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, imines/iminium salts, 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 treated and referred to as "carbanions." This is a convenient approximation, although these species are generally clusters or complexes containing highly polar, but still covalent bonds metal–carbon bonds (Mδ+–Cδ−) rather than true carbanions.
Geometry
Absent π delocalization, the negative charge of a carbanion is localized in an spx hybridized orbital on carbon as a lone pair. As a consequence, localized alkyl, alkenyl/aryl, and alkynyl carbanions assume trigonal pyramidal, bent, and linear geometries, respectively. By Bent's rule, placement of the carbanionic lone pair electrons in an orbital with significant s character is favorable, accounting for the pyramidalized and bent geometries of alkyl and alkenyl carbanions, respectively. Valence shell electron pair repulsion (VSEPR) theory makes similar predictions. This contrasts with carbocations, which have a preference for unoccupied nonbonding orbitals of pure atomic p character, leading to planar and linear geometries, respectively, for alkyl and alkenyl carbocations.
An alkyl carbanion is trigonal pyramidal.Vinyl anions are bent. 1,2-Disubstituted vinyl anions have E and Z isomers that undergo inversion through a linear transition state.
However, delocalized carbanions may deviate from these geometries. Instead of residing in a hybrid orbital, the carbanionic lone pair may instead occupy a p orbital (or an orbital of high p character). A p orbital has a more suitable shape and orientation to overlap with the neighboring π system, resulting in more effective charge delocalization. As a consequence, alkyl carbanions with neighboring conjugating groups (e.g., allylic anions, enolates, nitronates, etc.) are generally planar rather than pyramidized. Likewise, delocalized alkenyl carbanions sometimes favor a linear instead of bent geometry. More often, a bent geometry is still preferred for substituted alkenyl anions, though the linear geometry is only slightly less stable, resulting in facile equilibration between the (E) and (Z) isomers of the (bent) anion through a linear transition state.[2] For instance, calculations indicate that the parent vinyl anion or ethylenide, H2C=CH−, has an inversion barrier of 27kcal/mol (110kJ/mol), while allenyl anion or allenide, H2C=C=CH− ↔ H2C−−C≡CH), whose negative charge is stabilized by delocalization, has an inversion barrier of only 4kcal/mol (17kJ/mol), reflecting stabilization of the linear transition state by better π delocalization.[3]
Trends and occurrence
Carbanions are typically nucleophilic and basic. The basicity and nucleophilicity of carbanions are determined by the substituents on carbon. These include
the inductive effect. Electronegative atoms adjacent to the charge will stabilize the charge;
the extent of conjugation of the anion. Resonance effects can stabilize the anion. This is especially true when the anion is stabilized as a result of aromaticity.
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.
Carbanions, especially ones derived from weak carbon acids that do not benefit sufficiently from the two stabilizing factors listed above, are generally oxygen- and water-sensitive to varying degrees. While some merely degrade and decompose over several weeks or months upon exposure to air, others may react vigorously and exothermically with air almost immediately to spontaneously ignite (pyrophoricity). Among commonly encountered carbanionic reagents in the laboratory, ionic salts of hydrogen cyanide (cyanides) are unusual in being indefinitely stable under dry air and hydrolyzing only very slowly in the presence of moisture.
Organometallic reagents like butyllithium (hexameric cluster, [BuLi]6) or methylmagnesium bromide (ether complex, MeMg(Br)(OEt2)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. The more electropositive the attached metal atom, the closer the behavior of the reagent is to that of a true carbanion.
