Time‐Resolved Crystallography of the Reaction Intermediate of Nitrile Hydratase: Revealing a Role for the Cysteinesulfenic Acid Ligand as a Catalytic Nucleophile

The reaction mechanism of nitrile hydratase (NHase) was investigated using time‐resolved crystallography of the mutant NHase, in which βArg56, strictly conserved and hydrogen bonded to the two post‐translationally oxidized cysteine ligands, was replaced by lysine, and pivalonitrile was the substrate. The crystal structures of the reaction intermediates were determined at high resolution (1.2–1.3 Å). In combination with FTIR analyses of NHase following hydration in H₂¹⁸O, we propose that the metal‐coordinated substrate is nucleophilically attacked by the O(SO^?) atom of αCys114‐SO^?, followed by nucleophilic attack of the S(SO^?) atom by a βArg56‐activated water molecule to release the product amide and regenerate αCys114‐SO^?.     

Solvent effects on the asymmetric Pauson-Khand-type reaction by rhodium

The reaction efficiency and enantioselectivity of an asymmetric Pauson-Khand-type reaction catalyzed by cationic rhodium are heavily dependent on the solvent. Coordinating solvents, such as THF, provide a faster reaction and better stereoselectivity than non-coordinating solvents, such as toluene. These beneficial effects can be attributed to a significant increase in the more reactive catalytic species of [Rh(bisphosphane ligand)^∗(solvent)_n]⁺ (3) than of [Rh(bisphosphine ligand)^∗CO(solvent)]⁺(4) and [Rh(bisphosphine ligand)∗(CO)₂]⁺ (5) in a coordinating solvent.     

Dielectric polarization and non-linear dielectric effects in pivalonitrile solutions

Dielectric polarization and non-linear dielectric studies of solutions of pivalonitrile (CH₃)₃CN in carbon tetrachloride, cylohexane and n-hexane are presented. The contributions of Langevin orientation, dipolar coupling and pretransitional fluctuations have been separated from the observed dielectric non-linear effect.     

Competing Excited-State Hydrogen and Proton-Transfer Processes in 6-Azaindole-S3,4 and 2,6-Diazaindole-S3,4 Clusters (S=H2O,NH3)

Excited state hydrogen (ESHT) and proton (ESPT) transfer reaction pathways in the three and four solvent clusters of 6-azaindole (6AI-S_3,4) and 2,6-diazaindole (26DAI-S_3,4)(S=H₂O, NH₃) were computationally investigated to understand the fate of photo-excited biomolecules. The ESHT energy barriers in (H₂O)₃ complexes (39.6–41.3 kJmol⁻¹) were decreased in (H₂O)₄ complexes (23.1–20.2 kJmol⁻¹). Lengthening the solvent chain lowered the barrier because of the relaxed transition states geometries with reduced angular strains. Replacing the water molecule with ammonia drastically decreased the energy barriers to 21.4–21.3 kJmol⁻¹ in (NH₃)₃ complexes and 8.1–9.5 kJ mol⁻¹ in (NH₃)₄ complexes. The transition states were identified as H_a atom attached to the first solvent molecule. The formation of stronger hydrogen bonds in (NH₃)_3,4 complexes resulted in facile ESHT reaction than that in the (H₂O)_3,4 complexes. The ESPT energy barriers in 6AI-S_3,4 and 26DAI-S_3,4 were found to range between 40–73 kJmol⁻¹. The above values were significantly higher than that of the ESHT processes and hence are considered as a minor channel in the process. The effect of N(2) insertion was explored for the very first time in the isolated solvent clusters using local vibrational mode analysis. In DAI-S₄, the higher K_a(H_a⋯S_a) values depicted the increased photoacidity of the N(1)-H_a group which may facilitate the hydrogen transfer reaction. However, the increased N(6)⋯H_b bond length elevated the reaction barriers. Therefore, in the ESHT reaction channel, the co-existence of two competing factors led to a marginal/no change in the overall energy barriers due to the N(2) insertion. In the ESPT reaction pathway, the energy barriers showed notable increase upon N(2) insertion because of the increased N(6)⋯H_b bond length.     

