Reactions In Aprotic Solvents Research Articles

Dealkaline lignin–The waste from the pulp and paper industry as acid catalyst in biorefinery

Dealkaline lignin, a by-product from the pulp and paper industry, can be transformed into lignosulfonic acid catalysts with a simple ion exchange method. The catalyst exhibited much superior catalytic performance to the commercial resin catalyst for the conversion of furfuryl alcohol to ethyl levulinate and fructose to 5-hydroxymethylfurfural (yield: 97% over lignosulfonic acid catalyst versus 20% over the commercial resin catalyst under the similar reaction conditions). The excellent catalytic activity was due to the synergistic effects of carboxyl, hydroxyl and sulfonic acid groups. The abundant carboxyl and hydroxyl groups in catalyst could form hydrogen bond in local area of SO3H sites with the oxygen-containing substrates, facilitating their acid-catalyzed reactions, but only in the aprotic solvents. In protic solvents, the substrate will preferably form hydrogen bond with the solvent but not lignosulfonic acid, and thus the lignosulfonic acid catalyst is more advantageous for catalyzing the acid catalyzed reactions in aprotic solvents.

Reaction mechanisms of the oxygen reduction and evolution reactions in aprotic solvents for Li–O2 batteries

Abstract The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in aprotic solvents are elementary reactions for the discharging and charging processes on the cathode of the lithium-oxygen batteries, respectively. Understanding the mechanisms of these reactions at a molecular level has now become a bottleneck that hinders the development of the battery. This short article briefly reviews recent progresses in the studies of the ORR/OER mechanism in aprotic solvents. Two reaction mechanisms, the electrochemical pathway and chemical (disproportionation) pathway, will be discussed with their contribution to the ORR process on the cathode surface. Furthermore, the origin of the OER overpotential will also be discussed. The solutions to reduce the OER overpotential are noted with development of redox mediators.

Investigation of the complex influence of divalent cations on the oxygen reduction reaction in aprotic solvents

The resurgence of interest in oxygen reduction in non-aqueous electrolytes makes it necessary to develop a fundamental understanding of the factors influencing its kinetics and reversibility. One of these key factors is the cation of the conducting salt. This study focuses on the influence of the cation by extending research to the field of the alkaline earth cations. At gold electrodes, the main formation of the peroxides is observed for all the different cations. However, the peroxides do not lead to a deactivation of the electrode as opposed to the observations in the presence of lithium ions. Interestingly, at platinum and glassy carbon the cation exerts a more complex influence on the product distribution: The number of electrons transferred per oxygen molecule (z) increases in the order z(Ca2+)<z(Sr2+)<z(Ba2+)≈z(Mg2+). This is in good agreement with the acceptor numbers of the cations, with high acceptor numbers fostering the formation of peroxides. These results add an important aspect in understanding the oxygen reduction in non-aqueous solvents.

Calcium–Oxygen Batteries as a Promising Alternative to Sodium–Oxygen Batteries

The development of a reversibly working metal–O2 battery is an ongoing challenge. While lithium and sodium have been extensively used to study the oxygen reduction reaction in aprotic solvents, calcium did not receive attention for use in a metal–O2 battery. This paper aims at examining the oxygen reduction and evolution reaction in Ca2+-containing dimethyl sulfoxide at different electrode materials via the use of differential electrochemical mass spectrometry and rotating ring disc electrodes. The measurements reveal that superoxide is the main product of the oxygen reduction at various electrode materials. However, at gold electrodes the transfer of two electrons per oxygen molecule is observed. The superoxide is soluble and can be reoxidized almost completely from the solution to evolve oxygen, leading to roughly 90% true Coulombic efficiency. Furthermore, the oxygen evolution from species deposited during the oxygen reduction has been observed resulting in an overall reversibility of 95%. Our results ...

