Ion-pairing carbanions

There is evidence that some ElcB mechanisms can involve carbanion ion pairs,... 

Most of the reported secondary /3-deuterium KIEs attributed to negative ion hyperconjugation are for anion or carbanion ion-pair-forming hydron... 

Bertrand and coworkers have stndied the complexity of magnesinm enolates in solntion. The NMR and IR of the prodncts generated from alkyl- and dialkylmagnesinm reagents with enolizable ketones indicate that enolates as well as solvent-separated carbonyl conjn-gated carbanion ion pairs are formed. The analysis of the reaction mixture is complicated by the occurrence of ketone rednction . 

The ion pair status is not predictable only on the basis of the cation used. For highly delocalizable carbanions like 9-fluorenyl in THF, the CIP fraction was found to increase rapidly in the order Li < Na < K < Cs [22] and the caesium salt was found to be the most bathochromic in the series. The degree of association of carbanion ion pairs in non polar solvents is highly dependent upon the extent of charge delocalization [23]. With highly delocalizable anions SSIP will be formed with Li cation. A completely different sequence of ion pairs was shown in the case of naphtholate anions the CIP fraction increases in the order K < Na < Li [24], These anions are thus to be considered as more localized anions in order to agree with the preceding observations. 

Other illustrative examples of carbanionic ion-pair dissociation/aggregation are lithium triphenyhnethide, which exists as a tight ion pair in diethyl ether and as a solvent-separated ion pair in tetrahydrofuran, as shown by UV/Vis spectrophotometric measurements [287], and lithium 10-phenylnonafulvene-10-oxide, which exists as a tight ion pair (2b) in tetrahydrofuran solution and as a solvent-separated ion pair (3b) when hexamethylphosphoric triamide or dimethyl sulfoxide are added ( H and NMR measurements) [288]. 

Rotation around a carbon-carbon bond in the rj -allylmetals requires conversion to the r> -isomer. Compared to the free car-banion, this process is facilitated in the organometallics by the greater electrostatic attraction of the cation to the localized carbanion ion pair. Accordingly, the calculated rotation barriers decrease along the series Cs > Rb > K > Na as found by experiment.The increased electrostatic attraction of the smaller cations facilitates formation of the -isomer. The calculated barrier for lithium is higher than experiment and is attributed to dimerization in solution. ... 

Studies have shown that, in marked contrast to carbanionic polymerisation, the reactivity of the free oxonium ion is of the same order of magnitude as that of its ion pair with the counterion (6). On the other hand, in the case of those counterions that can undergo an equiUbrium with the corresponding covalent ester species, the reactivity of the ionic species is so much greater than that of the ester that chain growth by external attack of monomer on covalent ester makes a negligible contribution to the polymerisation process. The relative concentration of the two species depends on the dielectric constant of the polymerisation medium, ie, on the choice of solvent. 

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... 

The kinetic method of determining relative acidity suffers from one serious complication, however. This complication has to do with the fate of the ion pair that is formed immediately on removal of the proton. If the ion pair separates and difiuses into the solution rapidly, so that each deprotonation results in exchange, the exchange rate is an accurate measure of the rate of deprotonation. Under many conditions of solvent and base, however, an ion pair may return to reactants at a rate exceeding protonation of the carbanion by the solvent. This phenomenon is called internal return ... 

The pA of 1,3-dithiane is 36.5 (Cs" ion pair in THF). The value for 2-phenyl-1,3-dithiane is 30.5. There are several factors which can contribute to the anion-stabilizing effect of sulfur substituents. Bond dipole effects contribute but carmot be the dominant factor because oxygen substituents do not have a comparable stabilizing effect. Polarizability of sulfur can also stabilize the carbanion. Delocalization can be described as involving 3d orbitals on sulfur or hyperconjugation with the a orbital of the C—S bond. MO calculations favor the latter interpretation. An experimental study of the rates of deprotonation of phenylthionitromethane indicates that sulfur polarizability is a major factor. Whatever the structural basis is, there is no question that thio substituents enhance... 

The solid-state structures of several benzylic carbanion salts have been elucidated by X-ray analysis9 depending on the nature of the benzylic part, the cation, and the additives, the structures range from er-bonded organometallic compounds to delocalized ion pairs, from monomeric to dimeric and polymeric aggregates. Some compounds are listed together with leading references ... 

In reactions in which separated ion pairs are involved, e.g., R4N+, K or Na +, and as a borderline case, Li +, the cation does not contribute to the adjustment of the reaction partners in a dense, well-ordered transition state poor selcctivities arc usually the result of these carbanionic carbonyl additions. Further, the high basicity of such carbanionic species may cause decomposition or racemization of sensitive reactions partners. 

Alkenyllithium derivatives, carrying carbanion-stabilizing substituents, which facilitate the formation of solvent-separated ion pairs, can also exhibit preparatively useful configurational stability in respect to the double bond of the precursor. 

The electron transfer reaction from fluorenyl carbanions adjacent to sulfoxide or sulfone (9) to fluorenone (FIO) has been studied by means of flash photolysis37. For n — 1 as well as n — 2 the transient F10T, M + (contact ion pair) appeared in THF and F10T(free ion)... 

Polymerization of t-butyl methacrylate initiated by lithium compounds in toluene yields 100% isotactic polymers 64,65), and significantly, of a nearly uniform molecular-weight, while the isotactic polymethyl methacrylate formed under these conditions has a bimodal distribution. Significantly, the propagation of the lithium pairs of the t-Bu ester carbanion, is faster in toluene than in THF. In hydrocarbon solvents the monomers seem to interact strongly with the Li+ cations in the transition state of the addition, while the conventional direct monomer interaction with carbanions, that requires partial dissociation of ion-pair in the transition state of propagation, governs the addition in ethereal solvents. 

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. ... 

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 ... 

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