Allgemeine Basenkatalyse der Azokupplung von o-Diazophenolen. 19. Mitteilung zur Kenntnis der Azokupplungsreaktion

1. The kinetics and the mechanism of the diazo coupling reaction of 2-diazophenol-4-sulphonic acid with 1-naphthol-2-sulphonic acid have been investigated at 0° and ionic strength I=0.45. 2. The pK_a-value of the hydroxyl group in 2-diazophenol-4-sulphonic acid has been determined: pK_a=- 0.04 ± 0.10. It is the diazonium-phenolate anion which actually enters into the diazo coupling reaction. 3. The reaction is subject to general base catalysis. It is shown that no intermediate is enriched during the reaction at pH 11.3–11.6 which proceeds by a two-step mechanism with a steady state intermediate.     

Water-Tolerant Superbase Polyoxometalate [H2(Nb6O19)]6− for Homogeneous Catalysis

Here, doubly protonated Lindqvist-type niobium oxide cluster [H₂(Nb₆O₁₉)]^6–, fabricated by microwave-assisted hydrothermal synthesis, exhibited superbase catalysis for Knoevenagel and crossed aldol condensation reactions accompanied by activating C−H bond with pK_a >26 and proton abstraction from a base indicator with pK_a=26.5. Surprisingly, [H₂(Nb₆O₁₉)]⁶⁻ exhibited water-tolerant superbase properties for Knoevenagel and crossed aldol condensation reactions in the presence of water, although it is well known that the strong basicity of metal oxides and organic superbase is typically lost by the adsorption of water. Density functional theory calculation revealed that the basic surface oxygens that share the corner of NbO₆ units in [H₂(Nb₆O₁₉)]⁸⁻ maintained the negative charges even after proton adsorption. This proton capacity and the presence of un-protonated basic sites led to the water tolerance of the superbase catalysis.     

General Base Catalysis by Organic Bases of the Reaction of Diphenylphosphinic Hydrazide with Phenyl Isocyanate in Benzene

The kinetics of the reaction of diphenylphosphinic hydrazide with phenyl isocyanate in the presence of organic bases in benzene at 25°C were studied. The catalytic activity of the bases correlates with spectroscopic and thermodynamic Taft (pK _{H B}), Koppel-Palm (B), and Gutmann (DN) parameters. A common general base mechanism of the catalysis of semicarbazide formation by bases belonging to different classes of organic compounds is offered and discussed.     

Dediazoniation of Arenediazonium Ions in Homogeneous Solutions. Part XVII. Kinetics and mechanisms of dediazoniation of p-chlorobenzenediazonium tetrafluoroborate in weakly alkaline aqueous solutions in the presence of oxygen

The kinetics of dediazoniation of p-chlorobenzenediazonium tetrafluoroborate have been studied in buffer solutions in the pH-range 9.0–10.0, ionic strength I = 0.10, at 20.0° in glass and polytetrafluoroethylene vessels. The presence of oxygen (<5 ppb of O₂, 60 to 100 ppb of O₂, air, > 99% of O₂) has a decisive influence on the rate and kinetic order of the dediazoniation. Iodoacetic acid inhibits the reaction, whereas p-chlorophenol has a catalytic effect, and in air and >99% of O₂ it acts as an autocatalyst. The reaction is subject to general-base catalysis by water, hydroxyl ions, hydrogen carbonate and carbonate ions. The kinetic results are interpreted in conjunction with data concerning the reaction products [2] and a ¹⁵N-CIDNP. investigation of a related system [3]. Specific radical chain mechanisms are consistent with the results.     

