger electron-donating effect of the ethyl group. Intrinsic rate constants for the deprotonation of the carbene complexes by OH- (koOH) were estimated based on the Marcus equation, and intrins...

Aniline adducts of α-nitrostilbenes, PhCH(HNC 6 H 4 Z)CH(C 6 H 4 Y)NO 2 ha- ve been synthesized for a variety of Z/Y combinations. In several cases a mixture of two diastereomers was obtained. Structural assignment of the diastereomers was based both on 1 H NMR and reactivity data. The kinetics of the breakdown of the adducts into the respective benzalani- lines and arylnitromethane anions was studied in acidic 50% Me 2 SO-50% water solutions at 5 o C. The rate constants, k exp , were found to be independent of buffer concentration and pH, indicating a unimolecular mechanism. There is a strong dependence on the aniline substituent (ρ = -2.2) and the pK a of the arylnitromethanes (β 1g =-1.28).

A kinetic study of the reactions of substituted benzylidenemalonodialdehydes, ZC 6 H 4 CH=C(CHO) 2 with Z=p-NMe 2 , p-OMe, m-OMe, p-Cl, p-Br, m-NO 2 , and p-NO 2 , with OH - and water in aqueous solution is reported. The reactions lead reversibly to the formation of adducts, ZC 6 H 4 CH(OH)C-(CHO) 2 , although the kinetics are complex because of the formation of the enol form, ZC 6 H 4 CH-(OH)C(CHO)=CHOH, and OH - addition to one of the carbonyl groups. Analysis of the pH-dependence of the rates allows a determination of the rate and equilibrium constants for water and OH - addition as well as, in some cases, of the pKa of the enol form and the equilibrium constant of OH - addition to a carbonyl group. Furthermore, at low pH, where the p-NMe 2 derivative is protonated, k H2O 1 and K H2O 1 for this protonated form are also obtained. The rate and equilibrium constants correlate better with σ + than standard Hammett σ-values, indicating that a substantial amount of resonance stabilization is lost in converting the olefin to the adduct. BrOnsted-type plots of log k H2O 1 and log OH 1 vs log K H2O 1 show that the points for p-NMe 2 and p-OMe deviate negatively from the line defined by the others substituents. The observed deviations imply a reduced intrinsic rate constant (k o ) for these compounds, due to a transition state in which the loss of resonance is ahead of bond formation. This behavior is reminiscent of that in the reactions of substituted benzylidene Meldrum's acids and benzylidenemalononitriles with nucleophiles, but opposite to that found with substituted β-nitrostyrenes and other nitro-activated olefins

The hydrolysis of the title compound occurs by the four-step mechanism typical for the hydrolysis of olefins PhCH=CXY where X and/or Y are ele- ctron-withdrawing groups. These steps are nucleophilic attack by OH - or water on the olefin to form PhCH(OH)C(COMe) 2 (T - OH ); carbon pro- tonation of T - OH to form PhCH(OH)CH(COMe) 2 (T 0 OH ); oxygen depro- tonation of T 0 OH to form PhCH(O - )CH(COMe) 2 (T - O ); and collapse of T O - to PhCH=O and acetylacetonate anion. In strongly basic solution two kinetic processes were observed. To a good approximation the fast one can be assigned to reversible OH - addition to the substrate (k 1 - OH , k -1 H2 O ) to form T - OH and the slow one to irreversible con- version of T - OH to final products with rate-limiting carbon proto- nation of T - OH by water (k 2 H2 O ). In acidic solution only one kinetic process was observed; it corresponds to rate-limiting water addition to the substrate (k 1 H2 O )

A kinetic study of the reversible deprotonation of 9-cyanofluorene (2), 1,3-indandione (3), 4-nitrophenyl-acetonitrile (4), (3-nitrophenyl)nitromethane (5), and (4-nitrophenyl)nitromethane (6) by the anions of substituted benzylmalononitrile (1-X-) in 90M Me 2 SO-10% water (v/v) at 20 o C is reported. Intrinsic rate constants and intrinsic barriers of these reactions have been determined by extrapolation or interpolation of BrOnsted plots whose slopes (β) are all close to 0.5