In fact, true carbanions (i.e., a species not attached to a stabilizing covalently bound metal) without electron-withdrawing and/or conjugating 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 the methanide anion CH−3 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.[4] However, in 1978, the methanide anion was unambiguously synthesized by subjecting ketene to an electric discharge, and the electron affinity (EA) of •CH3 was determined by photoelectron spectroscopy to be +1.8kcal/mol, making it a bound species, but just barely so. The structure of CH−3 was found to be pyramidal (C3v) with an H−C−H angle of 108° and inversion barrier of 1.3kcal/mol, while •CH3 was determined to be planar (D3h point group).[5]
Simple primary, secondary and tertiary sp3 carbanions (e.g., ethanide CH3CH−2, isopropanide (CH3)2CH−, and t-butanide (CH3)3C− were subsequently determined to be unbound species (the EAs of CH3CH2•, (CH3)2CH•, (CH3)3C• are −6, −7.4, −3.6kcal/mol, 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 anions 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.[6]
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 ethersalt of the triphenylmethanide carbanion from triphenylmethane, n-butyllithium and 12-crown-4 (which forms a stable complex with lithium cations) at low temperatures:[7]
Formation of the triphenylmethane anion
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 [Li(12-crown-4)]+[CPh3]− precipitates at −20°C. The central C–C bond lengths are 145pm 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 ([Li(12-crown-4)]+[CHPh2]−), prepared form diphenylmethane (pKa in DMSO of CH2Ph2 = 32.3), was also obtained. However, the attempted isolation of a complex of the benzyl anion PhCH−2 from toluene (pKa in DMSO of CH3Ph ≈ 43) was unsuccessful, due to rapid reaction of the formed anion with the THF solvent.[8] The free benzyl anion has also been generated in the solution phase by pulse radiolysis of dibenzylmercury.[9]
Early in 1904[10] and 1917,[11]Schlenk prepared two red-colored salts, formulated as [NMe4]+[CPh3]− and [NMe4]+[PhCH2]−, 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,[12] the instability of the latter has so far precluded structural verification. The reaction of the putative "[NMe4]+[PhCH2]−" with water was reported to liberate toluene and tetramethylammonium hydroxide and provides indirect evidence for the claimed formulation.
One tool for the detection of carbanions in solution is proton NMR.[13] A spectrum of cyclopentadiene in DMSO shows four vinylic protons at 6.5ppm and two methylene bridge protons at 3 ppm whereas the cyclopentadienyl anion has a single resonance at 5.50ppm. The use of 6Li and 7Li NMR has provided structural and reactivity data for a variety of organolithium species.
Carbon acids
Any compound containing hydrogen can, in principle, undergo deprotonation to form its conjugate base. A compound is a carbon acid if deprotonation results in loss of a proton from a carbon atom. Compared to compounds typically considered to be acids (e.g., mineral acids like nitric acid, or carboxylic acids like acetic acid), carbon acids are typically many orders of magnitude weaker, although exceptions exist (see below). For example, benzene is not an acid in the classical Arrhenius sense, since its aqueous solutions are neutral. Nevertheless, it is very weak Brønsted acid with an estimated pKa of 49 which may undergo deprotonation in the presence of a superbase like the Lochmann–Schlosser base (n-butyllithium and potassium t-butoxide). As conjugate acid–base pairs, the factors that determine the relative stability of carbanions also determine the ordering of the pKa values of the corresponding carbon acids. Furthermore, pKa values allow the prediction of whether a proton transfer process will be thermodynamically favorable: In order for the deprotonation of an acidic species HA with base B− to be thermodynamically favorable (K > 1), the relationship pKa(BH) > pKa(AH) must hold.
These values below are pKa values determined in dimethylsulfoxide (DMSO), which has a broader useful range (~0 to ~35) than values determined in water (~0 to ~14) and better reflect the basicity of the carbanions in typical organic solvents. Values below less than 0 or greater than 35 are indirectly estimated; hence, the numerical accuracy of these values is limited. Aqueous pKa values are also commonly encountered in the literature, particularly in the context of biochemistry and enzymology. Moreover, aqueous values are often given in introductory organic chemistry textbooks for pedagogical reasons, although the issue of solvent dependence is often glossed over. In general, pKa values in water and organic solvent diverge significantly when the anion is capable of hydrogen bonding. For instance, in the case of water, the values differ dramatically: the pKa in water of water is 14.0,[14] while the pKa in DMSO of water is 31.4,[15] reflecting the differing ability of water and DMSO to stabilize the hydroxide anion. On the other hand, for cyclopentadiene, the numerical values are comparable: the pKa in water is 15, while the pKa in DMSO is 18.[15]
Carbon acid acidities by pKa in DMSO.[16] These values may differ significantly from aqueous pKa values.