Polyphenylens,Cross-Linked Through Pd-Carbene Complexes Formation,for Catalysis in Suzuki-Miyaura Reactions

Polyphenylenes supported N-heterocyclic carbene Pd-complexes were synthesized for catalysis of cross-coupling Suzuki-Miyaura reactions. Starting polyphenylenes were prepared by cyclocondensation reaction of diacetylaromatic with monoacetylaromatic compounds. N-metylimidazole has been involved to the polymer through the mono-functional acetyl monomer, in which in p-position to acetyl group the group of haloalkyl was situated, and haloalkyl group interacted with N-methylimidazole. N-heterocyclic carbene complexes of Pd were synthesized usually by the reaction of imidazolium salts with the salts of transition metals in the presence of a base, obtaining the complex (N-heterocyclic carbene)₂PdX₂. The catalysis reaction was carried out between arylhalides (iodo- or bromobenzene) and phenylboronic acid with the presence of 1 mol% of Pd. The yields of biphenyl are from 70 to 95%, which is comparable with homogeneous catalysis.     

The Mechanism of the Acid-catalysed Hydrolysis of 1-Aryl-2,2,2-trifluorodiazoethanes

The rates of the acid-catalysed hydrolysis of a series of 1-aryl-2,2,2-trifluorodiazoethanes la-d have been measured in dioxan/water/HClO₄. The results are well correlated with the Hammett equation when σ_p substituent constants are used (?_H = ?1.74 and ?_D = ?1.75). Kinetic solvent isotope effects, about 2.1, and general acid catalysis indicate that proton transfer is rate-determining (A-S_E2 mechanism). Rate measurements have also been made at pressures up to 1400 atm. The evaluated activation volumes, about ?20 cm³/mole, indicate that at least one water molecule is bound in the transition state of protonation. Rate measurements at low water concentrations indicate that no apparent change in mechanism has occured.     

Stepwise Versus Concerted Mechanisms in General‐Base Catalysis by Serine Proteases

General‐base catalysis in serine proteases still poses mechanistic challenges despite decades of research. Whether proton transfer from the catalytic Ser to His and nucleophilic attack on the substrate are concerted or stepwise is still under debate, even for the classical Asp‐His‐Ser catalytic triad. To address these key catalytic steps, the transformation of the Michaelis complex to tetrahedral complex in the covalent inhibition of two prototype serine proteases was studied: chymotrypsin (with the catalytic triad) inhibition by a peptidyl trifluoromethane and GlpG rhomboid (with Ser‐His dyad) inhibition by an isocoumarin derivative. The sampled MD trajectories of averaged pK_a values of catalytic residues were QM calculated by the MD‐QM/SCRF(VS) method on molecular clusters simulating the active site. Differences between concerted and stepwise mechanisms are controlled by the dynamically changing pK_a values of the catalytic residues as a function of their progressively reduced water exposure, caused by the incoming ligand.     

Catalysis in aromatic nucleophilic substitution. Note II. Piperidino substitution reactions of some 2-l-3-nitrothiophenes and 2-l-5-nitrothiophenes in methanol and benzene

The rates of piperidino substitution of some 2-L-3-nitrothiophenes (I) and 2-L-5-nitrothiophenes (II) (L = Cl, Br, I, OC₆H₄NO₂-p, and SO₂Ph) have been measured in methanol and in benzene at various piperidine concentrations. The reactivity of compounds (I) is not affected by the piperidine concentration in both methanol and benzene, except for the case of L = I (Ic). Probably due to association effects, the reactivity of Ic in benzene decreases as the piperidine concentration is increased. The reactions of compounds II follow overall second order kinetics in methanol while in benzene a different behaviour is observed as a function of the nature of the leaving group. In fact, the piperidino substitutions of IIa-c (L = Cl, Br, I) are mildly accelerated at high piperidine concentrations (a moderate solvent effect); on the contrary the reactivity of IId and e shows a strong dependence on the piperidine concentration, pointing out a genuine base catalysis.     