A kinetic study of the aminolysis of [methoxy(phenyl)carbene]pentacarbonylchromium(O) with five primary aliphatic amines in 20% acetonitril-80% water (v/v) at 25 o C is reported. Second-order rate constants (k A ) increase with amine and OH - concentration, which indicates general base catalysin. Plots of k A vs amine or OH - concentration are curved and level off at high concentration; this implies a change in rate-limiting step and demonstrates that the reaction is stepwise

An ab initio study of two series of carbon-to-carbon proton transfer reactions is reported. The first series refers to the heterocyclic C(4)H(5)X(+)/C(4)H(4)X (X = CH(-), NH, S, O, PH, CH(2), AlH, BH) systems, and the second to the linear [Formula: see text] (X = CH(-), NH, S, PH, O, CH(2), AlH, BH) reference systems . The major objective of this study was to examine to what degree the aromaticity of C(4)H(4)X (X = CH(-), NH, S, O, PH) and the antiaromaticity of C(4)H(4)X (X = AlH, BH) is expressed at the transition state of the proton transfer and how this affects the respective intrinsic barriers. From the differences in the barriers between a given cyclic system and the corresponding linear reference system , ΔΔH(++) = ΔH(++)(cyclic) - ΔH(++)(linear), it was inferred that in the cyclic systems both aromaticity and antiaromaticity lower ΔH(++)(cyclic). This conclusion was based on the assumption that the factors not associated with aromaticity or antiaromaticity such as resonance, inductive and polarizability effects in the protonated species, and charge delocalization occurring along the reaction coordinate affect ΔH(++) for the cyclic and linear systems in a similar way and hence offset each other in ΔΔH(++). The extent by which ΔH(++)(cyclic) is lowered in the aromatic systems correlates quite well with the degree of aromaticity of C(4)H(4)X as measured by aromatic stabilization energies as well as the NICS(1) values of the respective C(4)H(4)X. According to the rules of the principle of nonperfect synchronization (PNS), these results imply 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. These conclusions are consistent with the changes in the NICS(1) values along the reaction coordinate. Other points discussed in the paper include the complex interplay of resonance, inductive, and polarizability effects, along with aromaticity and antiaromaticity on the proton affinities of C(4)H(4)X.

Reactions of anions with acrylonitrile, acrolein, methyl vinyl ketone, and methyl acrylate at room temperature under SIFT conditions are described. Stabilized carbanions such as benzyl anion, cyanomethyl anion, cyclohexadienide, acetone enolate, and nitromethyl anion add to activated carbon-carbon double bonds in a process that is competitive with other modes of reaction. With acrylonitrile and methyl acrylate, addition is in a 1,4 or Michael fashion, while with acrolein and methyl vinyl ketone, 1,2 addition completes with 1,4 addition. Hydroxide, methoxide, fluoride, and cyanide do not add to activated olefins by either mode. A mechanism is proposed for hydride transfer from cyclohexadienide to activated olefins

