Linear Free Energy Correlations Research Articles

Aryl–Phenyl Scrambling in Intermediate Organopalladium Complexes: A Gas‐Phase Study of the Mizoroki–Heck Reaction

The intramolecular aryl-phenyl scrambling reaction within palladium-DPPP-aryl complex (DPPP=1,3-bis(diphenylphosphino)propane) ions was analyzed by state-of-the-art tandem MS, including gas-phase ion/molecule reactions. The Mizoroki-Heck cross-coupling reaction was performed in the gas phase, and the intrinsic reactivity of important intermediates could be examined. Moreover, linear free-energy correlations were applied, and a mechanism for the scrambling reaction proceeding via phosphonium cations was assumed.

Effect of the Identity of Xaa on the Fragmentation Modes of Doubly-Protonated Ala-Ala-Xaa-Ala-Ala-Ala-Arg

The product ion mass spectra resulting from collisional activation of doubly-protonated tryptic-type peptides Ala-Ala-Xaa-Ala-Ala-Ala-Arg have been determined for Xaa = Ala(A), Ser(S), Val(V), Thr(T), Ile(I), Phe(F), Tyr(Y), Sar, Met(M), Trp(W), Pro(P), and Gln(Q). The major fragmentation reaction involves cleavage of the second amide bond (counting from the N-terminus) except for Xaa = Ser and Thr where elimination of H(2)O from the [M + 2H](+2) ion forms the base peak. In general, the extent of cleavage of the second amide bond shows little dependence on the identity of Xaa and little dependence on whether the bond cleavage involves symmetrical bond cleavage to form a y(5)/b(2) ion pair or asymmetrically to form y (5) (+2) and a neutral b(2) species. Notable exceptions to this generalization occur for Xaa equal to Pro or Sar. For Xaa = Pro only cleavage of the second amide bond is observed, consistent with a pronounced proline effect, i.e., cleavage N-terminal to Pro. When Xaa = Sar considerably enhanced cleavage of the second amide bond also is observed, suggesting that at least part of the proline effect relates to the tertiary nature of the amide nitrogen. In the competition between symmetric and asymmetric bond cleavage an attempt to establish a linear free energy correlation in relating ln(y(5)(+2)/y(5)) to PA(H-Xaa-OH) did not lead to a reasonable correlation although the trend of increasing y(5)(+2)/y(5) ratio with increasing proton affinity of H-Xaa-OH was clear. Proline showed a unique behavior in giving a much higher y(5)(+2)/y(5) ratio than any of the other residues studied.

Kinetics and mechanism of the pyridinolyses of dimethyl and diethyl chloro(thiono)phosphates in acetonitrile

AbstractKinetic studies for the reactions of dimethyl chlorophosphate (1), dimethyl chlorothionophosphate (2), diethyl chlorophosphate (3), and diethyl chlorothionophosphate (4) with substituted (X) pyridines have been carried out in acetonitrile. The pyridinolyses of the P = O systems (1 and 3) show linear free energy correlations for substituent X variations, while those of the P = S systems (2 and 4) show biphasic concave upward Hammett and Brönsted plots with a break region between 3‐phenyl and 3‐methoxy pyridines. A concerted mechanism with both frontside and backside nucleophilic attack is proposed for the reactions of 1 and 3, but the fraction of a frontside attacks is somewhat larger than that of a backside attack. A concerted mechanism with a dominant backside nucleophilic attack for weakly basic pyridines and a concerted mechanism with a dominant frontside nucleophilic attack for strongly basic pyridines are proposed for the reactions of 2 and 4. The pyridinolysis rate does not depend solely on one major factor. The concave upward free energy relationship can be diagnostic of a change in the direction of the nucleophilic attack. Copyright © 2010 John Wiley & Sons, Ltd.

