Phenoxide Ion Research Articles

Linear Poly(arylene ether)s with Pendant Benzoyl Groups: Geometric Isomers of PEEK or Substituted Poly(phenylene oxide)?

The characterization and nucleophilic aromatic substitution (NAS) behavior of 3,5-difluorobenzophenone, 5, are reported. The reactivity of the electrophilic sites, activated by only a benzoyl group located in the meta position, in 5 was probed via NMR spectroscopy and model reactions and was determined to be sufficient to undergo typical NAS reactions. Displacement of the first fluoride atom in 5, with the phenoxide ion derived from p-cresol, required a reaction temperature of 150 °C for 21 h and resulted in a significant decrease in reactivity for displacement of the second fluoride atom, which required 185 °C for displacement. Confirmation that the displacement reaction of both fluorides occurred via an SNAr mechanism was provided by 1H and 13C NMR data on the model compounds. High molecular weight, amorphous poly(arylene ether)s, bearing a pendant benzoyl group, were prepared via the reaction of 5 with a variety of bis-phenols under typical NAS conditions. The glass transition temperatures (Tg) ranged from 111 to 133 °C while the 5% weight loss temperatures ranged from 488 to 494 °C under nitrogen and from 438 to 482 °C in air.

The bromination kinetics of phenolic compounds in aqueous solution

The purpose of this study was to investigate the bromination kinetics of selected phenolic compounds in aqueous solutions over the pH range of 5–11. The experiment results indicated that the reaction of hypobromous acid with the phenoxide ions controlled the overall reaction rate, whereas the reaction between hypobromite ion and the phenoxide ions and the reaction between hypobromous acid and the undissociated phenolic species were considered to be negligible respectively in the pH range of 7–9. The apparent second-order rate constants of the reaction of hypobromous acid with the phenoxide ions ranged from 7.9 × 10 6 M −1 s −1 for 3-chlorophenol to 6.5 × 10 8 M −1 s −1 for 3-methoxyphenol, respectively. The Hammett correlation could be successfully used to estimate the reactivity of bromine with substituted phenols and the linear regression was log( k 2) = −2.85б + 8.00. The rate constants of the reaction of bromine with phenol-like organic compounds were about three orders of magnitude higher than with chlorine and two to three orders of magnitude lower than with ozone.

Gas-Phase IR Spectroscopy of Deprotonated Amino Acids

Gas-phase infrared multiple photon dissociation (IRMPD) spectra have been recorded for the conjugate bases of a series of amino acids (Asp, Cys, Glu, Phe, Ser, Trp, Tyr). The spectra are dominated by strong symmetric and antisymmetric carboxylate stretching modes around 1300 and 1600 cm(-1), respectively. Comparison of the experimental spectra with spectra calculated at the DFT level suggests a carboxylate structure for all species investigated, which is in contrast with what has recently been suggested in this journal for deprotonated cysteine [J. Am. Chem. Soc. 2007, 129, 5403-5407]. In addition, the IR spectrum of the conjugate base of tyrosine is also unambiguously that of a carboxylate ion and not that of a phenoxide ion. In sharp contrast with the conjugate bases of other amino acids investigated here, the aspartate and glutamate anions show very broad, hardly resolved spectral features. We present qualitative experimental evidence indicating that this can be attributed to the formation of a proton bridge between the backbone and side chain carboxylate groups. The large amplitude motion of this shared proton, coupling to virtually all other vibrational modes, causes extensive spectral broadening.

Enantioselective Synthesis of Functionalized 1‐Benzoxepines by Phenoxide Ion Mediated 7‐endo‐tet Carbocyclization of Cyclic Sulfates

AbstractA new asymmetric synthesis of 2,3‐disubstituted 1‐benzoxepines is described. Key steps include Sharpless asymmetricdihydroxylation of trans‐α,β‐unsaturated esters and phenoxide ion mediated intramolecular 7‐endo‐tet carbocyclization of syn‐2,3‐dihydroxy ester derived cyclic sulfates. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Structure of phenol-formaldehyde polycondensates