Solvent effects on the rate of reaction of Cl2˙− and SO4˙− radicals with unsaturated alcohols

Rate constants have been measured in several aqueous/organic solvent mixtures for the addition reaction of Cl₂^˙? radicals with 2-propen-1-o1 and 2-buten-1-o1 as a function of temperature and with 2, 3-dimethyl-2-butene at room temperature. The rate constants were in the range of 10⁶–10⁹ L mol^?1 s^?1, the activation energies were relatively low (1–10 kJ mol^?1), and the pre-exponential factors varied over the range log A = 7.9 to 9.4. The rate constants (k) decreased (by up to a factor of 30) upon increasing the fraction of organic solvent and log k correlated linearly with the dielectric constant for a given water/organic solvent system, but the lines for the different solvent systems had different slopes. A better correlation of log k was found with a combination of the solvatochromic factor, E_T(30), and the hydrogen-bond donor acidity factor, α. This suggests that the rate of reaction is influenced by the solvent polarity and also by specific solvation of the ionic reactant and product. Solvent effect on the reaction of SO₄^˙? with 2-propen-1-o1 was studied for comparison. © 1993 John Wiley & Sons, Inc.     

How difficult are anion-molecule SNAr reactions of unactivated arenes in the gas phase,dimethyl sulfoxide,and methanol solvents?

Nucleophilic substitution on the aromatic ring (S_NAr) is a very important reaction for organic transformations. This kind of reaction is usually difficult to take place, requiring organometallic catalysis or activation of the ring by electron withdrawing groups to turn the nucleophilic attack possible. In this work, the relative importance of intrinsic gas phase barrier and the solvent effect on several S_NAr reactions using theoretical calculations were investigated. The reactions of the anions OH^?, CN^?, and CH₃O^? and the enolates CH₃COCH₂^? and CH₃COCHCOCH₃^? with bromobenzene and (o, m, p)-methoxy bromobenzene in methanol and dimethyl sulfoxide as solvents were considered. The OH^? and CH₃O^? ions are highly reactive in the gas phase. However, the solvent effect induces a high activation barrier in solution, turning the reaction difficult, although feasible. The CN^? and CH₃COCHCOCH₃^? ions have high activation barriers even in the gas phase. The interesting CH₃COCH₂^? ion has a moderate barrier in the gas phase, although the free energy barrier in DMSO solution reaches 33 kcal mol^?1. Our analysis suggests that decreasing the solvent effect, arylation of enolates with unactivated arenes could become possible.

     

Effect of solvent on the exchange of ethylene for propylene on cis-PtCl2(C3H6)(TPPTS), TPPTS = P(m-C6H4SO3Na)3

To further understand the effect of water as a solvent in organometallic reactions, the lability of η²-alkenes coordinated to platinum(II) phosphine complexes has been studied in water and chloroform as a comparison of solvent effects on the exchange kinetics and alkene complex stability. ¹H NMR techniques with both deuterated chloroform and a deuterium oxide/deuterated methanol mixture as solvent systems were used at temperatures as low as ?50°C. Reaction of cis-PtCl₂L(η²-C₃H₆) [L?=?PPh₃ (triphenylphosphine) (1a), TPPTS (tris(m-sulfonatophenyl)phosphine) 1b] with ethylene to form cis-PtCl₂L(η²-C₂H₄) (2?a, b) was observed with dependence on the rate by starting platinum complex and ethylene. The role of water on this reaction, as well as its effect on the equilibrium, will be discussed. The equilibrium constant shows preference for coordination of ethylene and the temperature dependence indicates the reaction is entropy controlled.     