Note that acetic acid, ammonia, aniline, ethanol, and hydrogen chloride are not carbon acids, but are common acids shown for comparison.
↑ The reported pKa in acetonitrile (MeCN) is −3.7.[20] The pKa in DMSO was estimated by the correlation pKaMeCN = 0.98 × pKaDMSO + 11.6.[21]
As indicated by the examples above, acidity increases (pKa decreases) when the negative charge is delocalized. This effect occurs when the substituents on the carbanion are unsaturated and/or electronegative. Although carbon acids are generally thought of as acids that are much weaker than "classical" Brønsted acids like acetic acid or phenol, the cumulative (additive) effect of several electron accepting substituents can lead to acids that are as strong or stronger than the inorganic mineral acids. For example, trinitromethaneHC(NO2)3, tricyanomethaneHC(CN)3, pentacyanocyclopentadiene C5(CN)5H, and fulminic acid HCNO, are all strong acids with aqueous pKa values that indicate complete or nearly complete proton transfer to water. Triflidic acid, with three strongly electron-withdrawing triflyl groups, has an estimated pKa well below −10. On the other end of the scale, hydrocarbons bearing only alkyl groups are thought to have pKa values in the range of 55 to 65. The range of acid dissociation constants for carbon acids thus spans over 70 orders of magnitude.
The acidity of the α-hydrogen in carbonyl compounds enables these compounds to participate in synthetically important C–C bond-forming reactions including the aldol reaction and Michael addition.
Chiral carbanions
With the molecular geometry for a carbanion described as a trigonal pyramid the question is whether or not carbanions can display chirality, because if the activation barrier for inversion of this geometry is too low any attempt at introducing chirality will end in racemization, similar to the nitrogen inversion. However, solid evidence exists that carbanions can indeed be chiral for example in research carried out with certain organolithium compounds.
The first ever evidence for the existence of chiral organolithium compounds was obtained in 1950. Reaction of chiral 2-iodooctane with s-butyllithium in petroleum ether at −70°C followed by reaction with dry ice yielded mostly racemic 2-methylbutyric acid but also an amount of optically active 2-methyloctanoic acid, which could only have formed from likewise optically active 2-methylheptyllithium with the carbon atom linked to lithium the carbanion:[22]
Optically active organolithium
On heating the reaction to 0°C the optical activity is lost. More evidence followed in the 1960s. A reaction of the cis isomer of 2-methylcyclopropyl bromide with s-butyllithium again followed by carboxylation with dry ice yielded cis-2-methylcyclopropylcarboxylic acid. The formation of the trans isomer would have indicated that the intermediate carbanion was unstable.[23]
Stereochemistry of organolithiums
In the same manner the reaction of (+)-(S)-l-bromo-l-methyl-2,2-diphenylcyclopropane with n-butyllithium followed by quenching with methanol resulted in product with retention of configuration:[24]
Optical stability of 1-methyl-2,2-diphenylcyclopropyllithium
Of recent date are chiral methyllithium compounds:[25]
Chiral oxy[H1]methyllithiums. Bu stands for butyl, i-Pr stands for isopropyl.
A carbanionic structure first made an appearance in the reaction mechanism for the benzoin condensation as correctly proposed by Clarke and Arthur Lapworth in 1907.[26] In 1904 Wilhelm Schlenk prepared [Ph3C]−[NMe4]+ in a quest for tetramethylammonium (from tetramethylammonium chloride and Ph3CNa)[10] and in 1914 he demonstrated how triarylmethyl radicals could be reduced to carbanions by alkali metals [27] The phrase carbanion was introduced by Wallis and Adams in 1933 as the negatively charged counterpart of the carbonium ion[28][29]
In chemistry, amines are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines, trimethylamine, and aniline. Inorganic derivatives of ammonia are also called amines, such as monochloramine.
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group attached to an R-group. The general formula of a carboxylic acid is often written as R−COOH or R−CO2H, sometimes as R−C(O)OH with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic acids occur widely. Important examples include the amino acids and fatty acids. Deprotonation of a carboxylic acid gives a carboxylate anion.
