Apolar Aprotic Solvents Research Articles

Selective aerobic oxidation of benzyl alcohol on inexpensive and reusable ZnO/MnCO3 catalyst

The selective synthesis of benzaldehyde (B) by oxidation of benzyl alcohol (BA) was studied at 373 K using ZnO-MnCO3 catalysts, O2 as oxidant and mild reaction conditions. An inexpensive commercial MnCO3 sample was impregnated with aqueous solutions containing different concentration of Zn2+ cations to obtain a series of ZnO-MnCO3 catalysts with 1.0–22.6 wt% Zn. Catalysts were characterized by N2 physisorption, atomic absorption spectrometry, X-ray diffraction and X-ray photoelectron spectroscopy techniques. Benzaldehyde was selectively formed during liquid phase oxidation of benzyl alcohol but the catalytic performance of the ZnO-MnCO3 materials depended on Zn loading. The maximum initial reaction rate value was measured on the most promising catalyst (15.4 wt% Zn) and a maximum of 88% benzaldehyde yield was achieved on this catalyst after 6 h of reaction using toluene as solvent. The catalytic activity of the ZnO-MnCO3 materials could be assigned to a synergistic contribution of Zn and Mn species. While the main role of ZnO would be related to its capacity to chemisorb the alcohol, the role of highly oxidized Mn surface species (mainly Mn4+) generated during catalyst preparation, is to participate in de redox mechanism involved in benzyl alcohol oxidation toward benzaldehyde. The effect of chemical nature of polar and apolar aprotic solvents was investigated during the aerobic oxidation over 15.4ZnMn. Apolar solvents, in which the oxygen solubility is higher than in the benzyl alcohol, are more suitable for the reaction. An optimum in solvent basicity requirements (hydrogen-bond-acceptance) in order to optimize the catalyst activity was found. The regeneration and reuse of the most promising catalyst were also investigated. A mild thermal treatment at 493 K in flowing air was enough to fully recover the activity of the 15.4ZnO-MnCO3 catalyst.

Substrate–Solvent Crosstalk—Effects on Reaction Kinetics and Product Selectivity in Olefin Oxidation Catalysis

In this work, we explored how solvents can affect olefin oxidation reactions catalyzed by MCM-bpy-Mo catalysts and whether their control can be made with those players. The results of this study demonstrated that polar and apolar aprotic solvents modulated the reactions in different ways. Experimental data showed that acetonitrile (aprotic polar) could largely hinder the reaction rate, whereas toluene (aprotic apolar) did not. In both cases, product selectivity at isoconversion was not affected. Further insights were obtained by means of neutron diffraction experiments, which confirmed the kinetic data and allowed for the proposal of a model based on substrate–solvent crosstalk by means of hydrogen bonding. In addition, the model was also validated in the ring-opening reaction (overoxidation) of styrene oxide to benzaldehyde, which progressed when toluene was the solvent (reaching 31% styrene oxide conversion) but was strongly hindered when acetonitrile was used instead (reaching only 7% conversion) due to the establishment of H-bonds in the latter. Although this model was confirmed and validated for olefin oxidation reactions, it can be envisaged that it may also be applied to other catalytic reaction systems where reaction control is critical, thereby widening its use.

Systematic study on solubility of chrysin in different organic solvents: The synergistic effect of multiple intermolecular interactions on the dissolution process

The solubility of chrysin in N, N-dimethylformamide (DMF), acetone, ethyl acetate (EA), n-butanol, isopropanol, n-propanol, ethanol, methanol, n-hexane, and water was measured by using the static equilibrium method or the dynamic method at temperatures ranging from 293.2 to 333.2 K. Chrysin shows the easiest solvation in the dipolar aprotic solvents, followed by the polar protic solvents (alcohols), and shows poor solvation in the apolar aprotic solvents (hexane) and the strong polar water. The solubility of chrysin increases with rising temperature. The inherent microscopic solvation mechanisms were investigated qualitatively and quantitatively using a combination of the KAT-LSER model as well as σ-profile theory and molecular interaction energies calculated by COSMO–RS model. The results show the solvation process of chrysin in the different solvents is a complex process which depends on the synergistic effects of multiple molecular interactions between the solute and solvent together with the solvent and solvent. Moreover, the experimental solubility data in all solvents could be reasonably correlated by the modified Apelblat equation, λh equation, van't Hoff equation, Wilson model, and NRTL model, whereas the NRTL model and the modified Apelblat equation showed the most satisfactory fitting results. The dissolution thermodynamic properties, including enthalpy, entropy and Gibbs energy of chrysin in the dissolution process were evaluated using the van't Hoff equation. Moreover, the overall dissolution trend and the temperature-dependence of experimental solubility of chrysin in the different classes of solvents can be predicted correctly by the COSMO–RS model.