Prepolymer resol resins with the molar ratio of phenol/formaldehyde 1/2.4 and different amounts of NaOH were synthesized and characterized using 13 C NMR spectra for samples in CD3OD solution. Nearly the full substitution in ortho and para aromatic positions occurred, and quite constant ratios of hydroxymethyl/methylene or o, p'/p, p'-methylene groups were obtained. The content of free formaldehyde strongly depends on the NaOH content in synthesis. The prepolymeric chains end mainly with aromatic rings substituted with two ortho-hydroxymethyl groups. The alkali-dependent content of phenoxide ions causes the downfield shifts of 13 C signals for ortho-hydroxymethyl groups from 62.8 ppm to 64.4 ppm and for aromatic carbon bearing the hydroxyl group from 157 ppm to 163 ppm. A comparison with industrial resol resin for the manufacture of plywood is presented.

Separation of liquefied product of Salix psammophila by column chromatography and structure analysis of its components

The liquefied product of Salix psammophila wood was separated by thin-layer chromatography (TLC) and column chromatography, and its structure was identified by nuclear magnetic resonance (NMR) spectra in our study. The separation result indicates that the sample of liquefied S. psammophila contained at least two categories of components. The structure of the main components was guaiacyl C-1, C-2 of the hydroxyphenyl propane, i.e., the aromatic nucleus protons of lignin. Degradation and polycondensation reactions occurred when the S. psammophila wood was liquefied in phenol. Polycondensation reactions occurred among the depolymerization products from cellulose, the aromatic depolymerization products from lignin and the products of the displacement reactions between phenoxide ion and cellulose.

Negative APCI-LC/MS/MS Method for Determination of Natural Persistent Halogenated Products in Marine Biota

A sensitive and selective method utilizing high performance liquid chromatography coupled to negative atmospheric pressure chemical ionization tandem mass spectrometry (APCI-LC/MS/MS) was developed to enable analysis of selected natural persistent organohalogens accumulated in marine biota. The analytes were three methoxylated tetrabromodiphenyl ethers (6-MeO-BDE47, 2'-MeO-BDE68, and 2',6-diMeO-BDE68), a dimethoxylated tetrabromobiphenyl (2,2'-diMeO-BB80), and two halogenated methyl bipyrroles (Cl(7)-MBP and Br(4)Cl(2)-DBP). These products were well resolved on a 150 mm reversed-phase column with methanol as the mobile phase. The fragmentation pathways of the Cl(7)-MBP and Br(4)Cl(2)-DBP produced characteristic multiple reaction monitoring (MRM) transitions. Determination was performed in the MRM mode using phenoxide ion [M-Br+O](-) and product Br(-) ions for MeO-BDE analogues, or the precursor [M-Cl+O](-) to Br(-) ion for Br(4)Cl(2)-DBP, and to C(4)NCl(4)(-) ion for Cl(7)-MBP. The APCI-LC/MS/MS method is acceptable for calibration of the linearity and repeatability of all products studied in the low ng/g (lipid weight) level and with similar sensitivity to the electron ionization (EI)-GC/MS method. The proposed method was applied for quantification of natural organohalogens accumulated in melon-headed whale (Peponocephala electra) blubber (N = 15) in the Asia-Pacific Ocean. The concentration was positively correlated between different groups of compounds except for 2'-MeO-BDE68. The use of the analytical method based on negative ion APCI-LC/MS/MS would provide a new way for rapid monitoring of halogenated natural products from marine biota, such as sponges or algae.

Evidence for Concerted Proton−Electron Transfer in the Electrochemical Oxidation of Phenols with Water As Proton Acceptor. Tri-tert-butylphenol.

Proton-coupled electron transfer oxidation of phenols play a prominent role in several natural processes, and one may wonder if their high efficiency is related to the possibility that the electron and proton transfer steps are concerted. The cyclic voltammetric observation of the electrochemical oxidation and reverse reaction has allowed, with the example of 2,4,6-tri-tert-butylphenol in nonbuffered aqueous media, the clear identification of a pathway in which a phenol is directly and reversibly converted into the phenoxyl radical while the generated proton is accepted by a water molecule in a concerted manner. In very basic media, a stepwise mechanism takes place in which the phenol is deprotonated by OH- and the resulting phenoxide ion rapidly oxidized into the phenoxyl radical. As the pH decreases, this pathway progressively shuts down to the advantage of the concerted pathway. The latter assignment is confirmed by the observation of a substantial H/D kinetic isotope effect. At moderately basic pH 10.5, the contributions of the two pathways are about equal and the occurrence of the two competing routes is directly visualized in the cyclic voltammetry response.