Experimental Evidence of the Occurrence of Intramolecular Electron Transfer in Catalyzed 1,2-Dioxetane Decomposition

The activation parameters for the thermal decomposition of 13 acridinium-substituted 1,2-dioxetanes, bearing an aromatic moiety, were determined and their chemiluminescence emission quantum yields estimated, utilizing in situ photosensitized 1,2-dioxetane generation and observation of its thermal decomposition kinetics, without isolation of these highly unstable cyclic peroxides. Decomposition rate constants show linear free-energy correlation for electron-withdrawing substituents, with a Hammett reaction constant of ρ = 1.3 ± 0.1, indicating the occurrence of an intramolecular electron transfer from the acridinium moiety to the 1,2-dioxetane ring, as postulated by the intramolecular chemically initiated electron exchange luminescence (CIEEL) mechanism. Emission quantum yield behavior can also be rationalized on the basis of the intramolecular CIEEL mechanism, additionally evidencing its occurrence in this transformation. Both relations constitute the first experimental evidence for the occurrence of the postulated intramolecular electron transfer in the catalyzed and induced decomposition of properly substituted 1,2-dioxetanes.

ChemInform Abstract: π-Acceptor Induced Reactions: Radicals vs. Ions in Thermally Induced Ether Cleavage Reactions.

Abstract The reactions of benzyl ether ( 1a ) and 4,4′-dicyanobenzyl ether ( 1b ) with a wide range of π-acceptors in acetonitrile at 190°C have been studied. The π-acceptors were chosen in order to systematically vary the ΔG° for hydride or electron transfer from the ethers to the π-acceptors. The products of thermal activation of the bond cleavage reaction are the aldehyde and the alcohol derived from the starting ethers. A linear free energy correlation of the relative thermal reaction rates of 1a and 1b with the reduction potentials of the quinones was obtained. This relationship indicates that the rate of radical pair separation becomes increasingly important relative to the rate of the second electron transfer and subsequent product formation through the hydride transfer intermediate as the electron affinity of the quinone decreases.

Arylethynyl Derivatives of the Dihydroazulene/Vinylheptafulvene Photo/Thermoswitch: Tuning the Switching Event

A selection of dihydroazulene (DHA) photoswitches incorporating an arylethynyl-substituent in the seven-membered ring was prepared by palladium-catalyzed Sonogashira cross-coupling reactions employing a suitable bromo-functionalized DHA. Shielding of the alkyne bridge and separating the aryl and DHA units, by sterically demanding groups, was required to obtain stable compounds. The DHAs underwent a light-induced ring-opening to vinylheptafulvenes (VHFs) which were thermally converted to a mixture of two DHA regioisomers, one of which was the original one. The influence of the aryl groups on the DHA and VHF absorptions and on their interconversion was investigated in detail. The rates of the switching events were finely tuned by the donor or acceptor strength of the aryl group. The thermal ring closure was found to proceed most readily in the presence of an electron-donating group on the seven-membered ring. The rate constant was found to follow a Hammett linear free energy correlation, which signals that stabilization of a positive charge in the seven-membered ring plays a crucial role in the ring-closure reaction. In view of these findings, it was possible to control the switching event by protonation/deprotonation of an anilino-substituted DHA. Also, the light-induced ring opening reaction was strongly controlled by acid/base. In addition to the mesomeric effects exerted by an arylethynyl group, the inductive effects exerted by different groups on the thermal ring closure were elucidated. Although the alkyne bridge transmits the electronic character of the aryl group, the ring-closure is retarded for all the ethynylated compounds relative to the parent unsubstituted compound. Along with our synthesis of suitable arylalkynes, we discovered an interesting byproduct in a Sonogashira cross-coupling reaction involving a nitrophenyl group, namely a diaryl azoxy compound. Its structure was confirmed by X-ray crystallographic analysis.

Transferência de elétron inversa na quimiexcitação da reação peróxi-oxalato usando ativadores facilmente redutíveis Inverse electron transfer in peroxyoxalate chemiexcitation using easily reducible activators

Chemiluminescence properties of the peroxyoxalate reaction in the presence of activators bearing electron withdrawing substituents were studied, to evaluate the possible occurrence of an inverse electron transfer, from the peroxide intermediate to the activator, in its chemiexcitation step. Relative catalytic rate constants and singlet quantum yields were obtained for the peroxyoxalate reaction, using 9-chloro, 9,10-dichloro, 9-cyano and 9,10-dicyanoanthracenes as activators. The linear free-energy correlation of the relative rate constants with the activators' reduction potentials and the dependence of the quantum yields on the released energy confirm, for the first time, the occurrence of this inverse electron transfer.