Effects of solvent on inclusion complexation of a chiral dipeptide toward racemic BINOL.

The effects of reaction solvent on inclusion complexation of a chiral dipeptide (3S,6S)-1 derived from (S)-proline toward racemic BINOL was investigated, discovering that the reaction solvent played a crucial role in determining the inclusion complexation behavior of dipeptide (3S,6S)-1 toward rac-BINOL. (3S,6S)-1 did not show any chiroselective or achiroselective complexation toward rac-BINOL in polar protic solvents such as methanol and ethanol, polar aprotic solvents including trichloromethane and THF, while in polar aprotic solvent ethyl acetate and apolar aprotic solvents benzene, (3S,6S)-1 displayed achiroselective complexation toward rac-BINOL. However, the resulting heterocomplex HC-2 from benzene and HC-3 from ethyl acetate have a different composition. Single crystal X-ray diffraction analysis demonstrates that the two heterocomplexes are formed via different H-bond interaction patterns, in which the reaction solvent has a dramatic effect. Furthermore, this work provides a relatively green method for quantitative enantiomeric enrichment of nonracemic BINOL, in which unacceptable and toxic benzene was replaced by ethyl acetate.

Solvation environment effects on the photoisomerization equilibrium of the model tannins catechin and epicatechin as natural sunscreens in aquatic systems

The photoisomerization equilibrium between the model tannins (-)-catechin and (-)-epicatechin in aqueous solution was investigated at the density functional level of theory to gain insights into the action of these compounds as natural sunscreens in aquatic systems. Increasing water temperature, as might be expected on seasonal and diurnal bases, is predicted to shift the equilibrium further in favor of catechin. The isomerization energy between catechin and epicatechin was also considered in a range of polar protic, polar aprotic, apolar protic, and apolar aprotic solvents using the solvation model based on density (SMD) and integral equation formalism polarizable continuum model (IEFPCM). The IEFPCM yielded a modest range in isomerization energies depending on solvent polarity or proticity, whereas a substantial variation was observed with the SMD. The SMD results suggest that the solvation environment around catechin and epicatechin will play a major role on the photoisomerization equilibrium between these two compounds. As the freely dissolved monomer in aquatic systems, the catechin–epicatechin photoisomerization equilibrium will be in the range of 11:1 to 14:1. In the less polar environments of associations with dissolved organic matter or within a larger tannin structural framework, the theoretical modeling efforts indicate that the catechin–epicatechin photoisomerization equilibrium could be as low as 3:1.

Consecutive Pictet–Spengler Condensations toward Bioactive 8‐Benzylprotoberberines: Highly Selective Total Syntheses of (+)‐Javaberine A, (+)‐Javaberine B, and (–)‐Latifolian A

Enantiopure 8‐benzylprotoberberines were synthesized by two consecutive Pictet–Spengler (PS) condensations with protected 3,4‐dihydroxyphenylacetaldehydes. The first PS to (+)‐(R)‐norprotosinomenine was optimized to 90 % ee with 5 mol‐% of (R)‐TRIP as chiral Brønsted acid (> 99 % ee after trituration). The second PS did not require any catalyst, and its regioselectivity was strongly dependent on the solvent: 99:1 para selectivity was obtained in trifluoroethanol leading to (+)‐javaberine A; 81:19 ortho selectivity was reached in apolar aprotic solvents for the synthesis of (+)‐javaberine B. Complete, natural diastereoselectivity was observed in the second PS. Through selective catechol oxidation the spirocyclic alkaloid (–)‐latifolian A was prepared from protected (+)‐javaberine A.