The role of imidazole in peptide cyclization by transesterification: parallels to the catalytic triads of serine proteases

The improved bioavailability, stability and selectivity of cyclic peptides over their linear counterparts make them attractive structures in the design and discovery of novel therapeutics. In our previous work, we developed an imidazole-promoted preparation of cyclic depsipeptides in which we observed that increasing the concentration of imidazole resulted in the concomitant increase in the yield of cyclic product and reduction in dimerization, but also resulted in the generation of an acyl-substituted side product. In this work, we used transition state analysis to explore the mechanism of the imidazole-catalyzed esterification of one such peptide, Ac-SAFYG-SCH2φ, and determined the acyl substitution product to be an intermediate in a competing reaction pathway involving acyl substitution of the thioester by imidazole. Our findings indicate that imidazole plays an essential role in this side-chain to C-terminal coupling, and by extension, in transesterifications in general, through a concerted mechanism wherein imidazole deprotonates the nucleophile as the nucleophile attacks the carbonyl. The system under study is identical to the histidine-serine portion of the catalytic triads in serine proteases and it is likely that these enzymes employ the same concerted mechanism in the first step of peptide cleavage. Additionally, relatively high concentrations of imidazole must be used to effectively catalyze reactions in aprotic solvents since the overall reaction involves imidazole acting both as an acid and as a base, existing in solution as an equilibrium distribution between the neutral form and its conjugate acid.

Kinetics and mechanism of quaternization of tertiary phosphines with unsaturated carboxylic acids. Kinetic studies of the reactions in aprotic solvents

Data obtained while studying the kinetics of quaternization of tertiary phosphines with the unsaturated carboxylic acids in the series of aprotic solvents indicate the participation of a second molecule of acid playing the part of the external proton donor in this process. Quantitative analysis of the effect of solvent within the frames of the Koppel-Palm equation showed that the main contribution in the reaction rate belongs to the nucleophilicity of medium while the effects of polarity and electrophilicity are smaller. The results obtained suggest the step-by-step mechanism of interaction including the formation of the zwitterionic intermediate on the reaction pathway common for the solvents with different proton activity.

Primary kinetic hydrogen isotope effects in deprotonations of carbon acids

AbstractThe mechanisms of deprotonation of carbon acids are reviewed to emphasise the important kinetic consequences associated with the internal and solvent reorganisation in forming the conjugate base. Some practicalities of measurements of kinetic hydrogen isotope effects are then considered to show how competing reactions, exchange and mechanistic branching might affect these measurements. Finally, isotopic rate ratios (kH/kD) have been collected for non‐enzymic deprotonations of carbon acids for which temperature dependences have also been determined. All examples contain nitro groups. For reactions in hydroxylic solvents, only two examples are found for which kH/kD &gt; 8 at 25 °C (28.8 and 10.7), possibly indicating a tunnelling contribution. In both cases, values of E–E are larger and values of AH/AD smaller than the range expected (E–E = 4.54 kJ mol−1 and 0.7 &lt; AH/AD &lt; 1.4) for effects arising from zero‐point energy differences with no tunnelling contributions. For reactions in aprotic solvents, isotope effects are larger (12.1 &lt; kH/kD &lt; 19.6), but again, the largest of these (19.6 and 15.8) are associated with elevated values of E–E and small values of AH/AD. It is pointed out that behaviour of this type is accommodated by corrections to the semi‐classical model with shallow tunnelling maximised by high symmetrical barriers. No examples have been found in which large effects are associated with large values of AH/AD and small values of E–E.Comparisons are made with two recently reported enzyme‐catalysed reactions involving deprotonation of nitroalkanes. Copyright © 2010 John Wiley &amp; Sons, Ltd.

