Quantification of Ion‐Pairing Effects on the Nucleophilic Reactivities of Benzoyl‐ and Phenyl‐Substituted Carbanions in Dimethylsulfoxide

Second‐order rate constants for the reactions of acceptor‐substituted phenacyl (PhCO?CH^??Acc) and benzyl anions (Ph?CH^??Acc) with diarylcarbenium ions and quinone methides (reference electrophiles) have been determined in dimethylsulfoxide (DMSO) solution at 20 °C. By studying the kinetics in the presence of variable concentrations of potassium, sodium and lithium salts (up to 10^?2 mol L^?1), the influence of ion‐pairing on the reaction rates was examined. As the concentration of K⁺ did not have any influence on the rate constants at carbanion concentrations in the range of 10^?4–10^?3 mol L^?1, the acquired rate constants could be assigned to the reactivities of the free carbanions. The counter ion effects increase, however, in the series K⁺<Na⁺<Li⁺, and the sensitivity of the carbanion reactivities toward variation of the counter ion strongly depends on the structure of the carbanions. The reactivity parameters N and s_N of the free carbanions were derived from the linear plots of log k₂ against the electrophilicity parameters E of the reference electrophiles, according to the linear‐free energy relationship log k₂(20 °C)=s_N(N+E). These reactivity parameters can be used to predict absolute rate constants for the reactions of these carbanions with other electrophiles of known E parameters.     

Electrophilic reactivities of 7-L-4-nitrobenzofurazans in σ-complexation processes: Kinetic studies and structure–reactivity relationships

The second-order rate constants (k) for reaction of 7-chloro-4-nitrobenzofurazan 1 and 7-methoxy-4-nitrobenzofurazan 2 with a series of nitroalkyl anions and several of para-substituted phenoxide anions in aqueous solution at 20 °C have been reported. On the basis of the linear novel approach recently designed by Mayr and coworkers, the electrophilicity parameters E at the C-5 position of the two nitrobenzofurazans 1 and 2 have been quantified and ranked on the comprehensive electrophilicity scale. Mayr's approach was found to correctly predict the rate constants for the addition of phenoxide anions at the C-5 position of 1 and 2 witting a factor of <2. Analysis of the kinetic measurements using Brønsted's model shows that β_nuc values remain remarkably constant for changes in the nature of the substituent and that the σ-complexation process is associated with high Marcus intrinsic barriers. In addition, satisfactory correlations between the log k_exp (k_exp values measured in this work for reactions of benzofurazans 1 and 2 with a series of phenoxide anions in aqueous solution at 20 °C) and log k_calcd (k_calcd values calculated from equation 1 using the electrophilicity parameters E of benzofurazans 1 and 2 and the previously published nucleophilicity parameters N and s_N of the phenoxide anions) with a slope very close to unity have been obtained and discussed.     

Kinetics and quantification of the electrophilic reactivities of substituted thiophenes and structure‐reactivity relationships

Second‐order rate constants (k₁) have been measured spectrophotometrically for reactions of 2‐methoxy‐3‐X‐5‐nitrothiophene 1a‐c (X = NO₂, CN, and COCH₃) with secondary cyclic amines (pyrrolidine 2a , piperidine 2b , and morpholine 2 c ) in CH₃CN and 91:9 (v/v) CH₃OH/CH₃CN at 20°C. The experimental data show that the rate constants (k₁) values exhibit good correlation with the parameters of nucphilicity (N) of the amines 2a‐c and are consistent with the Mayr's relationship log k (20°C) = s(E + N). We have shown that the electrophilicity parameters E derived for 1a–c and those reported previously for the thiophenes 1d‐g (X = SO₂CH₃, CO₂CH₃, CONH₂, and H) are linearly related to the pK_a values for their gem‐dimethoxy complexes in methanol. Using this correlation, we successfully evaluated the electrophilicity E values of 12 structurally diverse electrophiles in methanol for the first time. In addition, a satisfactory linear correlation (r² = 0.9726) between the experimental (log k_exp) and the calculated (log k_calcd) values for the σ‐complexation reactions of these 12 electrophiles with methoxide ion in methanol has been observed and discussed.     

