Carbanions halogen-stabilized

The effects of fluonnation on carbanion stability are largely deduced from C-H acidity data (p. 988) [64], a-Halogens stabilize carbanions in the order Br > Cl > F, which IS opposite the inductive electron-withdrawing order and reflects the... 

CHBr3 or iodoform, CHI3). Note that the second step of the reaction is a nucleophilic acyl substitution of CX3 by OH. That is, a halogen-stabilized carbanion acts as a leaving group. 

Carbanions bear many substituents that can affect their structure and reactivity and can influence the acidity of a parent C-H precursor. Halogens stabilize carbanions in the order Br > Cl > F. A prominent I—)i repulsion between the F and carbanionic center causes some destabilization in a-fluorinated carbanions. The magnitude of the destabilization depends on the carbanion structure. The destabilization is maximized as the carbanion structure approaches a planar configuration. Thus, fluorinated carbanions possess pyramidal structures with high barriers to inversion. 

The carbanion is stabilized by electron-mthdramng groups in the positions ortho and para to the halogen atom. If we examine the following resonance structures for a Meisenheimer intermediate, we can see how ... 

Carbanions are stabilized by inductive withdrawers of electron density such as halogens, but are destabilized by inductive donors such as alkyl groups. They are stabilized by resonance with i-electrons and aromatic character. 

Generally, the two-electron reduction of organic halides produces carbanion species. In fact, cathodic reduction of organic halides under certain conditions gives the product derived from the corresponding carbanion intermediates. Silicon is known to stabilize the carbanion at the a position by dn-pn interaction. Therefore, we can expect that silicon promotes the electron transfer from carbon-halogen bonds and the formation of the carbanion at the a position. 

The haloform reaction is a useful method of preparing a carboxylic acid (carboxylate ion) with one less carbon. It is one of the very few cases where carbanion loss occurs. It s only possible because the three halogen atoms are capable of stabilizing the negative charge. 

A fluorine substituent, however, has the opposite effect on geometry. Pyramidal ions are stabilized by fluorine and planar ions destabilized conjugation with the filled orbitals on fluorine is unfavorable. See A. Streitwieser, Jr., and F. Mares, J. Amer. Chem. Soc., 90, 2444 (1968). Chlorine, bromine, and iodine apparently stabilize an adjacent carbanion more than does fluorine, presumably because the destabilizing orbital overlap is less effective with the larger halogens (see Section 5.2, p. 227). J. Hine, N. W. Burske, M. Hine, and P. B. Langford, J. Amer. Chem. Soc., 79, 1406 (1957). 

Another common a-silyl anion is produced by die halogen exchange from a methyl (but not odier group) attached to silicon. Odier a-silyl carbanions can be generated by other processes. Such anions lack the resonance stabilization of an ester group seen in the previous example. They are consequently less stable and must be generated under carefully controlled conditions. They are good nucleophiles and add effectively to aldehydes and ketones. 

Since silicon stabilizes an a-carbanion by d jr-p jr interaction, silicon should promote electron transfer to a carbon-halogen bond which generates the a-carbanion. In fact,... 

The reaction of aryl and hetaryl halides with the nitrile-stabilized carbanions (RC -CN) leads to derivatives of the type ArCH(R)CN. Sometimes, however, dimeric products of the type ArCH(R)CH(R)Ar are formed (Moon et al. 1983). As observed, 1-naphthyl, 2-pyridyl, and 2-quinolyl halides give the nitrile-substituted products, while phenyl halides, as a rule, form dimers. The reason consists of the manner of a surplus electron localization in the anion radical that arises upon replacing halogen with the nitrile-containing carban-ion. If the resultant anion radical contains an unpaired electron within LUMO covering mainly the aromatic ring, such an anion radical is stable, with no inclination to split up. It is oxidized by the initial substrate and gives the final product in the neutral form, Scheme 1-7 ... 

The basicity of the anion CH2X , as a function of X, has been found to decrease in groups 16 (from OH to SH), 15 (from NH2 to PH2), and 14 (from CH3 to SiHs), because the a-stabilization of CH2X increases.5 In contrast, the basicity of CH2X along the series X = F, Cl, Br, I does not decrease, as commonly assumed. Fluorine has been found more effective than the heavier halogens for a-stabilization of carbanions. 

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