Kinetic studies on the addition of phenol to S-phenyl-S-vinyl-N-p-tosylsulfilimine derivatives

The rate constants of nucleophilic addition of phenyl to vinylsulfilimine(VSI) and its derivatives were determined by an ultraviolet spectrophotometer at 25°C, and the rate equations which can be applied over a wide pH range were obtained. On the basis of the pH-rate profile, product analysis, general base catalysis, and the substituent effect, a plausible mechanism of this addition reaction was proposed: Below pH 3.0, the reaction proceeded via the addition of a neutral molecule to the carbon-carbon double bond after protonation at the nitrogen atom of the sulfilimine. Above pH 10.0, the addition of a phenoxide ion to the carbon-carbon double bond was rate controlling. However, in the pH range of 3.0 to 10.0, these two reactions occurred competitively.

Does Electrospray Ionization Produce Gas-Phase or Liquid-Phase Structures?

Electrospray ionization of tyrosine from a 3:1 (v:v) CH3OH/H2O solution is found to afford an M - H ion which is a 70:30 mixture of phenoxide and carboxylate ions. This corresponds to the gas-phase equilibrium composition and not the liquid-phase proportions. In contrast, the carboxylate is produced as the dominant ion (approximately 95%) from anhydrous CH3CN and CH3CN/H2O mixtures. The addition of small amounts of CH3OH to the solvent, however, convert the M - H ion back into the gas-phase isomeric ratio. The isomeric structure therefore depends on the solvent system from which an ion is sprayed.

Determination of Gas-Phase Acidities of Dimethylphenols: Combined Experimental and Theoretical Study

The gas-phase acidities of the six dimethylphenol isomers were determined experimentally, by using the kinetic method, and theoretically, through quantum chemistry calculations. The experimental values, relative to the gas-phase acidity of phenol, are (in kJ mol(-1)): -1.76+/-0.76 (2,3-Me2C6H3OH), 1.78+/-0.29 (2,4-Me2C6H3OH), 0.83+/-0.58 (2,5-Me2C6H3OH), -4.39+/-0.89 (2,6-Me2C6H3OH), 5.38+/-1.08 (3,4-Me2C6H3OH), and 1.88+/-0.08 (3,5-Me2C6H3OH). This trend was discussed by considering the substituent effects on the thermodynamic stabilities both of the parent phenols and the corresponding phenoxide ions. The above acidity data, the literature values for 2-, 3-, and 4-methylphenol, and the substituent effects analysis allowed to develop a simple empirical method to estimate the acidity of any methyl-substituted phenol.

Structure of polycondensates from hydroxymethylphenols

AbstractThe structure of oligomers obtained from mono‐hydroxymethylphenols in melt condensation at 120°C was determined using 13C NMR spectra in CD3OD solution. Alongside of methylene region of spectrum, valuable information was obtained from signals of aromatic carbons. Noncatalytic conditions promote the formation of dihydroxydibenzyl ethers in equilibrium with ortho‐ and para‐benzoquinones of oxymethylene derivatives. The final methylene linked oligomers are formed, mainly, by splitting the ether intermediates with free aromatic positions. In alkaline conditions, highly nucleophilic phenoxide ions of ortho‐hydroxymethyl compounds are responsible for substitution in free aromatic positions. The most favored reaction in the mixture of both hydroxymethylphenols is the formation of p,p′‐methylene. In condensation of para‐hydroxymethylphenol, formation of p,p′‐methylene groups occurs with simultaneous release of formaldehyde. High content of alkali stabilized ortho‐hydroxymethyl groups of fully substituted methylene linked oligomers determines the curing behavior of resol phenol–formaldehyde resins. The role of hemiformals in reactions was insignificant. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008