Pushing the Upper Limit of Nucleophilicity Scales by Mesoionic N-Heterocyclic Olefins

A series of mesoionic, 1,2,3-triazole-derived N-heterocyclic olefins (mNHOs), which have an extraordinarily electron-rich exocyclic CC-double bond, was synthesized and spectroscopically characterized, in selected cases by X-ray crystallography. The kinetics of their reactions with arylidene malonates, ArCH=C(CO₂Et)₂, which gave zwitterionic adducts, were investigated photometrically in THF at 20 °C. The resulting second-order rate constants k₂(20 °C) correlate linearly with the reported electrophilicity parameters E of the arylidene malonates (reference electrophiles), thus providing the nucleophile-specific N and s_N parameters of the mNHOs according to the correlation lg k₂(20 °C)=s_N(N+E). With 21<N<32, the mNHOs are much stronger nucleophiles than conventional NHOs. Some mNHOs even excel the reactivity of mono- and diacceptor-substituted carbanions. It is exemplarily shown that the reactivity parameters thus obtained allow to calculate the rate constants for mNHO reactions with further Michael acceptors and predict the scope of reactions with other electrophilic reaction partners including carbon dioxide, which gives zwitterionic mNHO-carboxylates. The nucleophilicity parameters N correlate linearly with a linear combination of the quantum-chemically calculated methyl cation affinities and buried volumes of mNHOs, which offers a valuable tool to tailor the reactivities of strong carbon nucleophiles.     

SNAr reactions of substituted pyridines with secondary amines in aqueous solution: Kinetic and reactivity indices in density functional theory

A kinetic study is reported for the reactions of 2-methoxy-3-nitropyridine 1a and 2-methoxy-5-nitropyridine 1b with three secondary amines 2a–c (morpholine, piperidine, and pyrrolidine) in aqueous solution at 20°C. The Brønsted-type plots are linear with β_nuc = 0.52 and 0.55 for pyridines 1a and 1b , respectively, indicating that the reaction proceeds through a S_NAr mechanism in which the first step is the rate-determining step. Additional theoretical calculations using the DFT/B3LYP method confirm that the C-2 carbon being the most electrophilic center for the both pyridines 1a and 1b . The second-order rate constants have been used to evaluate the electrophilicity parameters E of 1a and 1b according to the linear free energy relationship log k (20°C) = s_N (N + E). The E parameters thus derived are compared with the electrophilic reactivities of a large variety of anisoles. The validity of these E values has been satisfactorily verified by comparison of calculated and experimental second-order rate constants for the reactions of pyridines 1a and 1b with anion of ethyl benzylacetate.     

Kinetic Studies on Nucleophilic Addition Reactions of 4‐X‐substituted Anilines to N1‐methyl‐4‐nitro‐2,1,3 benzothiadiazolium terafluoroborate in Acetonitrile: Reaction Mechanism and Mayr's Approach

The kinetics of the coupling of N1‐methyl‐4‐nitro‐2,1,3 benzothiadiazolium tetrafluoroborate 1 with a series of 4‐X‐substituted anilines 2a–f (X = OH, OMe, Me, H, Cl, and CN) have been investigated in acetonitrile at 20°C. The second‐order rate constants result in a nonlinear Brönsted‐type plot. The Hammett plot is also nonlinear, whereas the Yukawa–Tsuno plot exhibits an excellent linear correlation with ρ = –1.62 and r = 1.44. The large Brönsted (β_nuc = 1.24) and Hammett (ρ = –5.16) values suggest that the reactions proceed trough a single electron transfer mechanism. The finding of satisfactory correlation between the log k₁ of the reactions and the oxidation potentials (E°) of anilines 2a–d supports this mechanism. On the other hand, electrophilicity parameter E of benzothiadiazolium cation 1 as defined by the correlation log k_20°C = s(E + N) has been determined and compared with the electrophilic reactivities of a large variety of electrophiles.     