Water desorption isotherms of cellulose-acetate membranes

 The water desorption isotherms are determined in three cellulose acetate membranes with different acetyl content as a function of p/p ₀ at 10–40 °C. The partition coefficients (adsorbed water over water pressure) show a minimum at p/p ₀=0.5–0.6. This indicates a two energy mechanism. The agreement of our results with the BET adsorption isotherms only till p/p ₀<0.3 shows that a two energy adsorption mechanism is valid only for small water contents, probably one hydrate layer and a second more liquid-like water layer. At large p/p ₀, the adsorbed water becomes more and more liquid like by polarization of the hydrogen bonds. The heat of desorption is larger than the vaporization heat of water ΔH _vap(H₂O). It decreases with increasing water content asymptotically to ΔH _vap(H₂O). The cause may be a larger van der Waals interaction of the hydrate layer due to coordination numbers larger than 4.4 as in liquid water. Additionally, we found a hole adsorption process by sorbing unpolar solvents. The water and methonal adsorption are 100 times larger due to a swelling mechanism depending on the number of acetyl groups in the membranes. The amounts of n-alcohols sorbed decrease with their chain length. Received: 25 April 1997 Accepted: 10 June 1997     

Direct Estimation of the Surface Location of Immobilized Functional Groups for Concerted Catalysis Using a Probe Molecule

The location of active sites during concerted catalysis by a metal complex and tertiary amine on a SiO₂ surface is discussed based on the interaction between the functionalized SiO₂ surface and a probe molecule, p‐formyl phenylboronic acid. The interactions of the probe molecule with the surface functionalities, diamine ligand, and tertiary amine, were analyzed by FT‐IR and solid‐state ¹³C and ¹¹B MAS NMR. For the catalyst exhibiting high 1,4‐addition activity, the diamine ligand and tertiary amine base exist in closer proximity than in the catalyst with low activity.     

Synthesis and Structure of Bis(para‐methoxyphenyl)selenoxide and its Monohydrate. Theoretical Considerations of the Hydration of Diorganoselenium Oxides

The title compound was prepared by base hydrolysis of (p‐MeOC₆H₄)₂SeCl₂ in water and isolated as the crystalline monohydrate, (p‐MeOC₆H₄)₂SeO·H₂O, in which the water molecule is associated via hydrogen‐bonding. Water‐free (p‐MeOC₆H₄)₂SeO was obtained crystalline after drying and recrystallisation from toluene. Both crystal phases were investigated by single crystal X‐ray diffraction. Preliminary DFT calculations at the B3LYP/LANL2DZdp level of theory suggest that the hydrogen bonded complexes R₂SeO·H₂O (R = H, Me, Ph) are by 2.79, 3.36 and 11.10 kcal mol^?1 more stable than the corresponding elusive diorganoselenium dihydroxides R₂Se(OH)₂. The hydrogen bond energies of R₂SeO·H₂O (R = H, Me, Ph) are 5.98, 7.18 and 5.89 kcal mol^?1.     

Stereochemistry of the σ-Complex Intermediate in Sterically Hindered Electrophilic Aromatic Substitutions. (20th communication on diazo coupling reactions)

The kinetics of the diazo coupling reactions of diazotized sulfanilic acid with 9 derivatives of 2-naphthol, each containing a substituent in the 8-position, have been measured. The reactions proceed by general base catalysis. The rate constants (k₁) for the formation of the steady-state intermediate in the presence of varying concentrations of a base (pyridine) have been evaluated from rate measurements. Similarly the ratios k₂/k₁ (where k₂ is the rate constant for proton transfer from the steady-state intermediate to the base, and k_?1 is the rate constant for the dissociation of the intermediate) have been determined. The dependence of logk₁ on σ⁺_m and of the ratio k₂/k_?1 on a steric parameter R^f (defined in this paper) is interpreted as evidence for the steady-state intermediate existing as a benzodienone σ-complex with the sp³-bound hydrogen in a pseudo-equatorial position and the electrophile pseudo-axial. A sterically caused destabilisation of the intermediate can be excluded. The steric influence on the ratio k₂/k_?1 is therefore due to the steric influence on k₂ only. Diazo coupling of 8-(2′-pyridyl)-2-naphthol is subject to intramolecular base catalysis, as demonstrated by kinetic hydrogen isotope effects.     