Discrete Site Surface Complexation Constants for Lanthanide Adsorption to Bacteria As Determined by Experiments and Linear Free Energy Relationships

Bacteria are abundant in many natural and engineered environments where they are thought to exert important controls on the cycling, mobility, bioavailability, and toxicity of metal contaminants. In order to probe their role in moderating the behavior of lanthanides, pH-dependent adsorption edges of 13 individual lanthanides and yttrium to the Gram-negative bacterium Pantoea agglomerans were used to generate discrete site surface complexation constants. The calculated surface complexation constants were compared with stability constants estimated using linear free energy relationships based on a number of hydroxyl-containing ligands. The experimental data suggests that lanthanide adsorption edges below pH 6.5 are consistent with adsorption to phosphate groups for the light and some of the middle lanthanides (La to Gd), whereas some of the middle and heavy lanthanides appear to favor carboxyl co-ordination (Tb to Yb), although exceptions occur in each grouping. The experimentally derived surface complexation constants for carboxyl coordination were of similar magnitude to stability constants estimated from linear free energy correlations using fulvic acid stability constants. The implication is that the adsorption of lanthanides to bacterial surfaces could be modeled reasonably well using lanthanide stability constants for natural organic matter, except perhaps at low pH where phosphate binding dominates.

Direct Kinetic Observation of the Chemiexcitation Step in Peroxyoxalate Chemiluminescence

A high-energy intermediate in the peroxyoxalate reaction can be accumulated at room temperature under specific reaction conditions and in the absence of any reducing agent in up to micromolar concentrations. Bimolecular interaction of this intermediate, accumulated in the reaction of oxalyl chloride with hydrogen peroxide, with an activator (highly fluorescent aromatic hydrocarbons with low oxidation potential) added in delay shows unequivocally that this intermediate is responsible for chemiexcitation of the activator. Activation parameters for the unimolecular decomposition of this intermediate (DeltaH(double dagger) = 11.2 kcal mol(-1); DeltaS(double dagger) = -23.2 cal mol(-1) K(-1)) and for its bimolecular reaction with 9,10-diphenylanthracene (DeltaH(double dagger) = 4.2 kcal mol(-1); DeltaS(double dagger) = -26.9 cal mol(-1) K(-1)) show that this intermediate is much less stable than typical 1,2-dioxetanes and 1,2-dioxetanones and demonstrate its highly favored interaction with the activator. Therefore, it can be inferred that structural characterization of the high-energy intermediate in the presence of an activator must be highly improbable. The observed linear free-energy correlation between the catalytic rate constants and the oxidation potentials of several activators definitely confirms the occurrence of the chemically initiated electron-exchange luminescence (CIEEL) mechanism in the chemiexcitation step of the peroxyoxalate system.

Prediction of ion-interaction parameters of binary 2:1 and 2:2 ion-exchange systems by the coupling of the Margules model with a linear free energy correlation approach

The Gibbs free energy of exchange ( Δ G ∘ , T = 303 K; TN = 0.005 N) for aqueous binary mixtures of Na +, Cu 2+, Zn 2+, and Pb 2+ previously calculated using Pitzer’s and Margules (MM) models for aqueous ( γ i ’s) and solid phase ( A ij ) cationic parameters, respectively, were equated with a linear free energy correlation approach, Δ G LCA ∘ , which correlates thermodynamic parameters with cation properties. The resulting linear relationships between Δ G MM ∘ and Δ G LCA ∘ for the binary mixtures permitted the evaluation of the Margules parameters ( A 12, A 21) for other (1:2)- and (2:2)-exchange systems, as well as to predict the ion-exchange isotherm profile. These findings, consequently, allowed the prediction of selectivity trends of clinoptilolite for different groups of divalent cations without the need of empirical data. In general, clinoptilolite showed greater affinity for univalent cations of bigger ionic radii, which corresponds to its more negative non-solvation energies. The higher affinity of clinoptilolite for divalent cations of bigger ionic radii is probably owing to greater framework stabilization provided by higher charge density and polarizability. Homoionic Ca- and Mg-clinoptilolite showed greater affinity for any other divalent cation compared to the exhibited by its Na-form of the exchange, which is useful for practical applications.