In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula CH3. In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in many organic compounds. It is a very stable group in most molecules. While the methyl group is usually part of a larger molecule, bounded to the rest of the molecule by a single covalent bond, it can be found on its own in any of three forms: methanide anion, methylium cation or methyl radical. The anion has eight valence electrons, the radical seven and the cation six. All three forms are highly reactive and rarely observed.
In chemistry, a leaving group is defined by the IUPAC as an atom or group of atoms that detaches from the main or residual part of a substrate during a reaction or elementary step of a reaction. However, in common usage, the term is often limited to a fragment that departs with a pair of electrons in heterolytic bond cleavage. In this usage, a leaving group is a less formal but more commonly used synonym of the term nucleofuge. In this context, leaving groups are generally anions or neutral species, departing from neutral or cationic substrates, respectively, though in rare cases, cations leaving from a dicationic substrate are also known.
In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom to the substrates in synthetic steps, through nucleophilic addition or simple deprotonation. Organolithium reagents are used in industry as an initiator for anionic polymerization, which leads to the production of various elastomers. They have also been applied in asymmetric synthesis in the pharmaceutical industry. Due to the large difference in electronegativity between the carbon atom and the lithium atom, the C−Li bond is highly ionic. Owing to the polar nature of the C−Li bond, organolithium reagents are good nucleophiles and strong bases. For laboratory organic synthesis, many organolithium reagents are commercially available in solution form. These reagents are highly reactive, and are sometimes pyrophoric.
The 1,3-dipolar cycloaddition is a chemical reaction between a 1,3-dipole and a dipolarophile to form a five-membered ring. The earliest 1,3-dipolar cycloadditions were described in the late 19th century to the early 20th century, following the discovery of 1,3-dipoles. Mechanistic investigation and synthetic application were established in the 1960s, primarily through the work of Rolf Huisgen. Hence, the reaction is sometimes referred to as the Huisgen cycloaddition. 1,3-dipolar cycloaddition is an important route to the regio- and stereoselective synthesis of five-membered heterocycles and their ring-opened acyclic derivatives. The dipolarophile is typically an alkene or alkyne, but can be other pi systems. When the dipolarophile is an alkyne, aromatic rings are generally produced.
In organic chemistry, enolates are organic anions derived from the deprotonation of carbonyl compounds. Rarely isolated, they are widely used as reagents in the synthesis of organic compounds.
A 1,2-rearrangement or 1,2-migration or 1,2-shift or Whitmore 1,2-shift is an organic reaction where a substituent moves from one atom to another atom in a chemical compound. In a 1,2 shift the movement involves two adjacent atoms but moves over larger distances are possible. In the example below the substituent R moves from carbon atom C2 to C3.
In organic chemistry, a sulfoxide, also called a sulphoxide, is an organosulfur compound containing a sulfinyl functional group attached to two carbon atoms. It is a polar functional group. Sulfoxides are oxidized derivatives of sulfides. Examples of important sulfoxides are alliin, a precursor to the compound that gives freshly crushed garlic its aroma, and dimethyl sulfoxide (DMSO), a common solvent.
A nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring. Aromatic rings are usually nucleophilic, but some aromatic compounds do undergo nucleophilic substitution. Just as normally nucleophilic alkenes can be made to undergo conjugate substitution if they carry electron-withdrawing substituents, so normally nucleophilic aromatic rings also become electrophilic if they have the right substituents.
The E1cB elimination reaction is a type of elimination reaction which occurs under basic conditions, where the hydrogen to be removed is relatively acidic, while the leaving group is a relatively poor one. Usually a moderate to strong base is present. E1cB is a two-step process, the first step of which may or may not be reversible. First, a base abstracts the relatively acidic proton to generate a stabilized anion. The lone pair of electrons on the anion then moves to the neighboring atom, thus expelling the leaving group and forming double or triple bond. The name of the mechanism - E1cB - stands for Elimination Unimolecular conjugate Base. Elimination refers to the fact that the mechanism is an elimination reaction and will lose two substituents. Unimolecular refers to the fact that the rate-determining step of this reaction only involves one molecular entity. Finally, conjugate base refers to the formation of the carbanion intermediate, which is the conjugate base of the starting material.
