Anion interaction dimethylformamide

Like calix[4]arenes, calix[4]pyrroles are versatile ligands to the extent that the composition of the anion receptor complex is solvent dependent. A representative example is that involving 8 and the fluoride anion. As shown in Fig. 4b, the well-defined change in curvature observed at 1 1 ligand fluoride mole ratio indicates that in acetonitrile one fluoride anion interacts per unit of receptor 8. However, in moving from acetonitrile to A/,A/-dimethylformamide, the noticeable changes in curvature observed at a ligand/anion mole ratio of 0.5 and 1, indicate respectively the formation of a 1 2 and 1 1 anion complexes, respectively, in this solvent. 

Similarly, the reactivity of phenolate ions as the tetra- -butylammonium salt has been shown to be 3 10" times higher than that of the corresponding potassium salt in the Sn2 alkylation reaction with 1-halobutanes, carried out in 1,4-dioxane [340], Whereas the rate of alkylation of the potassium salt increases by a factor of ca. 10 on going from 1,4-dioxane (fir = 2.2) to iV,iV-dimethylformamide (e, = 36.7), the alkylation rate of the quaternary phenolate is essentially insensitive to the same solvent change. Obviously, the phenolate ion combined with the larger ammonium ion is already very reactive because of the relatively weak cation-anion interaction in the ion pair. In such cases, dissociation to a truly free anion does not seem to be required in order to explain the high reactivity [340]. 

It should be noted that at least in some cases the radical anions of the simplest carbonyl compounds (benzaldehyde, acetophenone) are probably secondary products formed as a result of interaction between the initial radical anion and dimethylformamide according to the following scheme ... 

It is well known that anionic samples tend to adsorb on poly(styrene-divinylbenzene) resins. However, cationic samples tend to be repelled from the resins. The mechanism seems to be an ionic interaction, although the poly(styrene-divinylbenzene) resin should be neutral. The reason is not well clarified. Therefore, it is recommended to add some salt in the elution solvent when adsorption or repulsion is observed in the analyses of polar samples. For example, polysulfone can be analyzed successfully using dimethylformamide containing 10 mM lithium bromide as an elution solvent, as shown in Fig. 4.42. 

Under ordinary conditions, reduction of these imines in dimethylformamide is a two-electron process involving saturation of the carbon-nitrogen double bond [181] because the radical from protonation of the radical-anion is more easily reduced than the starting imine. Immonium salts with two or more phenyl substituents are reduced reversibly in acetonitrile to the radical-zwitterion such as 42. Other immo-niura salts, e.g. 43, are reduced irreversibly to the dimer [182]. Radical-zwitterion intermediates generated from immonium salts exhibit nucleophilic character on carbon. Intramolecular interaction between the reduced immonium function and a... 

Acetonitrile offers the optimal conditions for the complexation of 8-oca/J/ and the fluoride anion (equation (12)) in that both, reactants and product favourably contribute to complex formation in this solvent. Indeed, on the one hand, acetonitrile interacts weakly with the reactants while these undergo strong interaction with A/,A/-dimethylformamide to an extent that both, fluoride and receptor are reluctant to interact strongly between themselves. On the other hand, acetonitrile is a better solvating medium for the anion complex than A/,A/-dimethylformamide. As a result, the enthalpic stability for the complexation of fluoride and 8-aajSjS in acetonitrile is quite high in contrast with the relatively low ACH° value observed for this system in A/,A/-dimethylformamide. 

The nucleophilic reactivity of the lithium salts changes in the same order as in protic solvents (I > Br > Cl cf. Table 5-15). However, the order is completely reversed for the ammonium salts (Cl > Br > I ), and this latter order is the same as that found in dipolar non-HBD solvents such as A,A-dimethylformamide [278]. The small lithium cation, with its high charge density, has a strong tendency to form ion pairs with anions, whereas the electrostatic interaction between the large tetraalkylammonium ion and anions is comparatively weak. Quaternary ammonium salts, therefore, should be practically fully dissociated in acetone solution. Thus, the reactivity order obtained with these salts corresponds to that of the free, non-associated halide ions. On the other hand, the sequence obtained with the lithium salts also reflects the dissociation equilibria of these salts in acetone solution [279]. 