Hydrogen bond interactions at the TiO 2 surface: Their contribution to the pH dependent photo-catalytic degradation of p-nitrophenol

We have obtained p K a values of p-nitrophenol–TiO 2 by measuring the adsorption equilibrium constants of p-nitrophenol (PNP) on the TiO 2 surface at different pH values. These values have been obtained from Langmuir isotherms and from a plot of 1/rate vs. 1/[PNP] o obtained during TiO 2 catalyzed solar light photo-degradation of PNP. Two limit equilibrium constants are readily obtained depending on the solution pH: at pH 5 at which the TiO 2 surface is mainly positively charged and at pH 8 when it is negatively charged. With these and other adsorption equilibrium constants and the PNP p K a value in solution, thermodynamic cycles are established in order to obtain the PNP p K a when it is adsorbed on positively charged, neutral and negatively charged TiO 2 surfaces. From these p K a values useful information on the PNP–TiO 2 interaction is readily obtained. For instance, the PNP nitro group interacts with the TiO 2 surface via a hydrogen bond, arising from the complex of water molecules with the Ti 4+ ions on its surface. The weaker the hydrogen bond donor, the stronger the oxygen nitro group basicity. Therefore, p K a changes on the phenolic hydroxyl group result from these interactions. Linear free energy correlations, maximum PNP adsorption capacity values ( Q L) and FTIR ATR, spectrum support this proposal. A k obs vs. pH degradation profile of p-nitrophenol is also provided.

Linear free-energy correlation analysis of the electronic effects of the substituents in the Sonogashira coupling reaction

Relative rate constants (krel) for the Sonogashira coupling were determined in competitive reactions between iodobenzene and a series of para- and meta-substituted iodobenzenes and compared to the charge on iodine and the z-component of the quadrupole moment of the iodine-bearing carbon. We use an Hammett correlation analysis and the computational data to provide further evidence that the rate limiting step of the Sonogashira reaction is the initial oxidative addition of Pd to the carbon-iodine bond.Key words: Sonogashira, competitive reactions, Hammett correlation.

Solubilization of nitrotoluenes in micellar nonionic surfactant solutions

A key factor in selecting surfactants to enhance chemical or biological transformation or physical removal of an organic pollutant from contaminated soil is knowledge of the pollutant's solubility behavior in the surfactant solution. This study investigated the influence of nonionic surfactant structure on the solubility of 4-nitrotoluene (NT), 2,3-dinitrotoluene, 2,4-dinitrotoluene, 2,6-dinitrotoluene, and 2,4,6-trinitrotoluene (TNT) at room temperature. For a series of alkyl phenol ethoxylates (Tergitol NP-8 to NP-40), decreasing the ethoxylate chain length increased the solubility of these nitrotoluenes by a factor of two or less in 10 g l(-1) surfactant solutions, but did not significantly change their molar solubilization ratios (MSR, e.g. 0.02 for TNT) or their micelle-water partition coefficients (K(m), e.g. 3.4 for TNT). For Tergitol NP-8 solutions ranging from 1.0 to 12.4 g l(-1), no enhancement in NT solubility was found, suggesting that the cloud point was reached. The MSRs for Tween 80 were higher than those of Tween 20 and the MSRs of Brij-58 were higher than those for Brij-35. When comparing solutes, NT had the highest solubility and MSR (0.28-0.41), while TNT had the lowest solubility and MSR (0.02-0.03). A linear relationship between K(m) values and octanol-water partition coefficients based on Triton X-100 predicted the logK(m) values within 0.5 of their measured values. A linear solvation free energy correlation for K(m) suggested the importance of solute volume and effective hydrogen bond basicity in the partitioning process while implying that the nitrotoluenes are solubilized in a polar portion of the micelle.