In organic chemistry the Brook rearrangement refers to any [1,n] carbon to oxygen silyl migration. The rearrangement was first observed in the late 1950s by Canadian chemist Adrian Gibbs Brook (1924–2013), after which the reaction is named. These migrations can be promoted in a number of different ways, including thermally, photolytically or under basic/acidic conditions. In the forward direction, these silyl migrations produce silyl ethers as products which is driven by the stability of the oxygen-silicon bond.
sec-Butyllithium is an organometallic compound with the formula CH3CHLiCH2CH3, abbreviated sec-BuLi or s-BuLi. This chiral organolithium reagent is used as a source of sec-butyl carbanion in organic synthesis.
The vinyl cation is a carbocation with the positive charge on an alkene carbon. Its empirical formula is C 2H+ 3. More generally, a vinylic cation is any disubstituted carbon, where the carbon bearing the positive charge is part of a double bond and is sp hybridized. In the chemical literature, substituted vinylic cations are often referred to as vinyl cations, and understood to refer to the broad class rather than the C 2H+ 3 variant alone. The vinyl cation is one of the main types of reactive intermediates involving a non-tetrahedrally coordinated carbon atom, and is necessary to explain a wide variety of observed reactivity trends. Vinyl cations are observed as reactive intermediates in solvolysis reactions, as well during electrophilic addition to alkynes, for example, through protonation of an alkyne by a strong acid. As expected from its sp hybridization, the vinyl cation prefers a linear geometry. Compounds related to the vinyl cation include allylic carbocations and benzylic carbocations, as well as aryl carbocations.
Organosodium chemistry is the chemistry of organometallic compounds containing a carbon to sodium chemical bond. The application of organosodium compounds in chemistry is limited in part due to competition from organolithium compounds, which are commercially available and exhibit more convenient reactivity.
Nitrile anions is jargon from the organic product resulting from the deprotonation of alkylnitriles. The proton(s) α to the nitrile group are sufficiently acidic that they undergo deprotonation by strong bases, usually lithium-derived. The products are not anions but covalent organolithium complexes. Regardless, these organolithium compounds are reactive toward various electrophiles.
The [2,3]-Wittig rearrangement is the transformation of an allylic ether into a homoallylic alcohol via a concerted, pericyclic process. Because the reaction is concerted, it exhibits a high degree of stereocontrol, and can be employed early in a synthetic route to establish stereochemistry. The Wittig rearrangement requires strongly basic conditions, however, as a carbanion intermediate is essential. [1,2]-Wittig rearrangement is a competitive process.
Desulfonylation reactions are chemical reactions leading to the removal of a sulfonyl group from organic compounds. As the sulfonyl functional group is electron-withdrawing, methods for cleaving the sulfur–carbon bonds of sulfones are typically reductive in nature. Olefination or replacement with hydrogen may be accomplished using reductive desulfonylation methods.
In organic chemistry, Wittig reagents are organophosphorus compounds of the formula R3P=CHR', where R is usually phenyl. They are used to convert ketones and aldehydes to alkenes:
A Fischer carbene is a type of transition metal carbene complex, which is an organometallic compound containing a divalent organic ligand. In a Fischer carbene, the carbene ligand is a σ-donor π-acceptor ligand. Because π-backdonation from the metal centre is generally weak, the carbene carbon is electrophilic.
↑ Ellison, G. Barney; Engelking, P. C.; Lineberger, W. C. (April 1978). "An experimental determination of the geometry and electron affinity of methyl radical". Journal of the American Chemical Society. 100 (8): 2556–2558. doi:10.1021/ja00476a054. ISSN0002-7863.
↑ Blanksby, S. J.; Bowie, J. H. (2005). "Carbanions: formation, structure and thermochemistry". The encyclopedia of mass spectrometry. Gross, Michael L., Caprioli, R. M. (1sted.). Amsterdam: Elsevier. ISBN9780080438504. OCLC55939535.