Proton transfer rate‐equilibria in apolar aprotic solvents: a historical perspective

Low dielectric constant apolar aprotic solvents, although employed on a limited scale for studying proton transfer reactions as compared with commonly used polar protic or dipolar aprotic ones, offer some particular advantages, namely, specific solute–solvent interactions are virtually eliminated and proton transfer occurs directly in an apolar aprotic solvent. An intriguing feature of these reactions is their general acid‐catalyzed/base‐catalyzed kinetics with a time scale over microseconds to minutes. In fact, the true or intrinsic relative strengths of acids/bases when measured in such solvents come to the fore much more clearly than those obtained in other classes of solvents. Recently, a review documenting the post‐1980 developments relating to proton transfer reactions in apolar aprotic solvents has been published. The present article is a commentary of the pre‐1980 developments in this area since the 1920s Brønsted–Lowry's “proton cult” of acid–base theory. Copyright © 2016 John Wiley & Sons, Ltd.

ChemInform Abstract: Proton Transfer Reactions in Apolar Aprotic Solvents

AbstractReview: 123 refs.

Singlet–triplet excitation energies of substituted phenyl cations: a G4(MP2) and G4 theoretical study

Singlet–triplet adiabatic excitation energies (AES−T) of the parent and variously substituted phenyl cations, as well as the parent benzannelated derivatives up to anthracenyl, were calculated at the G4(MP2) and G4 levels of theory. The G4(MP2)/G4 AES−T estimates range up to 40 kJ/mol higher than prior density functional theory (DFT)-based predictions for these cations and suggest that AES−T and ground state multiplicity structure–property trends for phenyl cations previously proposed in the literature need to be re-assessed at higher levels of theory. In general, Hartree–Fock, DFT, and semiempirical methods do a poor job describing the singlet–triplet excitation energetics of these systems. Only modest effects of different solvation models (SMD, IEF-PCM, and C-PCM) and different polar protic through apolar aprotic solvents are evident on the calculated AES−T of the phenyl cation. Electron-donating substituents on the phenyl cation substantially lower the AES−T to an extent where some functional groups (–NH2, N(CH3)2, OCH3, and SCH3) can result in triplet ground states depending on their position relative to the cation. In contrast to the phenyl and 1- and 2-naphthyl cations, which are predicted to be ground state singlets, the three parent anthracenyl cations will be ground state triplets.

Proton transfer reactions in apolar aprotic solvents

Proton transfer reactions are advantageously investigated in low‐dielectric‐constant apolar aprotic solvents where specific solute–solvent interactions are greatly minimized, if not eliminated, and proton transfer occurs directly. An intriguing feature of these reactions is their general acid/base‐catalyzed kinetics with a timescale over microseconds to minutes. Proton‐coupled electron transfer (PCET), a great promise in the development of renewable energy sources, is an emerging application of the reactions. This article is an updated review of the post‐1980 developments in understanding the mechanism of proton transfer reactions, quantitative structure–reactivity relationships, acid/base‐catalyzed molecular rearrangements, reverse trends between acidity parameters and 13C δco (a measure of electron population at the carboxyl carbon) in aromatic carboxylic acids, coupling of proton transfer with electron transfer, and PCET reactions in suitably designed model systems in apolar aprotic solvents for renewable energy devices. Copyright © 2015 John Wiley & Sons, Ltd.

