Carbanions solvation

Besides resonance stabilization of the carbanion, solvation has a major effect on k0 of proton-transfer reactions. In a first approximation, we only consider the solvation of ionic species. For the solvation of the carbanion, equation 5 takes on the form... 

Solvent for Displacement Reactions. As the most polar of the common aprotic solvents, DMSO is a favored solvent for displacement reactions because of its high dielectric constant and because anions are less solvated in it (87). Rates for these reactions are sometimes a thousand times faster in DMSO than in alcohols. Suitable nucleophiles include acetyUde ion, alkoxide ion, hydroxide ion, azide ion, carbanions, carboxylate ions, cyanide ion, hahde ions, mercaptide ions, phenoxide ions, nitrite ions, and thiocyanate ions (31). Rates of displacement by amides or amines are also greater in DMSO than in alcohol or aqueous solutions. Dimethyl sulfoxide is used as the reaction solvent in the manufacture of high performance, polyaryl ether polymers by reaction of bis(4,4 -chlorophenyl) sulfone with the disodium salts of dihydroxyphenols, eg, bisphenol A or 4,4 -sulfonylbisphenol (88). These and related reactions are made more economical by efficient recycling of DMSO (89). Nucleophilic displacement of activated aromatic nitro groups with aryloxy anion in DMSO is a versatile and useful reaction for the synthesis of aromatic ethers and polyethers (90). 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

Table 11 summarizes the relative conformation stabilities of various sulfmyl carbanions, based on the H/D exchange rates of the corresponding sulfmyl compounds 36-39. The results are in good agreement with the order of stabilities obtained from the MO calculations using the 3-21G basis set. This is remarkable, since the calculation did not take into consideration the solvent effect, despite the strong unsymmetrical solvation on the a-sulfmyl carbanion. 

The proposed intramolecular solvation is not the only feature differentiating between the polystyryl and polymethyl methacrylate salts. The former are classified as the salts of carbanions, whereas the latter are ambident salts having the character of allylic enolates with the cation interacting with the partially negatively charged carbon and oxygen atoms. The degree to which the one or the other interaction is favored is affected by the size of the cation. 

It is unlikely that free carbanions exist in solution. Like carbocations, they usually exist as either ion pairs or they are solvated. " Among experiments that demonstrated this was the treatment of PhCOCHMe with ethyl iodide, where was Li ", Na", or K" . The half-lives of the reaction were for Li, 31 x 10 Na, 0.39 X 10 and K, 0.0045 x 10 , demonstrating that the species involved were not identical. Similar results were obtained with Li, Na, and Cs triphenylmethides (PhsC M Where ion pairs are unimportant, carbanions are solvated. Cram " demonstrated solvation of carbanions in many solvents. There may be a difference in the structure of a carbanion depending on whether it is free (e.g., in the gas phase) or in solution. The negative charge may be more localized in solution in order to maximize the electrostatic attraction to the counterion. ... 

Ellison, G.B. Engelking, PC. Lineberger, W.C. J. Am. Chem. Soc., 1978, 100, 2556. Retention of configuration has never been observed with simple carbanions. Cram has obtained retention with carbanions stabilized by resonance. However, these carbanions are known to be planar or nearly planar, and retention was caused by asymmetric solvation of the planar carbanions (see p. 764). 

The SnI reactions do not proceed at bridgehead carbons in [2.2.1] bicyclic systems (p. 397) because planar carbocations cannot form at these carbons. However, carbanions not stabilized by resonance are probably not planar SeI reactions should readily occur with this type of substrate. This is the case. Indeed, the question of carbanion stracture is intimately tied into the problem of the stereochemistry of the SeI reaction. If a carbanion is planar, racemization should occur. If it is pyramidal and can hold its structure, the result should be retention of configuration. On the other hand, even a pyramidal carbanion will give racemization if it cannot hold its structure, that is, if there is pyramidal inversion as with amines (p. 129). Unfortunately, the only carbanions that can be studied easily are those stabilized by resonance, which makes them planar, as expected (p. 233). For simple alkyl carbanions, the main approach to determining structure has been to study the stereochemistry of SeI reactions rather than the other way around. What is found is almost always racemization. Whether this is caused by planar carbanions or by oscillating pyramidal carbanions is not known. In either case, racemization occurs whenever a carbanion is completely free or is symmetrically solvated. 

The oxidation potential of carbanions, ox> or the reduction potential of carbocations, red> could be a practical scale of stability as defined by (3). These potentials can be measured by voltammetry, although the scale is subject to assumptions regarding elimination of the diffusional potential and solvation effects. 

Winstein Robinson (1958) used this concept to account for the kinetics of the salt effects on solvolysis reactions. They considered that carbonium ions (cations) and carbanions could exist as contact ion-pairs, solvated ion-pairs and as free ions and that all these forms participated in the reactions and were in equilibrium with each other. These equilibria can be represented, thus ... 

In this respect such reactions are analogous to the S 1 or limiting reactions of compounds producing carbonium ions, although the intermediate is a solvated carbanion rather than a solvated carbonium ion. In the base-catalyzed halogenation of ketones, for example, the rate is independent of the halogen concentration and is the same for the reaction with bromine as for the reaction with chlorine.384... 

In the first case, SENAs in the presence of various catalysts (primarily salts containing the fluoride anion) generate the corresponding a-nitro carbanions, which are poorly solvated in aprotic solvents and, consequently, rapidly react with substrates R3— Y = X to give functionalized nitro compounds through the transition state A. 

The alkali metals in liquid ammonia give deep coloured solutions which have been shown to contain solvated electrons. The unsaturated system takes up an electron to give an anion radical. There is evidence for this species from electron spin resonance studies. It accepts a proton from the solvent to give a radical which is reduced to a carbanion by another sodium atom. Finally the addition of a proton gives the reduced product. This proton is supplied by a protic solvent like enthanol and not from NH3. 

In the light of the success of the Birch conditions for reducing organic compounds it is not surprising that epoxides can be opened by solvated electrons [6-9]. The initially formed radical is then further reduced to give carbanionic species, which do not display the reactivity of radicals. This concept has been extended by Bartmann [10], Cohen et al. [11], Conrow [12], and Yus et al. [13,14] who employed aromatic radical anions as the reduc-... 

Nucleophilic addition of phenolic nucleophiles to l,l-dicyano-2-arylethenes in the gas phase and in water has been studied theoretically" using the semiempirical AMI method and the Cramer-Truhlar solvation model SM2.1. The difference between the Brpnsted coefficients (a" = 0.81 and P" =0.65) determined for the gas-phase reaction is indicative of a small positive transition state imbalance of / = 0.16. For reaction in water the estimates (a" = 0.61 and P" = 0.36, giving I = 0.25) are close to the experimental values (a" = 0.55 and P" = 0.35) obtained with amine bases, and the small imbalance is as expected for a reaction involving no hybridization change at the incipient carbanion site. 

Reduction of benzenoid hydrocarbons with solvated electrons generated by the solution of an alkali metal in liquid ammonia, the Birch reaction [34], involves homogeneous electron addition to the lowest unoccupied 7t-molecular orbital. Protonation of the radical-anion leads to a radical intermediate, which accepts a further electron. Protonation of the delocalised carbanion then occurs at the point of highest charge density and a non-conjugated cyclohexadiene 6 is formed by reduction of the benzene ring. An alcohol is usually added to the reaction mixture and acts as a proton source. The non-conjugated cyclohexadiene is stable in the presence of... 

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