Nucleophilicities of Para‐Substituted Phenoxide Ions in Water and Correlation Analysis

Second‐order rate constants for the reactions of 2‐aryl‐4,6‐dinitrobenzotriazole 1‐oxides 1a‐d with some 4‐X‐substituted phenoxide ions 2a‐d (X = OCH₃, H, Cl, and CN) have been measured in aqueous solution at 20°C. The pK_a values for the σ‐complexation processes of a series of benzotriazole 1a‐d measured in water have been used to determine their electrophilicity parameters E according to the correlation E = –3.20 – 0.662 pK_a (F. Terrier, S. Lakhdar, T. Boubaker, and R. Goumont, J Org Chem, 2005 , 70, 6242–6253). For these reactions, plots of log k versus the electrophilicity parameters E of the benzotriazoles 1a‐d were linear, allowing to derive the nucleophilicity parameters N and s for phenoxide ions as defined by the Mayr equation log k₁ (20°C) = s (E + N) (H. Mayr, M. Patz. Angew Chem, Int Ed Engl 1994 , 33, 938–957). The N values are found to cover a range of nucleophilicity from 6.85 to 10.22, going from 4‐cyanophenoxide 2d for the least reactive ion to 4‐methoxyphenoxide 2a for the most reactive nucleophile. Good linear correlations were found between the nucleophilicity parameters N of phenoxide ions 2a‐d and the pK_a values of their conjugate acids (N = –3.05 + 1.25 pK_a) and the constants of the substituents X (N = 9.21 – 2.51).     

The ambident electrophilic behavior of 5‐nitro‐3‐X‐thiophenes in σ‐complexation processes

Kinetics of the reactions of 3,5‐dinitrothiophene 1 and 3‐cyano‐5‐nitrothiophene 2 with a series of parasubstituted phenoxide anions 3a–c have been investigated in aqueous solution at 20°C. Two unsubstituted electrophilic centers (C(2) and C(4)) of the two thiophenes have been identified. The Fukui functions correctly predict the C(2) and C(4) atoms as the most electrophilic centers of these electron‐deficient thiophenes 1 and 2 . Analysis of the experimental data in terms of Brønsted relationships reveals that the reaction mechanism likely involves a single‐electron transfer (SET) process. The excellent correlations upon plotting the rate constants versus the oxidation potentials E^o values is an additional evidence that reactions between thiophenes and phenoxide anions are proceeding through an initial electron transfer. It is of particular interest to note that the systems studied in this paper provide a rare example of a SET mechanism in σ‐complexation reactions. According to the free energy relationship log k = s(N + E) (Angew. Chem., Int. Ed. Engl., 1994, 33, 938–957), the electrophilicity parameters E of the C‐4 and C‐2 positions of the thiophenes have been determined and compared with the reactivities of other ambident electrophiles. On the other hand, the second‐order rate constants for the reactions of these thiophenes with the hydroxide ion has been measured in water and 50% water–50% acetonitrile and found to agree with those calculated theoretically using Mayr's equation from the E values determined in this work and from the previously published N and s parameters of OH⁻.     

Synthesis of new v-triazolopyrimidinium salts

New v-triazolo[1,5-α]- and v-triazolo[1,5-c]pyrimidinium salts 12a-e, 13a-c have been synthesized via oxidation (i.e. cyclodehydrogenation) of the appropriate pyrimidyl ketone arylhydrazones 3a-e, 6a-c using TBB (2,4,4,6-tetrabromocyclohexa-2,5-dien-1-one) as the reagent. The arylhydrazones were obtained by standard reactions; the Grignard reaction of 2-cyano- and 4-cyanopyrimidine 1a,b, 4a-c gave 2-pyrimidyl- and 4-pyrimidyl ketones 2a-e, 5a-c , which reacted with arylhydrazines to yield the desired ketone arylhydrazones 3a-e, 6a-c.     

Reactions of Substituted Phenylnitromethane Carbanions with Aromatic Nitro Compounds in Methanol: Carbanion Reactivity,Kinetic, and Equilibrium Studies