Rate constants (kB1 and kBH−1) for the reversible deprotonation of triphenyl(2-Z-9-fluorenyl)phosphonium ions (Z = H, Br, NO2) by piperidine, morpholine, n-butylamine, 2-methoxyethylamine, glycine ethyl ester, cyanomethylamine, OH− and water were determined in 50% Me2SO–50% (v/v) water at 20°C. Bronsted αCH (variation of carbon acid) and β values (variation of amine), and intrinsic rate constants [log k0 = log kB1/q when pKBHa - pKCHa + log(p/q) = 0] were obtained. αCH decreases with increasing basicity of the amine whereas βB decreases with increasing acidity of the carbon acid. These trends, which imply changes in the transition-state structure with reactivity, can be described by the interaction coefficient pxy = ∂βB/∂pKCHa = ∂αCH/ - ∂pKBHa = 0.03 (primary amines) and 0.01 (piperidine/morpholine). αCH is smaller than βB, indicating an imbalance due to a lag in the delocalization of the negative charge into the fluorenyl moiety at the transition state. The influence of the Ph3P+ group on the intrinsic rate constant is analyzed in terms of possible contributions by inductive/field (I), resonance (R), polarizability (P) and steric (S) effects. Using 9-carbomethoxyfluorene as a reference, it is shown that the stronger electron-withdrawing I effect of the Ph3P+ group relative to the COOMe group enhances log k0 substantially; the fact that the R effect of Ph3P+ is weaker than that of COOMe also contributes to an increase in k0, and so does the P effect of the phosphorus. All these increases are virtually completely offset by the rate-retarding S effect of the bulky Ph3P+ group. A similar analysis for the Me2S+ derivative studied by Murray and Jencks [J. Am. Chem. Soc. 112, 1880 (1990)] leads to similar conclusions except that the still smaller R effect is probably one of the main reasons why k0 for the Me2S+ derivative is more than ten times higher than for the Ph3P+ derivative; another potential reason is a difference in the steric effect.

The kinetics of reversible nucleophilic thiolate ion (RS - with R=n-Bu, HOCH 2 CH 2 , MeO 2 CCH 2 CH 2 and MeO 2 CCH 2 ) addition to Z-substituded β-nitrostyrenes (Z=4-Me 2 N, 4-MeO, 4-MeS, H, 3-Cl, 4-Cl, 3-CN, 4-CN, and 3-NO 2 ), to form ArCH(RS)CH=NO 2 - , have been measured in water at 20 o C. Rate constants in the forward (k 1 ) and reverse direction (k -1 ) and equilibrium constants (K 1 ) correlate reasonably well with Hammet σ-constants for the non-π-donor substituents but show deviations for the π-donors 4-Me 2 N, 4-MeO, and 4-MeS

The kinetics of nucleophilic addition of hydroxide (k 1 OH ) ion and nitromethide (k 1 NM ) ion to phenyl-substituted β-nitrostyrenes (Z=4-Me 2 N, 4-MeO, H, 3-C1, 4-Br, 4-CN, and 4-NO 2 ) have been measured in water, 50% Me 2 SO-50% water, 70% Me 2 SO-30% water (OH - reaction only), and 90% Me 2 SO-10% water

Rates of reversible deprotonation of 3,5-heptanedione (6b), 2,6-dimethyl-3,5-heptanedione (6c), and dibenzoylmethane (6d) by several primary aliphatic amines, by piperidine and morpholine, and by hydroxide ion (6b and 6d only) have been measured in 50% Me 2 SO-50% water (v/v) at 20 o C. Apparent pK a 's as well as the pK a values of the keto and the enol forms, and the enolization equilibrium constantes (K T ) were also determined. The pK a and K T values show the same trends observed previously in water. The intrinsic rate constants for the reactions of 6b and 6c with a given family of amines (primary aliphatic or secondary alicyclic) and the same and also equal to those for the reaction of acetylacetone (6a) with the same amines determined previously. These results indicate that steric effects play an insignificant role in the reactions of 6a, 6b, and 6c

The kinetics of the reaction of benzylidenemalonodialdehyde with piperidine, morpholine, n-butylamine, 2-methoxyethylamine, glycinamide, glycine ethyl ester, cyanomethylamine, and semicarbazide have been determined in 50% aqueous Me 2 SO at 20 o C. The reaction leads to a zwitterionic adduct, PhCH(RR'NH + )C(CHO) 2 − (T A ± ), that is in fast acid-base equilibrium with the anionic adduct, PhCH(RR'N)C(CHO) 2 − (T A − ). With strongly basic amines at high pH there is also attack of the amine on one of the carbonyl groups, which acts as a rapid preequilibrium. Rate constants for the formation of T A ± (κ 1 ) and its reverse (κ −1 ), as well as equilibrium constants (K 1 =κ 1 /κ −1 ) and the pK a of T A ± , were determined for all the amines. Intrinsic rate constants (κ 0 =κ 1 =κ −1 ) and the pK a of T A ± , were determined for all the amines. Intrinsic rate constants (κ 0 =κ 1 =κ −1 when K 1 =1) were calculated

The reversible deprotonation 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.