Density Functional Theory Study of the Oligomerization of Carboxylic Acids

We present a density functional theory [M06-2X/6-31+G(d,p)] study of the structures and free energies of formation of oligomers of four carboxylic acids (formic acid, acetic acid, tetrolic acid, and benzoic acid) in water, chloroform, and carbon tetrachloride. Solvation effects were treated using the SMD continuum solvation model. The low-lying energy structures of molecular complexes were located by adopting an efficient search procedure to probe the potential energy surfaces of the oligomers of carboxylic acids (CA)n (n = 2-6). The free energies of the isomers of (CA)n in solution were determined as the sum of the electronic energy, vibrational-rotational-translational gas-phase contribution, and solvation free energy. The assessment of the computational protocol adopted in this study with respect to the dimerization of acetic acid, (AA)2, and formic acid, (FA)2, located new isomers of (AA)2 and (FA)2 and gave dimerization constants in good agreement with the experimental values. The calculation of the self-association of acetic acid, tetrolic acid, and benzoic acid shows the following: (i) Classic carboxylic dimers are the most stable isomer of (CA)2 in both the gas phase and solution. (ii) Trimers of carboxylic acid are stable in apolar aprotic solvents. (iii) Molecular clusters consisting of two interacting classic carboxylic dimers (CA)4,(D+D) are the most stable type of tetramers, but their formation from the self-association of classic carboxylic dimers is highly unfavorable. (iv) For acetic acid and tetrolic acid the reactions (CA)2 + 2CA → (CA)4,(D+D) and (CA)3 + CA → (CA)4,(D+D) are exoergonic, but these aggregation pathways go through unstable clusters that could hinder the formation of tetrameric species. (v) For tetrolic acid the prenucleation species that are more likely to form in solution are dimeric and trimeric structures that have encoded structural motifs resembling the α and β solid forms of tetrolic acid. (vi) Stable tetramers of benzoic acid could form in carbon tetrachloride from the aggregation of trimers and monomers. (vii) Higher order clusters such as acetic acid pentamers and tetrolic acid hexamers are highly unstable in all solvation environments.

Solubility and Dissolution Thermodynamic Data of Mefenamic Acid Crystals in Different Classes of Organic Solvents

Different classes of solvents provide different polarity values, which influence the solubility of pharmaceutical solids. In this article, the solubility of mefenamic acid in different classes of organic solvents, including polar protic, dipolar aprotic, and apolar aprotics at a range of temperatures from (298 to 323) K are reported. It has been found that mefenamic acid shows high solubility in dipolar aprotic solvents (N,N-dimethylacetamide, N,N-dimethylformamide, ethyl acetate, and propanone), moderate solubility in polar protic solvents (ethanol and propan-2-ol), and poor solubility in apolar aprotic solvents (hexane, heptane, and cyclohexane) and water.

Hydroamination Reaction Catalyzed by Acid Activated Bentonite Under Atmospheric Pressure

Hydrochloric acid activated bentonite was used as a catalyst for hydroamination of cyclohexene with p-toluenesulfonamide under atmospheric pressure. The structure and surface acid property of the catalyst were studied by XRD and NH3-TPD. The effects of acid treatment condition and solvent on the yield of N-cyclohexyl-p-toluenesulfonamide were specifically studied. It was found that apolar aprotic solvents were the suitable solvents for this reaction. The bentonite activated by 2 mol/L hydrochloric acid showed the highest activity and the yield of N-cyclohexyl-p-toluenesulfonamide reached 83.7% after 2 hours reflux using toluene as the solvent. The different substituted location and different substituted group on benzene ring of the anilines have important effects on the yields of products and the presence of the electron-withdrawing substituent is in favor of the reaction. Keywords: Bentonite, hydroamination, p-toluenesulfonamide, cyclohexene, aniline

Solvent dependency of the yield of an N-monosubstituted diaziridine: a GC–MS study

Due to the growing importance of diaziridines in the drug design and the increasing popularity of diazirines as carbene precursors, the solvent influence on the yield of an N-monosubstituted diaziridine has been studied. It was observed that data points within every solvent class are clustering and the yield in general is the highest in apolar aprotic solvents.

Active Conformation in Amine-Thiourea Bifunctional Organocatalysis Preformed by Catalyst Aggregation

Self-activation: Takemoto's catalyst gains access to its active conformation by equilibrating between its hydrogen-bonded intra- and intermolecular interactions in apolar aprotic solvents. By destabilization of the inactive monomeric conformations, the extended anti-anti thiourea conformation is preformed in the assembly. On leaving the assembly, this transient conformation has a structural preference to become a catalytically active monomeric species that has the potency for dual activation (see scheme).

Influence of Aprotic Solvents on the Transmission of Anomalous Substituent Effects on 13C NMR Chemical Shifts at the Carboxyl Carbon (δco) in Meta-Substituted Benzoic Acids: A Strong Evidence for π-Polarization Mechanism

Anomalous (reverse) substituent-induced 13C nuclear magnetic resonance chemical shifts at the carboxyl carbon (δco) in meta-substituted benzoic acids have been studied for 11 substituents having varying electronic effects in 4 aprotic (nonhydroxylic) solvents of varying polarity by employing different dual substituent parameter models. The regression results for apolar aprotic solvents provide a strong evidence for through space π-polarization mode of transmission of reverse meta-substituent effects on the carboxyl carbon in benzoic acids. The results for dipolar aprotic solvents indicate significant specific solvation of π-polarized forms of the acids. The study showed further that an apolar aprotic solvent has a distinct preference over a dipolar aprotic one for investigating intrinsic substituent effects on chemical shifts in aromatic molecules.