The feasibility of carrying out nucleophilic addition from electron‐deficient heteroaromatics has been addressed through a detailed investigation of the interaction of a two 7‐substituted‐nitrobenzofurazan (R = OMe 2a ; R = Cl 2b ) with a series of substituted‐nitroaryl anions (X = 4‐NO₂ 1a ; X = 3‐NO₂ 1b ; X = 4‐CN 1c ; X = 4‐Br 1d ), all reactions first lead to the quantitative formation of the σ‐adducts 3a–d and 4a–d arising from covalent addition of the nucleophile to the C‐5 carbon. The rate and equilibrium constants for the formation of σ‐adducts 3a–d and 4a–d (k₅, K ₅ ) together with the rate constants for their decomposition (k_?5) have been determined in methanol at 25°C, allowing a determination of intrinsic rate constants, k₀ = 0.03, the lower k₀ value reflects the very strong salvation by methanol of the negative charge on the nitro group. The discovery of a linear correlation between the E and pK_a^MeOH parameters allows a calibration of the electrophilicity power of 2a and 2b , E = ?11.67 and ?10.29, respectively. Applying the general approach to nucleophilicity/electrophilicity recently developed by Mayr et al. through the relationship log k = s(E + N), a successful ranking of our nitroaryl anions 1a–d on the general nucleophilicity scale (N) has been carried out. The N values of 1a–d are found to cover a range from 15.78 to 16.69. The results are compared with previously reported data in water and DMSO.     

Structure-reactivity relationships on Michael additions of secondary cyclic amines with 3-cyanomethylidene-2-oxindoline derivatives

A kinetic study of the nucleophilic addition reactions of 3-cyanomethylidene-2-oxindoline derivatives with cyclic amines (namely: piperidine, morpholine and pyrrolidine) in MeCN solution at 20 °C is reported. The second-order rate constants showed of this process fit nicely the Brönsted equation log k₁ = β_nuc pK_a + C, allowing the determination of the β_nuc parameter in the range of 0.63 < β_nuc < 0.77 that indicates that the degree of formation of N–C bond in the transition state is more half complete. Moreover, the analysis of the kinetic measurements based on a good linear log k (20 °C) = s_N (E + N) free enthalpy relationship are used to assess the electrophilic reactivity in term of E parameter of these series of 2-oxindoline derivatives Michael acceptors. Of major interest is that the estimated E values were established to cover a domain of reactivity of ~3 units of E, ranging from −17.5 for 2-(5-chloro-2-oxindolin-3-ylidene)malononitrile (the most reactive electrophile) to −20.3 for ethyl 2-(5-chloro-1-methyl-2-oxindolin-3-ylidene)-2-cyanoacetate (the least reactive electrophile). The theoretical reactivity indices ω based on the conceptual Density Functional Theory (DFT) explains correctly the experimental electrophilicity E ordering founded in terms of experimental scales.     

Theoretical studies and rate constants calculation for the reactions of acetone with fluorine and bromine atoms

Theoretical investigations are carried out on the multichannel reactions CH₃COCH₃ + F (R1) and CH₃COCH₃ + Br (R2) by means of direct dynamics methods. The minimum energy path (MEP) is obtained at the MP2/6-31 + G(d,p) level, and energetic information is further refined at the MC-QCISD (single-point) level. The rate constants are calculated by the improved canonical variational transition-state theory (ICVT) with the small-curvature tunneling (SCT) contributions in a wide temperature range 200–1,500 K for the title reactions, H-abstraction channel is favored for the two reactions. The theoretical overall rate constants are in good agreement with the available experimental data and are found to be k _1a  = 3.22 × 10⁻¹⁵ T ^1.51exp(1,190.91/T) cm³ molecule⁻¹ s⁻¹, k ₂  = 5.95 × 10⁻¹⁸ T ^1.98exp(−4,622.45/T) cm³ molecule⁻¹ s⁻¹. Furthermore, the rate constants of reaction Cl + CH₃COCH₃ (R3) calculated in the other paper are added to discuss the reactivity trend of different halogen reaction with acetone on the rate constants of this class of hydrogen abstraction reactions.     

The influence of substituents on the kinetics of the reaction of carbonyl oxides with benzaldehyde

The reactivity of carbonyl oxides toward benzaldehyde was characterized by thek ₃₃/k ₃₃ ratio, wherek ₃₃ andk ₃₁ are the rate constants of the reactions of RCOO with PhCHO and diphenyldiazomethane Ph₂CN₂, respectively. Thek ₃₃/k ₃₁ ratios obtained at 60°C in acetonitrile range from 0.61·10⁻² (m-BrPh₂CN₂) to 20·10⁻² (Ph₂MeCHO). The reactions are probably preceded by the formation of a charge-transfer complex (CTC) with charge transfer from aldehyde to RCOO. The carbonyl oxide reacts with aldehydes by both the nucleophilic pathway (at the C atom of the—CHO group to form 1,2,4-trioxolane) and electrophilic pathway (by the attack at the aromatic ring with the intermediate formation of CTC). In the latter case, either 1,2,4-trioxolane or oxidation products of the phenyl ring are formed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 650–654, April, 2000.     