The role of negative hyperconjugation and anomeric and polar effects in stabilizing the XZHCbetaCalphaYY'- intermediates in SNV reactions was studied computationally by DFT methods. Destabilizing steric effects are also discussed. The following ions were studied: X = CH3O, CH3S, CF3CH2O and Y = Y' = Z = H (7b-7d), Y = Y' = H, Z = CH3O, CH3S, CF3CH2O (7e-7i), YY' = Meldrum's acid-like moiety (Mu), Z = H, (8b-8d), and YY' = Mu, Z = CH3O, CH3S, CF3CH2O (8e-8i). The electron-withdrawing Mu substituent at Calpha stabilizes considerably the intermediates and allows their accumulation. The hyperconjugation ability (HCA) (i.e., the stabilization due to 2p(Calpha) --> sigma*(Cbeta-X) interaction) in 8b-8d follows the order (for X, kcal/mol) CH3S (8.5) > CF3CH2O (7.6) approximately CH3O (7.5). The HCA in 8b-8d is significantly smaller than that in 7b-7d due to charge delocalization in Mu in the former. The calculated solvent (1:1 DMSO/H2O) effect is small. The stability of disubstituted ions (7e-7i and 8e-8i) is larger than that of monosubstituted ions due to additional stabilization by negative hyperconjugation and an anomeric effect. However, steric repulsion between the geminal Cbeta substituents destabilizes these ions. The steric effects are larger when one or both substituents are CH3S. The anomeric stabilization (the energy difference between the anti,anti and gauche,gauche conformers) in the disubstituted anions contributes only a small fraction to their total stabilization. Its order (for the following X/Z pairs, kcal/mol) is CF3CH2O/CH3S (8i, 4.9) > CF3CH2O/CH3O (8h, 3.9) > CH3O/CH3S (8g, 3.3) > CH3S/CH3S (8f, 2.9) > CH3O/CH3O (8e, 2.4). Significantly larger anomeric effects of ca. 8-9 kcal/mol are calculated for the corresponding conjugate acids.

The main question addressed in this paper is whether the nucleophilic substitution of the p-nitrophenoxy group in (CO)5Cr=C(OC6H4-4-NO2)Ph (1-NO2) by a series of substituted phenoxide ions is concerted or stepwise. Rate constants, kArO, for these substitution reactions were determined in 50% MeCN-50% water (v/v) at 25 degrees C. A Brønsted plot of log kArO versus pKa(ArOH) s consistent with a stepwise mechanism. This contrasts with reactions of aryl oxide ions with p-nitrophenyl acetate and with similar acyl transfers which are concerted. The reason for the contrast is that the tetrahedral intermediates formed in the reactions of 1-NO2 are much more stable than those in acyl transfers and the intrinsic barriers to their decomposition are higher than for the ester reactions. The points on the Brønsted plots for which pKa(ArOH)or = pKa(PNP) define a straight line with beta(nuc) = -0.39, suggesting that bond formation has made very little progress at the transition state and that partial desolvation of the nucleophile is part of the activation process. The hydrolysis of 1-NO2 and of the unsubstituted analogue (1-H) has also been studied over a wide pH range, providing rate constants for nucleophilic attack by hydroxide ion (kOH), by water (kH2O), and by general base-catalyzed reaction with water (kB). Furthermore, kH2O values were obtained for the hydrolysis of (CO)5Cr=C(OC6H4X)Ph (1-X) as a byproduct of the reactions of 1-NO2 with aryl oxide ions. Structure-reactivity relationships for these reactions are discussed in terms of inductive, pi-donor, and steric effects.