From Simple Diols to Carbohydrate Derivatives of Phenylarsonic Acid

A series of spiro-arsoranes bearing a phenyl moiety as the fifth substituent were synthesized applying open-chain, as well as cyclic vicinal diols, as chelating molecules by condensation reactions in aprotic solvents. The products synthesized are the spiro compounds of the general formula PhAs(DiolH(-2))(2) derived from the vicinal diols meso-2,3-butanediol, PhAs(meso-2,3-ButdH(-2))(2) (1), exo-cis-2,3-norbornanediol, PhAs{exo-cis-NobdH(-2)}(2) (2), cis-1,2-cyclopentanediol, PhAs(cis-1,2-CptdH(-2))(2) (3), anhydroerythritol, PhAs(AnErytH(-2))(2) (4), cis-1,2-cyclohexanediol, PhAs(cis-1,2-ChxdH(-2))(2) (5), and rac-trans-1,2-cyclohexanediol, rac-{PhAs(trans-1,2-ChxdH(-2))(2)} (6) which were identified as mononuclear compounds. A novel dimeric double-spiro environment for oxyarsoranes was found in the reaction products derived from the sterically demanding diols 1,1'-bicyclohexyl-1,1'-diol {PhAs(BhxdH(-2))O}(2) (7) and perfluorpinacol, {PhAs(FpinH(-2))O}(2) (8). The stability of the compounds in acidic and neutral aqueous media in the presence of organic co-solvents was investigated. A convenient synthetic procedure for spiro-oxyarsoranes, applying water as the solvent, was developed and proven to be advantageous. All of the compounds synthesized in this study were characterized by means of melting-point measurement, single-crystal X-ray analysis, NMR, IR, Raman, UV/vis, and mass spectrometry. The principles found for these reactions were valid for the methyl glycosides of beta-D-ribofuranose and alpha-D-mannopyranose. The spiro-arsorane derived from methyl alpha-D-mannopyranoside, PhAs(Me-alpha-D-Manp2,3H(-2))(2) (9), is the first example of a structurally characterized carbohydrate-arsenic(V) compound.

A New Reaction Pathway in the Ester Aminolysis Catalyzed by Glymes and Crown Ethers

Butylaminolysis of p-nitrophenyl acetate in chlorobenzene in the presence of different kinds of phase-transfer catalysts (crown ethers and glymes) supports the existence of a reaction pathway exhibiting a first-order dependence on the concentration of the phase transfer catalyst and a second-order dependence on the concentration of butylamine. This novel reaction pathway must be included in the mechanism traditionally accepted for the catalysis by phase-transfer agents of aminolysis reactions in aprotic solvents.

Stereoselective Michael addition of benzylamines to homochiral methylenebutanedioates

The Michael addition of benzylamine to the homochiral methylenebutanedioate 10 gave an adduct 11 in good yield with high stereoselectivity. By performing the reaction in methanol the (2R,3R) diastereoisomer 11 was obtained in 88% de, which was increased to 98% de after recrystallisation of the primary amine derivative 13. The ratio of diastereoisomers was reversed by performing the reaction in aprotic solvents, with the (2R,3S) diastereoisomer 12 being obtained in 40% de in tetrahydrofuran. The Michael adduct 11 is formed under kinetic control. The primary amine 15 is a key intermediate in the synthesis of novel matrix metalloproteinase inhibitors.

Aggregation effects in the reactions of 2,4-dinitrochlorobenzene with aniline in aprotic solvents

The kinetics of the reaction of 2,4-dinitrochlorobenzene with aniline have been studied in toluene and in toluene–methanol binary solvents and compared with the reactions in benzene and chloroform. The reactions in aprotic solvents exhibit a rate dependence that is third-order in amine consistent with aggregates of the aniline acting as the nucleophile. Molecular complexes between the aniline and the substrate were also detected spectrophotometrically. Additionally, an inhibitory effect of toluene complexes was observed, which is similar to the previously reported effect of benzene in the reaction of the same substrate with n-butylamine. The reaction in the presence of pyridine shows an amine dependence which indicates formation of ‘mixed aggregates’ between aniline and pyridine. All the above results, as well as recently reported studies of other reactions different from SNAr, are fully interpreted within the ‘dimer nucleophile’ mechanism.

Influence of magnesium(II) ions on cathodic reactions in aprotic solvents—carboxylation of methyl aryl ketones

The presence in the medium of free MgII leads to a complete change in the voltammetry for the reduction of 2-acetylnaphthalene in DMF–Bu4NBF4 saturated with carbon dioxide. The mechanism for these changes has been investigated and it is confirmed that the MgII interacts with the radical anions formed in the initial electron transfer reaction. The relevance of these conclusions to electrocarboxylations carried out in undivided cells with a sacrificial Mg anode is discussed. It is concluded that all the MgII formed at the anode is complexed strongly by the final carboxylated products from the cathode reaction and therefore the cathode reaction occurs in an environment where no free MgII ions are available. On the other hand, the complexation of the carboxylated products stabilises them to unwanted further reactions, thereby improving the selectivity of the electrosynthesis.