↑ Olmstead, Marilyn M. (1985). "The isolation and X-ray structures of lithium crown ether salts of the free phenyl carbanions [CHPh2]− and [CPh3]−". Journal of the American Chemical Society. 107 (7): 2174–2175. doi:10.1021/ja00293a059.
↑ Bockrath, Bradley; Dorfman, Leon M. (2002-05-01). "Submicrosecond formation and observation of reactive carbanions". Journal of the American Chemical Society. 96 (18): 5708–5715. doi:10.1021/ja00825a005.
↑ Kasmai, Hamid S. (June 1999). "A Simple and Convenient Method for Generation and NMR Observation of Stable Carbanions". Journal of Chemical Education. 76 (6). doi:10.1021/ed076p830.
↑ Silverstein, Todd P.; Heller, Stephen T. (2017-04-17). "pKa Values in the Undergraduate Curriculum: What is the Real pKa of Water?". Journal of Chemical Education. 94 (6): 690–695. Bibcode:2017JChEd..94..690S. doi:10.1021/acs.jchemed.6b00623.
↑ Bordwell, Frederick G. (1988). "Equilibrium acidities in dimethyl sulfoxide solution". Accounts of Chemical Research. 21 (12): 456–463. doi:10.1021/ar00156a004.
↑ Bordwell, G. F.; Matthews, Walter S. (2002-05-01). "Equilibrium acidities of carbon acids. III. Carbon acids in the membrane series". Journal of the American Chemical Society. 96 (4): 1216–1217. doi:10.1021/ja00811a041.
1 2 Russell, Jamie; Roques, Nicolas (1998-11-05). "Effective nucleophilic trifluoromethylation with fluoroform and common base". Tetrahedron. 54 (45): 13771–13782. doi:10.1016/S0040-4020(98)00846-1. ISSN0040-4020.
↑ Kütt, Agnes; Rodima, Toomas; Saame, Jaan; Raamat, Elin; Mäemets, Vahur; Kaljurand, Ivari; Koppel, Ilmar A.; Garlyauskayte, Romute Yu.; Yagupolskii, Yurii L.; Yagupolskii, Lev M.; Bernhardt, Eduard; Willner, Helge; Leito, Ivo (2011). "Equilibrium Acidities of Superacids". The Journal of Organic Chemistry. 76 (2): 391–395. doi:10.1021/jo101409p. PMID21166439.
↑ Ding, Feizhi; Smith, Jeremy M.; Wang, Haobin (2009). "First-Principles Calculation of pKa Values for Organic Acids in Nonaqueous Solution". The Journal of Organic Chemistry. 74 (7): 2679–2691. doi:10.1021/jo802641r. PMID19275192.
↑ Letsinger, Robert L. (1950). "Formation of Optically Active 1-Methylheptyllithium". Journal of the American Chemical Society. 72 (10): 4842. doi:10.1021/ja01166a538.
↑ Applequist, Douglas E. (1961). "The Configurational Stability of cis- and trans-2-Methylcyclopropyllithium and Some Observations on the Stereochemistry of their Reactions with Bromine and Carbon Dioxide". Journal of the American Chemical Society. 83 (4): 862–865. doi:10.1021/ja01465a030.
↑ Walborsky, H. M. (1964). "Cyclopropanes. XV. The Optical Stability of 1-Methyl-2,2-diphenylcyclopropyllithium". Journal of the American Chemical Society. 86 (16): 3283–3288. doi:10.1021/ja01070a017.
↑ Kapeller, Dagmar (2007). "Preparation of Chiral α-Oxy-[2H1]methyllithiums of 99% ee and Determination of Their Configurational Stability". Journal of the American Chemical Society. 129 (4): 914–923. doi:10.1021/ja066183s. PMID17243828.
↑ Wallis, E. S.; Adams, F. H. (1933). "The Spatial Configuration of the Valences in Tricovalent Carbon Compounds1". Journal of the American Chemical Society. 55 (9): 3838. doi:10.1021/ja01336a068.
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