Correlation between proton transfer and solvent motion in the (H3O2)− species

The (H₃O₂)⁻ and (H₃O₂)⁻(H₃O)₂ species have been studied by ab initio 4–31G calculations. The solvent parameters have been found to intervene in the reaction coordinate, showing a correlation between the proton-transfer process and the solvent motion.     

Hexamethylphosphoramide

Growing interest is being shown in the role of the solvent (particularly polar solvents) in chemical reactions^[1–4]. The most remarkable polar aprotic solvent appears to us to be the hexamethylated triamide of orthophosphoric acid, OP(N(CH₃)₂)₃, which is known as hexamethylphosphoramide or tris(dimethylamino)phosphine oxide. Hexamethylphosphoramide^[5–7] has recently been studied with respect to its solvent properties for gases^[5], for many organic and inorganic salts^[7], and for polymers^[5], as well as its use as a polymerization co-catalyst. However, these aspects are not considered in the present paper, which deals with the physical and chemical properties of hexamethylphosphoramide, and in particular with its use as a reaction medium. Our own work on hexamethylphosphoramide began in 1961, and until then very little work had been done on this compound. Even in a paper published by Parker^[1b] in 1965, very little reference is made to hexamethylphosphoramide.     

New heavy metal ion-selective macrocyclic ligands with mixed-donor atoms and their extractant properties

Synthetic procedures for new macrocyclic diamides with N₂S₄O₃- and N₂S₅O₃-donors were given. The corresponding macrocyclic ligands were prepared by reaction of NaBH₄ with the macrocyclic diamides in the presence of boron triflouride ethyl etherate in dry tetrahydrofuran. The solvent extraction method was used to evaluate metal-ion binding properties of the new ligands. The values of the extraction constants (log K _ex) and the complex compositions were determined for the extracted complexes. The solvent extraction experiments suggested that the reduced N₂S₅O₃-donor macrocycle has Ag⁺ selectivity compared to Pb²⁺, Co²⁺, Ni²⁺, Mn²⁺, and Cd²⁺ for chloroform as organic solvent.     

Global Reaction Pathways in the Photodissociation of I3− Ions in Solution at 267 and 400 nm Studied by Picosecond X-ray Liquidography

The mechanism of a photochemical reaction involves the formation and dissociation of various short-lived species on ultrafast timescales and therefore its characterization requires detailed structural information on the transient species. By making use of a structurally sensitive X-ray probe, time-resolved X-ray liquidography (TRXL) can directly elucidate the structures of reacting molecules in the solution phase and thus determine the comprehensive reaction mechanism with high accuracy. In this work, by performing TRXL measurements at two different wavelengths (400 and 267 nm), the reaction mechanism of I₃⁻ photolysis, which changes subtly depending on the excitation wavelength, is elucidated. Upon 400 nm photoexcitation, the I₃⁻ ion dissociates into I₂⁻ and I. By contrast, upon 267 nm photoexcitation, the I₃⁻ ion undergoes both two-body dissociation (I₂⁻+I) and three-body dissociation (I⁻+2I) with 7:3 molar ratio. At both excitation wavelengths, all the transient species ultimately disappear in 80 ns by recombining to form the I₃⁻ ion nongeminately. In addition to the reaction dynamics of solute species, the results reveal the transient structure of the solute/solvent cage and the changes in solvent density and temperature as a function of time.     

Tris[bis(trimethysilyl)amido]zinkate von Lithium und Calcium

Tris[bis(trimethylsilyl)amido]zincates of Lithium and Calcium Calcium-bis[bis(trimethylsilyl)amide] and Bis[bis(trimethylsilyl)amido]zinc yield in 1,2-dimethoxyethane quantitatively Calcium-bis{tris[bis(trimethylsilyl)- amido]zincate} · 3DME. When THF is chosen as a solvent, the two reactants and the zincate form a temperature-independent equilibrium, whereas in benzene no reaction occurs. The tris[bis(trimethylsilyl)amido]zincate anion displays characteristic ¹³C{¹H) and ²⁹Si{¹H] chemical shifts of 7 and ?8 ppm, respectively; the nature of the solvent, the cation and the complexating ligands don't influence the IR nor NMR data of the zincate anion and thus verify that [Ca(DME)₃]²⁺ and {Zn[N(SiMe_{3 2}]₃}^? appear as solvent separated ions, which is also confirmed by their insolubility in hydrocarbons.     

