Carbanion

A carbanion is an anion in which carbon is trivalent (forms three bonds) and bears a formal negative charge in at least one significant mesomeric contributor (resonance form). Absent π delocalization, carbanions assume a trigonal pyramidal, bent, or linear geometry when the carbanionic carbon is bound to three (e.g., methyl anion), two (e.g., phenyl anion), or one (e.g., acetylide anion) substituents, respectively. Formally, a carbanion is the conjugate base of a carbon acid: A carbanion is an anion in which carbon is trivalent (forms three bonds) and bears a formal negative charge in at least one significant mesomeric contributor (resonance form). Absent π delocalization, carbanions assume a trigonal pyramidal, bent, or linear geometry when the carbanionic carbon is bound to three (e.g., methyl anion), two (e.g., phenyl anion), or one (e.g., acetylide anion) substituents, respectively. Formally, a carbanion is the conjugate base of a carbon acid: where B stands for the base. Carbanions have a concentration of electron density at the negatively charged carbon, which, in most cases, reacts efficiently with a variety of electrophiles of varying strengths, including carbonyl groups, halogenating reagents (e.g., N-bromosuccinimide and diiodine), and proton donors. A carbanion is one of several reactive intermediates in organic chemistry. In organic synthesis, organolithium reagents and Grignard reagents are commonly regarded as carbanions. This is a convenient approximation, although these species are almost always multinuclear clusters containing polar covalent bonds rather than true carbanions. Carbanions are typically nucleophilic and basic. The basicity and nucleophilicity of carbanions are determined by the substituents on carbon. These include Geometry also affects the orbital hybridization of the charge-bearing carbanion. The greater the s-character of the charge-bearing atom, the more stable the anion. Organometallic reagents like butyllithium (hexameric cluster, 6) or methylmagnesium bromide (ether complex, MeMgBr(OEt)2) are often referred to as 'carbanions,' at least in a retrosynthetic sense. However, they are really clusters or complexes containing a polar covalent bond, though with electron density heavily polarized toward the carbon atom. In fact, true carbanions without stabilizing substituents are not available in the condensed phase, and these species must be studied in the gas phase. For some time, it was not known whether simple alkyl anions could exist as free species; many theoretical studies predicted that even methanide anion CH3– should be an unbound species (i.e., the electron affinity of CH3• was predicted to be negative). Such a species would decompose immediately by spontaneous ejection of an electron and would therefore be too fleeting to observe directly by mass spectrometry. However, in 1978, methyl anion was unambiguously synthesized by subjecting ketene to electric discharge, and the electron affinity (EA) of CH3• was determined by photoelectron spectroscopy to be +1.8 kcal mol−1. The structure of CH3– was found to be pyramidal with a 1.3 kcal mol−1 inversion barrier, while CH3• was determined to be planar. Most simple primary, secondary and tertiary sp3 carbanions (e.g., CH3CH2–, (CH3)2CH–, and (CH3)3C–) were subsequently determined to be unbound species (EA of CH3CH2•, (CH3)2CH•, (CH3)3C• = −6, –7.4, –3.6 kcal mol−1, respectively) indicating that α substitution is destabilizing. However, relatively modest stabilizing effects can render them bound. For example, cyclopropyl and cubyl anions are bound due to increased s character of the lone pair orbital, while neopentyl and phenethyl anion are also bound, as a result of negative hyperconjugation of the lone pair with the β-substituent (nC → σ*C-C). The same holds true for anions with benzylic and allylic stabilization. Gas-phase carbanions that are sp2 and sp hybridized are much more strongly stabilized and are often prepared directly by gas-phase deprotonation. In the condensed phase only carbanions that are sufficiently stabilized by delocalization have been isolated as truly ionic species. In 1984, Olmstead and Power presented the lithium crown ether salt of the triphenylmethanide carbanion from triphenylmethane, n-butyllithium and 12-crown-4 (which forms a stable complex with lithium cations) at low temperatures: Adding n-butyllithium to triphenylmethane (pKa in DMSO of CHPh3 = 30.6) in THF at low temperatures followed by 12-crown-4 results in a red solution and the salt complex +– precipitates at −20 °C. The central C–C bond lengths are 145 pm with the phenyl ring propellered at an average angle of 31.2°. This propeller shape is less pronounced with a tetramethylammonium counterion. A crystal structure for the analogous diphenylmethanide anion (+–), prepared form diphenylmethane (pKa in DMSO of CH2Ph2 = 32.3), was also obtained. However, the attempted isolation of a complex of the benzyl anion – from toluene (pKa in DMSO of CH3Ph ≈ 43) was unsuccessful, due to rapid reaction of the formed anion with the THF solvent. The free benzyl anion has also been generated in the solution phase by pulse radiolysis of dibenzylmercury. Early in 1904 and 1917, Schlenk prepared two red-colored salts, formulated as +– and +–, respectively, by metathesis of the corresponding organosodium reagent with tetramethylammonium chloride. Since tetramethylammonium cations cannot form a chemical bond to the carbanionic center, these species are believed to contain free carbanions. While the structure of the former was verified by X-ray crystallography almost a century later, the instability of the latter has so far precluded structural verification. The reaction of the putative '+–' with water was reported to liberate toluene and tetramethylammonium hydroxide and provides indirect evidence for the claimed formulation.