Hydrolytic Reactions of Thymidine 5′‐O‐Phenyl‐N‐Alkylphosphoramidates, Models of Nucleoside 5′‐Monophosphate Prodrugs

To obtain detailed data on the kinetics of hydrolytic reactions of triester-like nucleoside 5'-O-aryl-N-alkylphosphoramidates, potential prodrugs of antiviral nucleoside monophosphates, the hydrolysis of diastereomeric (Rp/Sp) thymidine 5'-{O-phenyl-N-[(1S)-2-oxo-2-methoxy-1-methylethyl]phosphoramidate} (3), a phosphoramidate derived from the methyl ester of L-alanine, has been followed by reversed-phase HPLC over the range from Ho=0 to pH 8 at 90 degrees C. According to the time-dependent product distributions, the hydrolysis of 3 proceeds at pH<4 by two parallel routes, namely by nucleophilic displacement of the alaninyl ester moiety by a water molecule and by hydrolysis of the carboxylic ester linkage that allows intramolecular attack of the carboxy group on the phosphorus atom, thereby resulting in the departure of either thymidine or phenol without marked accumulation of any intermediates. Both routes represent about half of the overall disappearance of 3. The departure of phenol eventually leads to the formation of thymidine 5'-phosphate. At pH>5, the predominant reaction is hydrolysis of the carboxylic ester linkage followed by intramolecular displacement of a phenoxide ion by the carboxylate ion and hydrolysis of the resulting cyclic mixed anhydride into an acyclic diester-like thymidine 5'-phosphoramidate. The latter product accumulated quantitatively without any indication of further decomposition. Hydroxide-ion-catalyzed P--OPh bond cleavage of the starting material 3 occurred as a side reaction. Comparative measurements with thymidine 5'-{N-[(1S)-2-oxo-2-methoxy-1-methylethyl]phosphoramidate} (4) revealed that, under acidic conditions, this diester-like compound is hydrolyzed by P--N bond cleavage three orders of magnitude more rapidly than the triester-like 3. At pH>5, the stability order is reversed, with 3 being hydrolyzed six times as rapidly as 4. Mechanisms of the partial reactions are discussed.

Electrochemical and FTIR spectroscopic studies of tyrosine oxidation at polycrystalline platinum surfaces in alkaline solutions

The electrochemical oxidation of tyrosine has been studied on polycrystalline platinum in sodium hydroxide using cyclic voltammetry and in situ Fourier transform infrared spectroscopy (FTIRS). According to infrared data, the first interaction of tyrosine through the carboxylate group on platinum is proposed. The results showed an onset for tyrosine oxidation at ca. 0.5 V with the formation of soluble formate ion as product. The sharp increase in the current vs potential slope from ca. 1.1 V obeys the detection of cyanate ions together with p-(methyl)-phenoxide ions. No adsorbed carbon monoxide as well as carbon dioxide was detected by this methodology in the whole studied potential range. These results contrast with those obtained in acid solution where two parallel reactions, one yielding the γ-lactone of hydroxy benzofurane and the other producing p-benzoquinone occurs for the oxidation of tyrosine on platinum.

Dissociation of chloride from ortho-, meta- and para-chloromethylphenoxides: The enthalpy of formation of meta-quinomethane

The enthalpy of formation of m-quinomethane has been determined by energy-resolved collision-induced dissociation measurements involving chloro-substituted phenoxide ions. The chloride affinity of m-quinomethane is measured to be 202 ± 9 kJ/mol, whereas the chloride affinities of o- and p-quinomethane are both 57 ± 5 kJ/mol. The difference in the chloride affinities reflects the difference in the stabilities of the Kekule and non-Kekule structures. Combination of the dissociation energy of the meta-isomer with the gas-phase acidity of 3-(chloromethyl)phenol, measured by using the kinetic method to be 1432 ± 6 kJ/mol, results in an enthalpy of formation of 166 ± 13 kJ/mol for the m-quinomethane diradical. The measured value agrees with that predicted by bond additivity. The low bond dissociation energies for the o- and p-isomers, along with electronic structure calculations, suggest that the corresponding chlorine-substituted ions are best described as halide/diradical complexes.