Aquation kinetics of chloro- and bromo-pentamminecobalt(III) ions in water-acetone mixtures

Summary The aquation kinetics of [Co(NH₃)₅Cl]²⁺ and [Co(NH₃)₅Br]²⁺ ions in acetone-water have been investigated over a wide range of solvent compositions and temperatures. The variation of thermodynamic properties of the activated complex with the mole fraction of acetone reveals the existence of specific solvation. The correlation of the rate constant with dielectric constant of the medium is examined and discussed. A reaction mechanism which describes the solvent effect on reaction rate is proposed.     

Mechanism and Selectivity of the Oxidative Ring Contraction of Cyclic α‐Formyl Ketones

The oxidative contraction of α‐formal ketone to form continuous all carbon chiral centers promoted by H₂O₂ is widely used in natural product total synthesis. Typically, using this transformation, chiral cyclic ketones are obtained as the major products and ring‐opening products as the minor products. Herein, DFT calculations have been used to investigate the detailed reaction mechanism and chemoselectivity. In addition, with the widely accepted mechanism of H₂O₂‐promoted transformation, our systematic investigation with various explicit‐solvent‐model calculations for the first time shows that H₂O and H₂O₂ are comparable at catalyzing the rate‐determining step of this reaction, which emphasis the importance of solvent effect in such transformations. It is found that both the less ring‐constrain and a later transition state in an exothermic reaction account for the origin why the reaction favors ring‐contraction pathway rather than ring‐opening one. By a comprehensive analysis for the substituted groups, it has been disclosed that the steric effects of the substituted groups on R² and R³ contribute to the selectivity with larger steric hindrance favoring the chiral cyclic products. Moreover, the electronic effects on R¹ but not R³ affect the selectivity with electron‐donating groups leading to the cyclic products. Based on our calculations, some predictions for higher selectivity have been made.     

LIGAND-EXCHANGE REACTION BETWEEN TRIS(1,10-PHENANTHROLINE)METAL(II) AND TRIS(4,7-DIPHENYL-1,10-PHENANTHROLINE)METAL(II) IONS

Abstract

The ligand exchange reaction between [M(phen)₃]²⁺ and [M(DIP)₃]²⁺ (where M is the same and M = Fe^II or Ni^II, phen = 1,10-phenanthroline, DIP = 4,7-diphenyl-1,10-phenanthroline) has been investigated by reversed phase ion-paired chromatography (RP-IPC). The effect of pH and solvent on the ligand-exchange reaction is studied by monitoring the variation in chromatograms with time after mixing. The results have shown that the ligand exchange reaction between [M(phen)₃]²⁺ and [M(DIP)₃]²⁺ takes place in the pH range of 3–8 and the rate of reaction for nickel(II) complexes is about two times slower than that for iron(II) complexes. Experiments on the effect of various solvents on the ligand-exchange reaction have revealed that the rate of reaction is enhanced by the solvent in the following order: (CH₃)₂CO > CHCl₃ ≥ CH₂Cl₂ > CH₃CN > CH₃OH. Elemental analysis and UV-visible spectroscopy confirmed that the products obtained from the ligand-exchange reaction are mixed-ligand complexes containing phen and DIP ligands, i.e., [M(phen)₂(DIP)]²⁺ and [M(phen)(DIP)₂]²⁺.     