Solvent sensitivity of ortho substituent effect on 13C NMR chemical shift of the carboxyl carbon (δco) in benzoic acid

A reverse ortho effect is observed for the (13)C NMR chemical shifts of the carboxyl carbon (δ(co)) in benzoic acids measured in aprotic solvents of varying polarity. The ortho effect on δ(co) is best described by a combination of the reverse field and steric accelerating effects of the substituent in an 80: 20 pattern in apolar aprotic solvents and a 60: 40 pattern in dipolar aprotic ones. Interestingly, no good enough correlation was found between δ(co) and log k(1) of the acids measured in similar solvents. A critical analysis of the results clearly indicates the use of an apolar aprotic solvent and not a dipolar aprotic one as the solvent of choice for investigating intrinsic substituent effects on δ(c) in an aromatic system.

Kinetics of Proton Transfer between Ortho Substituted Benzoic Acids and the Carbinol Base of Crystal Violet in Toluene. Ortho Effect on the Reactivity of Benzoic Acids in Apolar Aprotic Solvents

Apolar aprotic solvents are particularly advantageous for investigating the intrinsic ortho effect free from complications of specific solvent effects. A kinetic study for toluene-phase proton transfers between ortho F, Cl, Br, I, OMe, OEt, OPh, OAc, Me, NO(2), COMe, COPh, OH, NH(2), and H benzoic acids and crystal violet carbinol base has shown the forward rate constant (log k(+1)) is the most appropriate reactivity parameter in toluene. log k(+1) (toluene) as compared to other reported reactivity parameters in benzene, toluene, or chlorobenzene has been found more sensitive to the ortho substituent effect. The regression results of the correlation of log k(+1) (toluene) of the acids (except OH and NH(2) substituted ones) according to seven ortho effect models are all very significant, and the best result is given by Fujita-Nishioka's model. The overall analysis reveals that a substituent's ortho effect pattern is a 58:24:18 ratio of its ordinary electrical, proximity electrical, and steric effects and that the proximity electrical effect is the major component to account for the peculiarity of the substituent's ortho effect. The results further favor the transmission of this effect mainly through the molecular cavity. The effect may, however, be outweighed by the steric component for bulky enough substituents, e.g., Me. The enhanced strength exhibited by salicylic acid in toluene has been quantitatively described using Pytela-Liška's σ(HB)(i) parameter. The abnormally high log k(+1) observed for anthranilic acid in toluene has been ascribed to a very extensive homoconjugation in its acid-acid anion complex induced by the acid's three hydrogen bond donors.

Non-specific solvent effects on proton transfer equilibrium between toluic acids and carbinol base of crystal violet

Acid-base reactions in apolar aprotic media provide a sufficiently close measure of intrinsic acid/base strengths. A study on reaction equilibria between o- and m-methyl benzoic (toluic) acids with the carbinol base of crystal violet in five apolar aprotic solvents of similar Kirkwood polarity function (e - 1)/(2e + 1) and significantly different Dimroth-Reichardt's E T (30) and Kamlet-Taft's polarity-polarisability π * parameter indicates strongly the overall non-specific solvent effects are contributed by both electrostatic and dispersion interactions, the latter having a more dominant role therein.

Enantioselective Decarboxylative Protonation

A base-mediated enantioselective decarboxylative protonation of acyclic, cyclic, and bicyclic α-aminomalonate hemiesters is described. Presumably, the tertiary amine moiety of the thiourea-containing cinchona alkaloid acts as a base, promoting the formation of an enolate via decarboxylation. While binding the enolate through urea-mediated hydrogen bonds, the chiral catalyst may enantioselectively protonate this intermediate. Enantioenriched aminoesters were obtained in high yields and enantioselectivities. Both enan­tiomeric forms could be produced by using either base A or pseudoenantiomeric base B. Interestingly, a wide range of polar as well as apolar aprotic solvents afforded high selectivities.