Quantification of the Electrophilicities of Diazoalkanes: Kinetics and Mechanism of Azo Couplings with Enamines and Sulfonium Ylides

Kinetics and mechanism of the reactions of methyl diazoacetate, dimethyl diazomalonate, 4-nitrophenyldiazomethane, and diphenyldiazomethane with sulfonium ylides and enamines were investigated by UV-Vis and NMR spectroscopy. Ordinary alkenes undergo 1,3-dipolar cycloadditions with these diazo compounds. In contrast, sulfonium ylides and enamines attack at the terminal nitrogen of the diazo alkanes to give zwitterions, which undergo various subsequent reactions. As only one new bond is formed in the rate-determining step of these reactions, the correlation lg k₂(20 °C)=s_N(N+E) could be used to determine the one-bond electrophilicities E of the diazo compounds from the measured second-order rate constants and the known reactivity indices N and s_N of the sulfonium ylides and enamines. The resulting electrophilicity parameters (−21<E<−18), which are 11–14 orders of magnitude smaller than that of the benzenediazonium ion, are used to define the scope of one-bond nucleophiles which may react with these diazoalkanes.     

Kinetics of the polycondensation and copolycondensation of bis(3‐hydroxypropyl) terephthalate and bis(4‐hydroxybutyl) terephthalate

The kinetics of the polycondensation and copolycondensation reactions of bis(3‐hydroxypropyl) terephthalate (BHPT) and bis(4‐hydroxybutyl) terephthalate (BHBT) as monomers were investigated at 270 °C in the presence of titanium tetrabutoxide as a catalyst. BHPT was prepared by the ester interchange reaction of dimethyl terephthalate and 1,3‐propanediol (1,3‐PD). Through the same method adopted for BHPT synthesis, BHBT was prepared with 1,4‐butanediol instead of 1,3‐PD. With second‐order kinetics applied for polycondensation, the rate constants of the polycondensation of BHPT and BHBT, k₁₁ and k₂₂, were calculated to be 4.08 and 4.18 min^?1, respectively. The rate constants of the cross reactions in the copolycondensation of BHPT and BHBT, k₁₂ and k₂₁, were calculated with results obtained from proton nuclear magnetic resonance spectroscopy analysis. The rate constants during the copolycondensation of BHPT and BHBT at 270 °C decreased in the order k₁₂ > k₂₂ > k₁₁ > k₂₁, indicating that the reactivity of BHBT was larger than that of BHPT at 270 °C. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2435–2441, 2002     

Temperature dependences of the reactions of O(3P) atoms with selected chlorofluoroethenes between 298 and 359 K

The rate constants for the gas‐phase reactions of ground‐state oxygen atoms with CF₂?CFCl (1), (E/Z)‐CFCl?CFCl (2), CFCl?CH₂ (3), and (E/Z)‐CFH?CHCl (4) have been measured directly using a discharge flow tube coupled to a chemiluminescence detection system. The experiments were carried out under pseudo‐first‐order conditions with [O³P)]₀ ? [ethene]₀. The temperature dependences of the reactions were studied for the first time in the range 298–359 K. The proposed Arrhenius expressions (in units of cm³ molecule^?1 s^?1) were k₁ = (1.07 ± 0.32) × 10^?11 exp{?(8000±1600)/RT}, k₂ = (0.56 ± 0.10) × 10^?11 exp{?(8700±500)/RT}, k₃ = (4.23 ± 1.25) × 10^?11 exp{?(12,700 ± 800)/RT}, and k₄ = (1.13 ± 0.62) × 10^?11 exp{?(10,500 ± 1500)/RT}. All the rate coefficients display a positive temperature dependence, which highlights the importance of the irreversibility of the addition mechanism for these reactions. Halogen substitution in the ethene is discussed in terms of reactivity with O(³P). © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 763–769, 2005     