A one-pot multi-component reaction for the facile synthesis of some novel 2-aryl thiazolidinones bearing benzimidazole moiety using La(NO<Subscript>3</Subscript>)<Subscript>3</Subscript>·6H<Subscript>2</Subscript>O as an efficient catalyst

A series of new derivatives of 3-benzimidazolyl-2-aryl thiazolidinones, 4a–j are synthesized via a rapid, one-pot, three-component reaction by using La(NO₃)₃·6H₂O as an efficient catalyst from the reaction of 2-aminobenzimidazole, aromatic aldehydes and thioglycolic acid in ethanol at room temperature. These new compounds were characterized by IR, ¹H, ¹³C NMR and mass spectroscopies. An inexpensive and available catalyst, short reaction time, easy workup, good to excellent yields and nontoxic solvent are the advantages of this reaction.     

Kinetic Studies of the Ligand Substitution Reaction of Some New Uranyl Schiff Base Complexes with Tri‐n‐butylphosphine

Kinetics of the substitution reaction of solvent molecule in uranyl(VI) Schiff base complexes by tri‐n‐butylposphine as the entering nucleophile in acetonitrile at 10–40°C was studied spectrophotometrically. The second‐order rate constants for the substitution reaction of the solvent molecule were found to be (8.8 ± 0.5) × 10^?3, (5.3 ± 0.2) × 10^?3, (7.5 ± 0.3) × 10^?3, (6.1 ± 0.3) × 10^?3, (13.5 ± 1.6) × 10^?3, (13.2 ± 0.9) × 10^?3, (52.9 ± 0.2) × 10^?3, and (88.1 ± 0.6) × 10^?3 M^?1 s^?1 at 40°C for [UO₂(Schiff base)(CH₃CN)], where Schiff base = L1–L8, respectively. In a temperature dependence study, the activation parameters ΔH^# and ΔS^# for the reaction of uranyl complexes with PBu₃ were determined. From the linear rate dependence on the concentration of PBu₃, the span of k₂ values and the large negative values of the activation entropy, an associative (A) mechanism is deduced for the solvent substitution. By comparing the second‐order rate constants k_2, it was concluded that the steric and the electronic properties of the complexes were important for the rate of the reactions.     

Solvent-free sonochemical kabachnic-fields reaction to synthesize some new α-aminophosphonates catalyzed by nano-BF3•SiO2

To obtain a rapid, efficient synthesis of some new α-aminophosphonates, ultrasonic irradiation has been applied to the reaction mixtures containing amine, aromatic or heteroaromatic aldehydes and triethyl phosphite. The Kabachnik-Fields reaction was performed by using nano-BF₃?SiO₂ as a recyclable catalyst under solvent free conditions. Key advantages of this procedure consist in the eco-friendly and highly efficient reaction conditions, high yields, an easy work-up procedure, short reaction times and solvent free conditions. All title compounds were characterized by spectral and elemental analysis. They were further screened for their in vitro antioxidant activity by the DPPH, O^2? and NO methods. The majority of the title compounds showed good antioxidant activity when compared with the standard antioxidants.     

Electronic spectral studies and solvent effects on the reaction of iodine with 1,4,8,11-tetraazacyclotetradecane

The reaction between iodine and 1,4,8,11-tetraazacyclotetradecane (TACTD) is studied photometrically in various solvents like CCl₄, CHCl₃, CH₂Cl₂ and 1,2-dichloroethane. The results reveal that in each solvent the (TACTD):I₂ ratio is 1:2 and the iodine complex is formulated as (TACTD)I⁺·I₃⁻. The obtained values of the formation constant (K), extinction coefficient (ε) and oscillator strength (ƒ) for the iodine complex are shown to be strongly dependent on the polarity of the solvent. A linear correlation is obtained between either (ƒ) or (ε) and the dielectric constant (D) of the solvent. The important role of the solvent is mainly suggested to be due to the interaction of the ionic iodine complex with the solvent.