Dipolar aprotic solvent cation solvation

Reactions of this charge type appear to be rather insensitive to solvent polarity and to protic-dipolar aprotic solvent effects. Solvation of the trimethylsulphonium cation is expected to decrease quite strongly in... 

Lil to LiCl. Parker has shown that if the ability of dipolar aprotic solvents to solvate cations varies in the order DMSO = DMA > DMF > AC = TMSO2 > AN = NM > benzonitrile = NB, many properties of electrolytes in these solvents can be explained. This ordering coincides with that found by Drago and co-workers for the electron donating power of dipolar aprotic solvents. It is interesting to note that the anomalies in conductance behaviour have been encountered most frequently in the solvents having lower cation solvating power. 

Ji et al. (2010,2012) discuss a number of types of reactions of organic substances in liquid ammonia, and provide many references to recent and earlier work. They compare the equilibrium constants of phenols and carbonyl-activated carbon adds in ammonia and water, and describe work that is part of an ongoing study of the kinetics of a variety of reactions, including aromatic substitutions and solvolyses in ammonia. They point out that owing to its weakness as an add and as a hydrogen-bond donor, in many respects ammonia behaves as a dipolar aprotic solvent. It solvates cations strongly, but anions hardly at all (Marcus, 1983,1985). The low value of the autoprotolysis constant ( 10 at -33°C) is chiefly due to the weakness of ammonia as an acid. The mobility of the NH ion in liquid ammonia is not anomalous (Lagowski,... 

Water has high permittivity and moderate acidity and basicity. Thus, in water, many cations and anions are easily solvated (hydrated) and many electrolytes are highly soluble and dissociate into ions. Water has fairly wide pH and potential ranges and a convenient liquid temperature range. Of course, water is an excellent solvent. However, as in Table 1.7, the reaction environment can be expanded much wider than in water by use of a solvent of weak acidity and/or basicity. This is the reason why dipolar aprotic solvents, which are either protophilic or protophobic, are used in a variety of ways in modern chemistry. 

Most of the so-called dipolar aprotic solvents have appreciable Lewis base character with donor numbers greater than 10 and autoprotolysis constants smaller than 10 20 (pK > 20).20 They solvate cations better than anions, and radical anions often have appreciable stability in the rigorously purified solvents. 

Reaction between 2 -hydroxyacetophenone and benzaldehyde (Claisen-Schmidt condensation) in the absence of a solvent at 423 K giving 2 -hydroxy chalcones and flavanones has been successfully performed with MgO as a solid base catalyst/581 A conversion of 40 % after 1 h with 67 % selectivity to chalcone was achieved. The influence of the solvent and the effects of a substituent on the aromatic ring were investigated by Amiridis et a//59,6"1 The reaction was carried out on MgO at 433 K. Dimethyl sulfoxide (DMSO) showed a strong promoting effect on the reaction, which was attributed to the ability of this dipolar aprotic solvent to weakly solvate anions and stabilize cations so that both become available for reaction. In this case, a conversion of 2-hydroxyacetophenone of 47 % with a selectivity to flavanone of 78 % was achieved after 30 min in a batch reactor. Further investigations1611 showed that DMSO significantly increases the rate of the subsequent isomerization of the 2 -hydroxychalcone intermediate to flavanone. 

An important lesson from this is that the idea of nucleophilicity in the real world of organic reactions is not easy to pigeonhole. Polarizability is important, but basicity is also very important and can be influenced by solvation. Values of the pKa of a given compound vary as a function of solvent, and so does basicity. You can make a species, anions in particular, more reactive by putting them in solvents that don t solvate them very well. Dipolar aprotic solvents interact nicely with cations, but not so well with anions. Polar protic solvents (e. g., water, alcohols) can hydrogen bond to anions, diminishing their basicity and literally blocking them sterically. 

The observation that protic solvents are far better anion solvators than dipolar aprotic solvents, and that the reverse is true for cation solvation, has led to extremely valuable rules for the selection of solvents for specific reactions [73, 92-97]. 

A further instructive study on the influence of protic and dipolar aprotic solvents on the rate of SnI heterolysis of tertiary R3C-X (with X = Cl, Br, I, 2,4-dinitrophenolate) shows that the rate-accelerating anion solvation due to H-bonding by protic solvents decreases dramatically on increasing the radius of the halide ions. Therefore, the differential solvation transferred from anion-solvating methanol to cation-solvating dimethyl sulfoxide as solvent is reversed on going from the chloro- to the iodoalkane at 60 °C /ji(DMSO)/ki(MeOH) = 0.05 (t-BuCl) < 0.57 (t-BuBr) < 6.9 (t-BuI) the 2,4-dinitrophenolate ion behaves like the iodide [830]. 

