Salts carbanion

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 ... 

A few observations of the isolation of carbocation-carbanion salt containing heteroatoms, such as oxygen and nitrogen, have been reported. Over 30 years ago LeGoff and LaCount (1963) reported the formation of a... 

Alkenes frequently have two kinds of C—H bonds, vinyl and allyl, that are generally more acidic than the C—H bonds of saturated alkanes. Quantitative measures of acidity are related to the chemistry of the corresponding carbanions and carbanion salts or organometallic compounds. Several methods have been used for the study of anions in the gas phase1. For many acids it is possible to measure equilibrium constants for equilibria of the type in equation 1. From such equilibrium constants with compounds RH of independently known gas-phase acidity, it has been possible to determine the acidities of a wide range of compounds2. 

A common technique for measuring the values has been to employ species that produce anions with useful ultraviolet (UV) or visible (vis) absorbances and then determine the concentrations of these species spectropho tome trie ally. Alternatively, NMR measurements could be employed, but generally they require higher concentrations than the spectrophotometric methods. A hidden assumption in Eq. 5 is that the carbanion is fully dissociated in solution to give a free anion. Of course, most simple salts do fully dissociate in aqueous solution, but this is not necessarily true in the less polar solvents that are typical employed with carbanion salts. For example, dissociation is commonly observed for potassium salts of carbanions in DMSO because the solvent has an exceptionally large dielectric constant (s = 46.7) and solvates cations very well, whereas dissociation occurs to a small extent in common solvents such as DME and THE (dielectric constants of 7.2 and 7.6, respectively). In these situations, the counterion, M+, plays a role in the measurements because it is the relative stability of the ion pairs that determines the position of the equilibrium constant (Eq. 6). 

Acid is then added to neutralize the carbanion salt. 

Fluorenyl Carbanions. Salts of fiuorene (pAa = 22.6) (6) are more hindered and less reactive than many other organometallic initiators. These carbanions can be readily formed by reaction with alkali metal derivatives as shown in equation 19 for 9-methylfiuorene (99). Carbanion salts of 9-methylfiuorene are preferable to fiuorene, since the latter generate chain ends which retain reactive, acidic fluorenyl hydrogens which can participate in chain-transfer reactions (100,101). Fluorenyl salts are useful initiators for the polymerization of alkyl methacrylates, epoxide, and thiirane monomers. 

The solvent effects on the absorption spectra of ion pairs were studied by many authors and the direction of the observed shift depends on the change (increase or decrease) of dipole moment upon the electronic transition [25]. Generally a bathochromic shift is observed with an increase of solvent polarity. When going from a polar solvent to a less polar one, the association in the ground state increases more strongly than in the excited state this may be understood if the ion pair switches progressively from SSIP to CIP status. Observations of this type were often made, together with cation effects, as for instance in the case of alkali phenolates and enolates [7], fluorenyl and other carbanion salts [22] or even for aromatic radical anions [26, 27],... 

Synthesis of optically active alcohols. For asymmetric synthesis of alcohols, the carbanion salt (2) is generated from optically active (R)-melhyl p-tolyl sulfoxide (1). The optically active carbanion reacts with bcnzaldehyde (3) to give a I I diastereomeric mixture of 2-hydroxy-2-phenylethyl p-tolyl sulfoxides (4a) and (4b) in 84% yield-These can be separated by silica gel chromatography and fractional crystallizations to give (4a, oq, -t- 91.7°. 17% yield) and (4b, at, + 202.8", 15.5% yield). Raney nickel desulfurization of (4a) and (4b) gives (S)-( )-l-phcnylcthanol (5a, — 42.6 ) and... 

Reetz, M.T., Knauf, T., Minet, U., and Bingel, C. (1988) Metal-ftee carbanion salts as initiators for the anionic polymerization of acrylic and methacryUc acid esters. Angewandte Chemie, International Edition in English, 27,1373-1374. 

Scholz R, Hellmann G, Rohs S, Ozdemir D, Raabe G, Ver-meeren C, Gais H-J. Enantioselective synthesis, configurational stability, and reactivity of lithium a-tert-butylsulfonyl carbanion salts. Eur J. Org. Chem. 2010 (24) 4588 616. 

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