Reduction in dry aprotic solvents allows the formation of ion pairs between the radical anion and a countercation. Where the cation can interact simultaneously with two radical species, dimerization favors the ( )-product because there are fewer steric interactions, in the corresponding transition state, between groups attached to the reacting radicals. Solvent and supporting electrolyte combinations that give a high yield of ( )-pinacol are acetonitrile with tetraethylammonium bromide [82], dimethylformamide with sodium perchlorate [83], and dimethylformamide with europium(III) chloride. Europium(II) is formed in the last... 

The detailed studies of the surface of CdS nanocrystallites prepared in N,N-dimethylformamide (CdS-DMF) by means of emission measurements, in-situ Cd K-edge EXAFS analysis, and theoretical MO calculations reveal the correlation of the photocatalysis of CdS-DMF and the formation of sulfur vacancies on its surface. It has been experimentally proved that CO2 interacts with the sulfur vacancies and is converted into its radical anion under irradiation as an intermediate in the photocatalysis. The knowledge on the photocatalysis obtained above has led to the achievement of the photofixation of CO2 into benzophenone, acetophenone and benzyl halides under visible light irradiation in the presence of TEA as an electron donor. 

A comparison of anion solvation by methanol, a protic solvent, and dimethylformamide, a dipolar aprotic solvent, is instructive. The electrostatic contribution, d/i , to the Gibbs free energy of solvation per mole of an ion is sometimes estimated quite successfully (Stokes, 1964) from the Bom model, in which a charged sphere of radius r is transferred from vacuum to a medium of uniform dielectric constant, c. The Bom equation (17) suggests that an anion should be similarly solvated in methanol and in DMF, because these solvents have effectively the same dielectric constant (33-36). The Born equation makes no allowance for chemical interactions, such as hydrogen-bonding and mutual... 

Nucleophilic fluorination using [18F]fluoride ion is a typical displacement reaction that is generally performed in a polar aprotic solvent such as acetonitrile, dimethyl sulfoxide, or dimethylformamide [100], In polar aprotic solvents, the nucleophilicity of anions including [18F]fluoride is enhanced by the selective solvation of the cations. Conversely, solvation in protic solvents likely reduces an anion s nucleophilicity by interaction with the partial... 

The solvents that interact with anions and as a consequence favor carbocationic processes are called anion-stabilizing solvents. The anion-stabilizing ability makes, for example, methanol a better solvent for carbocationic solvolysis than acetonitrile or dimethylformamide, although the latter solvents are more polar (50a, 50b). 

Considering ion-solvent interactions other than hydrogen bonding, we expect ion-dipole forces to be stronger for cations, which are attracted by the exposed negative end of the CH3CN dipole, than for anions. Acetonitrile, to judge by its refractive index, is less polarisable than dimethylformamide, more nearly comparable with methanol. Dispersion interactions in acetonitrile will therefore be appreciable but not as prominent as in dimethylformamide. 

In formamide and dimethylformamide solutions the cation is primarily bound to the oxygen of the amide group [Pe 68], but in formamide there is also an interaction with the nitrogen atom of the amide. On the other hand, the hydrogen atoms of the amide group of formamide may form hydrogen bonds with the electronegative anions. 

The reduction of CO2 at metallic cathodes has been studied with almost every element in the periodic table °. This reaction can be driven electrochemi-cally or photochemically " and semiconductors have been used as cathodic materials in electrochemical or photoelectrochemical cells . The aim of these studies has been to find cathodes that discriminate against the reduction of H2O to H2 and favor the reduction of CO2 and also to find a cathode selective for one product in the reduction of CO2. A fundamental requirement is that the latter process occurs at a lower overpotential on such electrodes. However the purposes mentioned before in metallic cathodes depends on a series of factors such a solvent, support electrolyte, temperature, pressure, applied overpotential, current density, etc. (we will see the same factors again in macrocyclic electro-catalysis). For instance when protons are not readily available from the solvent (e.g., A,A -dimethylformamide), the electrochemical reduction involves three competing pathways-oxalate association through self-coupling of COj anion radicals, production of CO via O-C coupling between and COj and CO2, and formate generation by interaction of C02 with residual or added water. ... 

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