Kinetics of Reactions of CCN Radical with Alcohols

The reaction kinetics of cyanomethylidyne radical, CCN(X2), with a series of primary alcohols were studied at about 1.33 kPa total pressure and room temperature using pulsed laser photolysis/laser-induced fluorescence (LP/LIF) technique. The CCN radical was produced via laser photolysis of CCl3CN with the fourth harmonic output of a Nd: YAG laser (266 nm). The relative concentration of the CCN(X2) radical was monitored by LIF in the (0, 0) band of the CCN(A2 X2) transition at 470.9 nm. Under pseudo-first-order conditions, the reaction rate constants of CCN(X2) with a series of primary alcohol molecules (n-CnH2n+1OH, n=1-6) were determined by measuring the time evolution of the relative concentration of CCN(X2i). The measured rate constants increased monotonously with the number of carbon atoms in the alcohols, and the values for reactions of CCN(X2) with alcohols were larger than those for reactions of CCN(X2) with alkanes (C1-C5). Based on the bond dissociation energies and linear free energy correlations, it was believed that the reactions of CCN(X2) with alcohols proceeded via a hydrogen abstraction mechanism that was analogous to CCN(X2) with alkanes. The experimental results indicated that the H atoms on the C-H bonds were activated at the presence of the OH group in alcohol molecules and the hydrogen abstraction from the C-H bonds in the alcohol molecules was the dominant reaction pathway. The relation between the rate constants and the long-distance attractive potentials between the CCN radical and the alcohol molecules was discussed.

Oxidative N-Dealkylation Reactions by Oxoiron(IV) Complexes of Nonheme and Heme Ligands

Nonheme and heme iron monooxygenases participate in oxidative N-dealkylation reactions in nature, and high-valent oxoiron(IV) species have been invoked as active oxidants that effect the oxygenation of organic substrates. The present study describes the first example of the oxidative N-dealkylation of N,N-dialkylamines by synthetic nonheme oxoiron(IV) complexes and the reactivity comparisons of nonheme and heme oxoiron(IV) complexes. Detailed mechanistic studies were performed with various N,N-dialkylaniline substrates such as para-substituted N,N-dimethylanilines, para-chloro-N-ethyl-N-methylaniline, para-chloro-N-cyclopropyl-N-isopropylaniline, and deuteriated N,N-dimethylanilines. The results of a linear free-energy correlation, inter- and intramolecular kinetic isotope effects, and product analysis studied with the mechanistic probes demonstrate that the oxidative N-dealkylation reactions by nonheme and heme oxoiron(IV) complexes occur via an electron transfer-proton transfer (ET-PT) mechanism.

Studies on the Autoxidation of Aryl Alkenes

Autoxidation activity of substituted styrenes (aryl alkenes) and their product formation were studied in the temperature range of 65–125 °C using cumene or chlorobenzene as solvent. It was observed that higher reaction temperatures, inert solvent, electron donating substituents, and bulky sterical groups at α-, ortho- and para- positions promoted radical formation of epoxides and led to C=C oxidative cleavage. In contrast, electron accepting substituents and using cumene as the solvent favored the formation of oligomeric peroxides. The relative activity of all investigated styrenes could be correlated to the linear free energy (LFE) relation and the ionization potential. For α-substituted styrenes and ring substituted α-methylstyrenes LFE correlation for the reactivity of aryl alkenes and formation of epoxides was found to depend on the polar and steric factors. A general pathway for product formation by autoxidation of aryl alkenes has been proposed.

Linear kinetic and free energy correlations for intra molecular dissociative electron transfers – Estimation of standard potentials and cleavage rate constants of radical anions of aromatic halides

A Hammett-type linear kinetic and free energy correlations pertaining to the intramolecular dissociative electron transfer in radical anions of aromatic halides have been formulated and studied using the reaction series of aromatic chlorides, bromides and iodides. The correlation evinces the higher reactivity of carbon–chlorine bond to the polar effects of the substituents in comparison to carbon–bromine and carbon–iodine bonds. The standard potentials and cleavage rate constants of radical anions of some aromatic iodides have been predicted using the Hammett-type kinetic and free energy relationships.