The effect of electrolytes on the reaction rates and acid-base equilibria in ionic micelles

The influence of a number of electrolytes on the micellar effect of cetyltrimethylammoniurn bromide (CTAB) in the hydrolysis ofp-nitrophenyl acetate (1) and bis(p-nitrophenyl) methylphosphonate (2) and in the course of the acid-base dissociation of thep-nitroanilide of bis(chloromethyl)phosphinic acid (3) has been examined. The activity of the salts studied increases in the following order: MeCOOK2CO₃3<p-MeC₆H₄SO₃K. It has been found that in the presence of electrolytes the catalytic effect of CTAB micelles in the hydrolysis reactions of esters1 and2 decreases, and the pK _a value of anilide3 increases. The results obtained are interpreted in terms of the pseudophase model of micellar catalysis. The analysis of the experimental data carried out using logarithmic coordinates revealed a relationship between phase transitions in micellar catalysis and in micellization.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1396–1400, August, 1993.     

Ionic Equilibria of Chromophoric Reagents in Microemulsions

Protolytic equilibria of twenty chromophoric acid–base indicators from different classes, namely, sulfophthaleins, hydroxyxanthenes, azo compounds, and others, were studied in direct microemulsions of benzene–pentanol-1–surfactant–water at volume fractions of the dispersed phase of up to 13%. Cetylpyridinium chloride, sodium dodecylsulfate, and certain nonionic surfactants were used. Apparent acidity constants (pK _a) of indicators were determined under the conditions of their complete binding by microdroplets (pK _a ^ac). The strong differentiating effect of the dispersed phase of microemulsions of different types on the acid–base properties of indicators was revealed. The effect of the ionic strength of the continuous phase (0.025–1.0) on the values of pK _a ^ac was studied. The partition constants of Bromothymol Blue anions between water and negatively charged microdroplets were estimated. Similarities and differences were revealed in the action of microemulsions and micellar solutions of the corresponding surfactants on the state of the studied chromophoric reagents.     

Structure and reactivity in cationic polymerization of butadiene derivatives. IV. 1-alkoxybutadienes

The reactivity of trans-1-alkoxybutadienes in cationic homopolymerization and copolymerizations and structure of the polymers produced were investigated. 1-Ethoxybutadiene is polymerized easily at ?78°C by various acidic catalysis. The reactivity of 1-ethoxybutadiene was similar to that of ethyl vinyl ether. The polymers produced possessed molecular weights of several thousands, and were composed of 70–95% 1,4 structure and 5–30% 3,4 structure. In the copolymerization of ethyl vinyl ether (M₁) with 1-ethoxybutadiene at ?78°C in toluene by boron trifluoride diethyl etherate, r₁ = 1.15, r₂ = 2.62. From the Hammett plot of the relative reactivities of alkoxybutadienes (alkoxy: CH₃O, C₂H₅O, i-C₃H₇O), the reaction constant _p^* was determined to be ?2.9. Results of the present study were compared with those of various butadiene derivatives.     

Studies in nuclear magnetic resonance—IX. Rotational barriers in substituted N,N-dimethylbenzamides

The barriers to rotation about the C? N bond in eighteen substituted N,N-dimethylbenzamides have been determined by complete line shape analysis of the NMR spectra of the N,N-dimethyl protons. The barriers have been correlated with the substituent constants σ and σ⁺. It has been shown that polar solvents increase the barrier in N,N-dimethylbenzamide. Acid catalysis of rotation about the amide C? N bond in N-(p-N,N-dimethylcarboxamidobenzyl)-pyridinium bromide has been investigated. ¹⁸O exchange studies show that catalysis is due to N-protonation rather than the formation of a tetrahedral intermediate. The rate of rotation is a function of the Hammett acidity function, H₀, and the water activity, and it is shown that proton exchange between the N- and O-protonated species involves the intermediacy of a water molecule. The differences in chemical shifts for the non-equivalent N, N-dimethyl groups of the benzamides are also a function of the substituents. Possible explanations of this phenomenon are discussed.     