Nucleophilic reactivities of Schiff base derivatives of amino acids

Treatment of α-imino esters derived from glycine esters and benzophenone or benzaldehydes with potassium tert butoxide in DMSO gave persistent solutions of carbanions at 20?°C. The kinetics of their reactions with quinone methides and benzylidene malonates (reference electrophiles) have been followed photometrically under pseudo-first order conditions. The reactions followed second-order rate laws. Since addition of 18-crown-6 ether did not affect the reaction rates, the measured rate constants correspond to the reactions of the non-paired carbanions. Plots of the second-order rate constants against the electrophilicity parameters E of the electrophiles are linear, which allowed us to derive the nucleophile-specific parameters N and s_N, according to the linear Gibbs energy relationship lg k₂(20?°C)?=?s_N(N + E). The Ph₂C?=?N- and PhCH?=?N- groups act as very weak electron acceptors with the consequence that Ph₂C?=?N-CH^–-CO₂R and PhCH?=?N-CH^–-CO₂R have a similar nucleophilicity as Ph-CH^–-CO₂Et, the anion of ethyl phenylacetate.     

Reaction of the NO3 radical with some thiophenes: Kinetic study and a correlation between rate constant and EHOMO

The rate constants and activation energies for the reactions of some thiophenes with the NO₃ radical were measured using the absolute fast‐flow discharge technique at 263–335 K and low pressure. The proposed Arrhenius expressions for 2‐ethylthiophene, 2‐propylthiophene, 2,5‐dimethylthiophene, and 2‐chlorothiophene are k = (4.2 ± 0.28) ×10^?16 exp[(2280 ± 70)]/T, k = (7.0 ± 2) × 10^?18 exp[(3530 ± 70)]/T, k = (1 ± 1) × 10^?14 exp[(1648 ± 240)]/T, and k = (8 ± 2) × 10^?17 exp[(2000 ± 200)]/T (k = cm³ molecule^?1 s^?1), respectively. The reactions of this radical with 2‐chlorothiophene and 3‐chlorothiophene were also studied by a relative method in a Teflon static reactor at room temperature and atmospheric pressure. The effect of substitution on thiophene reactivity is discussed, and a relationship between the rate constants and the ionization potential (IP = ?E_HOMO) has been proposed. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 570–576, 2006     

Reactions of the HO2 radical with OH,H, Fe2+ and Cu2+ at elevated temperatures

The temperature dependence of the rate constant for the reactions of HO₂ with OH, H, Fe²⁺ and Cu²⁺ has been determined using pulse radiolysis technique. The following rate constants, k (dm³ mol⁻¹ s⁻¹) at 20°C and activation energies, E_a (kJ mol⁻¹) have been found. The reaction with OH was studied in the temperature range 20–296°C (k=7.0×10⁹, E_a=7.4) and the reaction with H in the temperature range 5–149°C (k=8.5×10⁹, E_a=17.5). The reaction with Fe²⁺ was studied in the temperature range 16–118°C (k=7.9×10⁵, E_a=36.8) and the reaction with Cu²⁺ in the temperature range 17–211°C (k=1.1×10⁸, E_a=14.9).     

Electrophilic Reactivities of Azodicarboxylates

The kinetics of the reactions of the azodicarboxylates 1 with the enamines 2 have been studied in CH₃CN at 20 °C. The reactions follow a second‐order rate law and can be described by the linear free energy relationship log k₂(20 °C)=s(N+E) (E=electrophilicity parameter, N=nucleophilicity parameter, and s=nucleophile‐specific slope parameter). With E parameters from ?12.2 to ?8.9, the electrophilic reactivities of 1 turned out to be comparable to those of α,β‐unsaturated iminium ions, amino‐substituted benzhydrylium ions, and ordinary Michael acceptors. While the E parameters of the azodicarboxylates 1 determined in this work also hold for their reactions with triarylphosphines, they cannot be used for estimating rate constants for their reactions with amines. Comparison of experimental and calculated rate constants for cycloadditions and ene reactions of azodicarboxylates provides information on the concertedness of these reactions.