Photochemical and Redox Switching in a Ring-closed Isomer of Thioindigo-extended Quinone

Abstract A new dispiro π-system having a bis(2,6-di-tert-butylcyclohexadienone) unit was synthesized and characterized. Photoirradiation gave an ESR-active bis(phenoxy) radical and electrochemical reduction formed a bis(phenoxide) ion both by ring-opening.

Kinetics of the Reactions of [p-Nitrophenoxy(phenyl)carbene]pentacarbonylchromium(0) with Aryloxide Ions, Hydroxide Ion, and Water in Aqueous Acetonitrile. Concerted or Stepwise?

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.

The hydrophobic effect in reaction mechanism studies and in catalysis by artificial enzymes

AbstractWater is an environmentally benign solvent, easily cleaned and with a harmless vapor, so there is great interest in using water in chemical manufacturing. However, water also has important special properties as a solvent, with the hydrophobic effect promoting rapid and selective reactions in addition reactions, atom transfer reactions, and substitutions. The presence of the hydrophobic effect can be determined with the addition of prohydrophobic and antihydrophobic materials to the water solution. Using such materials, it is possible to determine the detailed geometry of transition states for a number of classical reactions, including alkylation of phenoxide ions on oxygen and carbon. The factors involved, particularly the effect of antihydrophobic agents such as ethanol in lowering the free energy of non‐polar reactants, have not been taken into account in previous well‐known mechanistic studies. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Linear Poly(arylene ether)s with Pendant Phenylsulfonyl Groups: Nucleophilic Aromatic Substitution Activated from the Meta Position

The synthesis, characterization, and nucleophilic aromatic substitution (NAS) behavior of 3,5-difluorophenyl sulfone, 3, are reported. The reactivity of the electrophilic sites, activated by only a phenylsulfonyl group located in the meta position, in 3 was probed via NMR spectroscopy and model reactions and was determined to be sufficient to undergo NAS reactions. Displacement of the first fluoride atom in 3, with the phenoxide ion derived from p-cresol, resulted in a significant decrease in reactivity for displacement of the second fluoride atom, however, not to such an extent as to prevent its displacement under more vigorous conditions. Confirmation of an SNAr mechanism was provided by 1H and 13C NMR data on the model compounds. High molecular weight, amorphous poly(arylene ether)s, bearing a pendant phenylsulfonyl group, were prepared via the reaction of 3 with a variety of bisphenols under typical NAS conditions. The glass transition temperatures (Tg) ranged from 136 to 175 °C while the 10% weight l...

DFT/B3LYP Study of the Substituent Effect on the Reaction Enthalpies of the Individual Steps of Single Electron Transfer−Proton Transfer and Sequential Proton Loss Electron Transfer Mechanisms of Phenols Antioxidant Action

The reaction enthalpies related to the individual steps of two phenolic antioxidants action mechanisms, single electron transfer-proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET), for 30 meta and para-substituted phenols (ArOH) were calculated using DFT/B3LYP method. These mechanisms represent the alternative ways to the extensively studied hydrogen atom transfer (HAT) mechanism. Except the comparison of calculated reaction enthalpies with available experimental and/or theoretical values, obtained enthalpies were correlated with Hammett constants. We have found that electron-donating substituents induce the rise in the enthalpy of proton dissociation (PDE) from ArOH+* radical cation (second step in SET-PT) and in the proton affinities of phenoxide ions ArO- (reaction enthalpy of the first step in SPLET). Electron-withdrawing groups cause the increase in the reaction enthalpies of the processes where electron is abstracted, i.e., in the ionization potentials of ArOH (first step in SET-PT) and in the enthalpy of electron transfer from ArO- (second step in SPLET). Found results indicate that all dependences of reaction enthalpies on Hammett constants of the substituents are linear. The calculations of liquid-phase reaction enthalpies for several para-substituted phenols indicate that found trends hold also in water, although substituent effects are weaker. From the thermodynamic point of view, entering SPLET mechanism represents the most probable process in water.