Dimerization of methylstyrenes by acetyl perchlorate and trifluoromethanesulfonic acid

Ring-substituted methylstyrenes (p-, m-, and o-methylstyrenes) in conjunction with acetyl perchlorate (AcClO₄) or trifluoromethanesulfonic acid as catalysts gave their linear unsaturated dimer in high yield in benzene at temperatures from 50 to 70°C. In particular, the yield of o-methylstyrene dimer was as high as 90% in the AcClO₄ catalysis at 50°C. The dimer yield depended on solvent and catalyst. The terminal structures of the dimers and higher oligomers were analyzed by NMR spectroscopy. Oligomers with a cyclic terminal structure increased in the products at higher temperature. The dimer yield was improved by codimerizing p-methylstyrene with less reactive m-methylstyrene or styrene with AcClO₄ catalyst. The dimers obtained partly consisted of linear unsaturated codimers.     

Confusing Quantitative Descriptions of BrønstedLowry AcidBase Equilibria in Chemistry Textbooks – A Critical Review and Clarifications for Chemical Educators

In chemistry textbooks, the pK value of water in the solvent water at 25 °C is sometimes given as 14.0, sometimes as 15.7. This is confusing. The particular chemical reaction considered is the one in which water as Brønsted? Lowry acid reacts with water as Brønsted? Lowry base in water as solvent to yield equal concentrations of hydrated oxonium and hydroxide ions, H₃O⁺(aq) and HO^?(aq), respectively. This reaction is also known as the ‘self‐ionization’ of water for which the equilibrium constant is abbreviated as K_w with its known value of 10^?14.0 at 25 °C, i.e., pK_w(25 °C)=14.0. Identical values for pK and pK_w at a fixed temperature appear reasonable, since K and K_w refer to one and the same reaction. Therefore, reasons for the apparent disagreement between the ‘thermodynamically correct’ pK_a value for water (14.0 at 25 °C) and the value reported in most organic chemistry textbooks (15.7) should be discussed when teaching acid? base chemistry. There are good arguments for introducing, from the very beginning, the concepts of activity and thermodynamic standard states when teaching quantitative aspects of chemical equilibria. This also explains in a straightforward way why all thermodynamic equilibrium constants, including K_w, are dimensionless, and why pK(25 °C)=0.     

Cleavage of α-halo-substituted alkyl groups from silicon. The mechanism of catalysis by ammonia buffer

Kinetic studies have been made of the base-catalysed cleavage of α-dihalo-substituted alkyl groups from silicon in n-propanol—water in the presence of ammonia buffer. The separate rate constants for the concurrent base-catalysed and base-plus-nucleophile-catalysed processes have been determined for the compounds XC₆H₄(Me₂)SiCHCl₂ with X = H, p-MeO, p-Me, p-Cl, m-Cl, m-CF₃ and a good correlation with σ-constants found for both types of catalysis. Solvent isotope effects and steric effects of substituents have also been studied. Possible mechanisms are discussed.     

Basicity of some aliphatic amines in mixed H2O?CH3CN solvents

The values of pK_a^ms (K_a^ms represents ionization constant of conjugate acid of amine base in mixed water–acetonitrile solvent) for all amines, except for charged amine bases, show a mild decrease (ca. 0.1–0.4 pK units) with the increase in CH₃CN content from 2 to ∼60% v/v. However, the pK_a^ms values at 70% v/v CH₃CN become nearly equal or slightly larger (by ≤0.7 pK units) than the corresponding pK_a^ms at 2% v/v CH₃CN for all neutral and charged amines. The values of pK_a^ms for phenol increase from 10.17 to 13.38 with the increase in the content of CH₃CN from 2 to 70% v/v in mixed aqueous solvent. Taft reaction constants, ρ*, obtained from the plots of pK_a^ms against ∑σ* for primary and secondary amines decrease by ca. 0.8 ρ* units with the increase in the CH₃CN content from 2 to 70% v/v. The values of pK_a^ms show an empirical linear relationship with the corresponding values of pK_a^w (where pK_a^w represents the pK_a obtained in aqueous solvent containing 2% v/v CH₃CN), which allows the estimation of a pK_a in mixed H₂O CH₃CN solvents from that in water. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 146–152, 2000