Influence of magnesium(II) ions on cathodic reactions in aprotic solvents: the reduction of benzoate esters

The use of an Mg anode in an unidivided cell alters the products from many electrosynthetic reactions. In an attempt to understand this phenomenon, the influence of MgII ions on the mechanism for the reduction of benzoate esters in DMF has been investigated. It is confirmed that the presence of MgII ions in solution leads to a change in the major reaction pathway from cleavage of the ester linkage (to give benzoate ion) to dimerisation (to diketone). The dimer is then reduced further at the same potential. Cyclic voltammetry shows that in the absence of MgII, the also, the ester radical anion is stable, While in the presence of MgII, the radical anion dimerises rapidly and the MgII also catalyses further reduction. Voltammetry within the synthesis cell, however, shows the much of the MgII formed by dissolution of the anode is tied up by anion products and that the dimerisation of the radical anion must occur well from the cathode surface.

The reaction of imidazole with some 1-halogeno-2,4-dinitrobenzenes in aprotic solvents

The reactions of imidazole with 1-chloro- and 1-fluoro-2,4-dinitrobenzenes in dimethyl sulfoxide are not catalysed by imidazole. DABCO or imidazolium perchlorate. In acetonitrile these reactions are not catalysed by the nucleophile or DABCO but the second-order rate constant kA has a linear dependence on the concentration of tetrabutylammonium chloride and is depressed by the addition of imidazolium perchlorate. In this solvent the reaction of imidazole with 1-bromo-2,4-dinitro-benzene is not catalysed by the nucleophile and the kABr/kACl ratio is 0.90. These results are rationalised in terms of the conventional mechanism of aromatic nucleophilic substitution reactions in aprotic solvents of high relative permittivity, with chloride ion catalysing the formation of the intermediate in acetonitrile solution.In benzene the reaction of imidazole with 1-chloro-2,4-dinitrobenzene is strongly catalysed by the nucleophile and by tetrabutylammonium chloride and weakly by pyridine. The corresponding reaction with 1-bromo-2,4-dinitrobenzene is also strongly catalysed by imidazole and there is little difference in the reactivity of the two substrates. The catalysis of the reactions by amines observed here, and that previously reported in the literature, is explained as being due to reaction taking place via complexes formed from the reactants and catalysts.

Solvent effects on kinetics and mechanism of acid-catalyzed decomposition of 1,3-bis(4-methylphenyl)-triazene II. Reactions in aprotic solvents

The effect of aprotic solvents (hexane, cyclohexane, dichloromethane, 1,2-dichloroethane, benzene, acetonitrile, acetone, 1,2-dimethoxyethane, ethyl acetate, dioxane) on kinetics and mechanism of acid-catalyzed decomposition of 1,3-bis(4-methylphenyl)triazene has been studied with trichloroacetic acid as the acid catalyst. It has been found that beside the non-dissociated monomer of the acid also its dimer acts as the catalytic species. With regard to the results obtained in protic solvents (the catalysis by proton and general acid) three cases can be encountered of the dependence of observed rate constant on analytical concentration of the acid. The effect of solvents (inclusive of the protic ones) on the catalytic rate constant of the reaction with the non-dissociated monomer of acid is best interpreted by the equation suggested by Koppel and Palm and by the solvent scale suggested by us earlier. The solvent acidity and polarity have positive effect, whereas its basicity has negative effect. The catalytic rate constant of the reaction with the acid dimer decreases with increasing solvent basicity and polarity, due predominantly to the decrease in the equilibrium constant of dimerization.