Experimental study of neptunyl adsorption onto Bacillus subtilis

The subsurface mobility of Np is difficult to predict in part due to uncertainties associated with its sorption behavior in geologic systems. In this study, we measured Np adsorption onto a common gram-positive soil bacterium, Bacillus subtilis. We performed batch adsorption experiments with Np(V) solutions as a function of pH, from 2.5 to 8, as a function of total Np concentration from 1.29 × 10 −5 M to 2.57 × 10 −4 M, and as a function of ionic strength from 0.001 to 0.5 M NaClO 4. Under most pH conditions, Np adsorption is reversible and exhibits an inverse relationship with ionic strength, with adsorption increasing with increasing pH. At low pH in the 0.1 M ionic strength systems, we observed irreversible adsorption, which is consistent with reduction of Np(V) to Np(IV). We model the adsorption reaction using a nonelectrostatic surface complexation approach to yield ionic strength dependent NpO 2 +-bacterial surface stability constants. The data require two bacterial surface complexation reactions to account for the observed adsorption behavior: R-L 1 − + NpO 2 + ↔ R-L 1-NpO 2 ° and R-L 2 − + NpO 2 + ↔ R-L 2-NpO 2 °, where R represents the bacterium to which each functional group is attached, and L 1 and L 2 represent the first and second of four discrete site types on the bacterial surface. Stability constants (log K values) for the L 1 and L 2 reactions in the 0.001 M system are 2.3 ± 0.3 and 2.3 ± 0.2, and in the 0.1 M system the values are 1.7 ± 0.2 and 1.6 ± 0.2, respectively. The calculated neptunyl-bacterial surface stability constants are not consistent with values predicted using the linear free energy correlation approach from Fein et al. (2001), suggesting that possible unfavorable steric interactions and the low charge of NpO 2 + affects Np-bacterial adsorption.

Cyclopentadienyl and Olefin Substituent Effects on Insertion and β-Hydrogen Elimination with Group 4 Metallocenes. Kinetics, Mechanism, and Thermodynamics for Zirconocene and Hafnocene Alkyl Hydride Derivatives

Reactions of group 4 metallocene dihydrides, (RnCp)2MH2 (RnCp = alkyl-substituted cyclopentadienyl; M = Zr, Hf), with olefins afford stable metallocene alkyl hydride complexes of the general formula (RnCp)2M(CH2CHR‘2)(H) (R‘ = H, alkyl). For sterically crowded, monomeric dihydrides, Cp*2ZrH2 (Cp* = η5-C5Me5), Cp*(η5-C5Me4H)ZrH2, Cp*(η5-C5Me4Et)ZrH2, Cp*2HfH2, and Cp*(η5-C5H3-1,3-(CMe3)2)HfH2, second-order rate constants for olefin insertion have been measured. For Cp*2HfH2, the relative rates of olefin insertion have been found to be 1-pentene > styrene ≫ cis-2-butene > cyclopentene > trans-2-butene > isobutene. The rate of isobutene insertion into Cp*(η5-C5Me4H)ZrH2 is 3.8 × 103 times greater than that for Cp*2ZrH2 at −63 °C, demonstrating the striking steric effect for isobutene insertion imposed by a tenth methyl substituent on the two cyclopentadienyl ligands. A primary kH/kD of 2.4(3) at 23 °C and a linear free energy correlation to σ (ρ = −0.46(1)) for para-substituted styrene insertion indicate tha...

Electronic substituent effects in the electron impact mass spectrometry of 2‐(arylazo)‐4‐phenylphenols

A mass spectrometric study of a set of six novel 2-(arylazo)-4-phenylphenols 1-6 was performed. The electron impact spectra were acquired and analyzed for five of the compounds in order to establish a fragmentation pattern. The suggested pathways were investigated and confirmed by means of tandem mass spectrometry (MS/MS) experiments together with high-resolution accurate mass data. However, the sixth molecule, a sodium sulfonate salt, was studied using fast atom bombardment (FAB) ionization in positive and negative modes. In addition, some electronic substituent effects were investigated by analyzing Hammett-McLafferty linear free energy correlations for some peaks derived from the corresponding molecular ions. Also, the role of the O-H...N hydrogen bond present in the target compounds was analyzed. The roles of these H-bonds were consistent with the corresponding acidity constant values obtained experimentally as well as by theoretical quantum chemistry calculations using HF/6-31 + G(d,p) and B3LYP/6-31G(d,p). Some spectrometric data were correlated with topological properties derived from the atoms-in-molecules (AIM) theory.