It should be mentioned briefly that solvation phenomena should also influence the outcome in the case of ambident nucleophiles, at least to the extent to which these reagents are sensitive to solvent effects. With an ambident anion, which is not manipulated by countercations (formation of ion pairs), the more electronegative center should attack preferentially. The more this area is blocked by hydrogen bridges formed in protic solvents, or shielded by countercations, the more likely it is that the less electronegative end will react. If dipolar aprotic solvents are used, which can only solvate the cations, a preferential attack of the nonshielded more electronegative center is to be expected. It must be realized, however, that in substitution reactions employing cyanide ions, dipolar aprotic solvents have not been reported to enhance the formation of isonitriles. " ... 

Particularly good results have been reported (Table 1) with dipolar aprotic solvents like DMSO, DMF, formamide or HMPA, which exclusively solvate cations and thus generate highly reactive naked cyanide anions. As a consequence a remarkable increase in reaction rates is observed. The most... 

In the particular case of sodamide containing complex bases where the base to be activated is insoluble, alcoholates can complex sodamide with help of two ionic sites (ROs and Nas+). This must be more favorable for dissolution of NaNH2 than simple solvation of the cation by dipolar aprotic solvent (solvation of anions being low). However, in solution because of the double complexation the basic power of complex bases must be lower than the basic power of NaNH2 in a dipolar aprotic solvent if this latter was able to dissolve it substantially. 

Polymerization of epoxides occurs readily under the influence of strong bases in both protic and aprotic solvents, propagation involving stepwise growth of alkoxide ions. Dimethyl sulfoxide (DMSO) is the most useful of the dipolar aprotic solvents and shows a marked ability to solvate cations (especially K ) whilst leaving anions essentially unsolvated. As a consequence nucleophilic reactivity of anions is greater in solvents such as DMSO. 

Anions from the Schiffs base (78) can be C- or A -alkylated with ethyl iodide or diethyl sulphate. The ratio of the products depends both on the solvent and on the presence of 18-crown-6. In non-polar solvents, the crown ether increases the solubility of the base, and C-alkylation is the major pathway. In dipolar aprotic solvents, the 18-crown-6 breaks up ion pairs by solvation of the Na" cation, and favours A -alkylation. A nerylsulphonamide, formed from (79), undergoes regiospecific reductive desulphonylation to give nerol (80), which implies that (79) is an effective synthon for cisoid iso-prenoids. Chiral complexes of crown ethers, e.g. (81), catalyse the Michael addition reaction of j3-keto-esters and methyl vinyl ketone to give adducts in high optical yields. ... 

A dissection of the influence of solvent and base should be possible from studies in dipolar aprotic solvents containing various bases in which the two reactants can be varied independently. Although bases whose conjugate acids have pATa values of less than 11 rarely induce elimination in protic media , halide ions have been used successfully in this connection in dipolar aprotic solvents . Of course in the latter solvents, which lack exchangeable hydrogens but possess atoms with lone pairs capable of solvating cations, pATa values may differ appreciably from those already derived in protic sol-vents . There is obviously ample scope for future research in this field. 

The success of the Born treatment, in accounting for cation solvation in water and dipolar aprotic solvents, when modified with a field-dependent dielectric constant, but its failure for anion solvation because of hydrogen bonding, suggests that the Born equation with allowance for dielectric saturation, should account for anion solvation in dipolar aprotic media where the hydrogen-bonding complication is absent. 

Because of the high anion activity in dipolar aprotic solvents, there seems to be great variety in the stoichiometry of ion association aggregates, especially when highly charged cations are involved. Here even the aggregates carry substantial positive charge and are thus well solvated by these solvents. Lo in a recent study of the anation reaction ... 

Closely related to supported PTC are the so-called polymer-supported dipolar aprotic solvents [99]. These are polymers carrying strongly dipolar species which are able to solvate alkali metal cations in hydrocarbon solvents and hence function like solvents. This then activates the attendant anion so the effect overall is very similar to that of PTC. Bound phosphoramides were reported some time ago [103] and more recently bound formamides were shown to be effective [104]. Perhaps somewhat surprisingly phosphine oxide groups have now also proved to form the basis of useful polymer cosolvents [105]. This could prove to be a significant advance because the triphenylphosphine oxide residue is particularly stable and the polymeric cosolvent has been readily re-used with good retention of activity. 

All these facts are interconnected in the sense that both the size of the cation and the cation-solvating power of dipolar aprotic solvents have the effect of disrupting ion pairs and hence rendering the thiolate ion more... 

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