The ‘dimer mechanism’ in aromatic nucleophilic substitution by amines in aprotic solvents

AbstractThe mechanism of aromatic nucleophilic substitutions by amines in protic solvents is well established; on the contrary the mechanism/s of the reactions in aprotic solvents is/are still subject of controversy. The present paper describes several systems for which fourth‐order kinetics (third‐order in amine) were observed. A mechanism is proposed to account for this as well as other observation such as: overall negative energies of activation, quadratic dependence of kA with non‐nucleophilic tertiary bases, spectacular effects of hydrogen‐bond donor (HBD) and hydrogen‐bond acceptor (HBA) catalysts, etc. Other alternative mechanisms are also discussed.

Selective Conversion of 2-Mercaptoalkanols to Thiirans with Orthocarbonates

Abstract Tetraalkyl orthocarbonates 1 were demonstrated to serve as potent cyclodehydrating agents for 2-mercaptoalkanols 2 to give the corresponding thiirans in good yields in acid-catalyzed reactions in aprotic solvents. Effects of catalysts, solvents, and alkyl substituents of 1 were examined. Activity of the acid catalysts depended on their acidity (pKa), and the strong Lewis acid BF3·OEt2 also had high catalytic activity. Effects of solvents and alkyl substituents of 1 were little observed.

Kinetics and mechanism of photoinduced and thermal proton-transfer processes in o-hydroxybenzaldehyde and o-hydroxyacetophenone. A remarkable temperature dependence of the reaction rate

Time-resolved electronic emission and absorption spectroscopy of photoexcited solutions of o-hydroxybenzaldehyde (OHBA) and o-hydroxyacetophenone (OHAP) are reported. When these molecules are brought to their first excited single state an adiabatic excited-state phototautomerization reaction takes place and subsequently the tautomer in its lowest electronic triplet state and in its ground state may be observed as transient species. The return of the ground-state tautomer in solutions in aprotic solvents to the stable form of the molecule may be catalysed by concentrations of alcohols as small as 10–4 mol dm–3. The catalysis involves a protonation and a subsequent deprotonation step. The rate of the uncatalysed reaction in aprotic solvents exhibits a remarkable temperature dependence. The rate constant behaves in an Arrhenius manner, but with a negative activation energy in the high-temperature range and a normal positive activation energy in the low-temperature range. The explanation for this behaviour involves a fast unimolecular dynamic equilibrium between two forms of the ground-state tautomer. In one form there is an intramolecular hydrogen bond, and in the other form this bond is broken. Only the form with the hydrogen bond leads directly to the stable form of the molecule. In the other form of the tautomer the proton donor and acceptor sites are far apart as a consequence of rotational isomerization, which accompanies the breaking of the intramolecular hydrogen bond. It is shown that a dynamic equilibrium between two rotameric forms, as in the case of OHBA, may explain the small pre-exponential factors for the observed normal Arrhenius behaviour of a number of other intramolecular proton-transfer reactions.

Phosphoryl and Thiophosphoryl Transfer Reactions: Stereochemical Imperatives for Metaphosphate and Thiometaphosphate

Abstract The stereochemical courses of phosphoryl transfer reactions in aprotic solvents and thiophosphoryl transfer reactions in protic solvent have been determined. The extensive racemisation observed in both instances is discussed in terms of metaphosphate and thiometaphosphate intermediates of significant life-times.

ChemInform Abstract: Factors Influencing Kinetic Isotope Effects in Some Proton‐Transfer Reactions in Aprotic Solvents.

Chemischer InformationsdienstVolume 17, Issue 50 Preparative Organic Chemistry ChemInform Abstract: Factors Influencing Kinetic Isotope Effects in Some Proton-Transfer Reactions in Aprotic Solvents. P. PRUSZYNSKI, P. PRUSZYNSKISearch for more papers by this authorA. JARCZEWSKI, A. JARCZEWSKISearch for more papers by this author P. PRUSZYNSKI, P. PRUSZYNSKISearch for more papers by this authorA. JARCZEWSKI, A. JARCZEWSKISearch for more papers by this author First published: December 16, 1986 https://doi.org/10.1002/chin.198650137AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume17, Issue50December 16, 1986 RelatedInformation