Organic chemistry carbanion

As one of the fundamental reactive intermediate classes in organic chemistry, carbanions have always played a central role in organic synthesis. The advances in the field that enable enantioselective synthesis with chiral unstabilized carbanions have considerably expanded the synthesis armamentarium. The advances in transition metal catalysis provide new avenues for the construction of stereochemically challenging structures of natural and anthropogenic origin. Additional revolutionary discoveries can be anticipated as the discipline continues to expand on the knowledge of the structure and reactivity of carbanions. 

You have already had considerable experience with carbanionic compounds and their applications in synthetic organic chemistry The first was acetyhde ion m Chapter 9 followed m Chapter 14 by organometallic compounds—Grignard reagents for example—that act as sources of negatively polarized carbon In Chapter 18 you learned that enolate ions—reactive intermediates generated from aldehydes and ketones—are nucleophilic and that this property can be used to advantage as a method for carbon-carbon bond formation... 

The present chapter extends our study of carbanions to the enolate ions derived from esters Ester enolates are important reagents m synthetic organic chemistry The stabilized enolates derived from p keto esters are particularly useful... 

Organic halides play a fundamental role in organic chemistry. These compounds are important precursors for carbocations, carbanions, radicals, and carbenes and thus serve as an important platform for organic functional group transformations. Many classical reactions involve the reactions of organic halides. Examples of these reactions include the nucleophilic substitution reactions, elimination reactions, Grignard-type reactions, various transition-metal catalyzed coupling reactions, carbene-related cyclopropanations reactions, and radical cyclization reactions. All these reactions can be carried out in aqueous media. 

Carbocations, carbon radicals, and carbanions are important reactive carbon intermediates in organic chemistry and their interconversions could be effected, in principle, by redox processes. With the cation pool method at hand, we next examined the redox-mediated interconversions of such reactive carbon species. 

In this section we shall examine in detail the role of n—a interactions in carbanion chemistry. Carbanion stability as well as relative acidities of organic molecules are important probes of n—a interactions. 

One of the most efficient structure building principles in lithium organic chemistry is the Lis triangle /zs-capped by a carbanionic Ca atom. This structural motif can further be aggregated to build deltahedral metal cores. The Li4 tetrahedron is found in various lithium organic tetramers while the Lie octahedron is present in many hexamers (Figure 4). 

In organic chemistry this stabilizing effect is well known the stability of carbanions is known to be enhanced by nitro groups. The stability of the cyclopentadienide anion is increased by complexing with a typical Lewis acid so that it becomes less reactive. For example, ferrocene is not ionized in nitromethane solution. Addition of a Lewis acid such as aluminum chloride facilitates the occurrence of intramolecular race-mization (75) a process which is believed to involve ionic intermediates [16). This belief is supported by kinetic evidence and the failure of the reaction to occur in nearly inert solvents like methylene chloride and in those of high donidty. Whereas the former do not support the solvation of the cation formed in the process of ionization, the latter will react preferentially with the Lewis acid, which is then no longer available for the stabilization of the carbanion. 

Electro-organic chemistry at the cathode is essentially that of radicals, radical-anions, carbanions, and polyanions (which range from dianions to hexa-anions (Scheme 1). The anions may behave as nucleophiles, bases, and as single electron reductants the factors governing the competition between these roles are not yet fully understood. 

When, in 1832, Wohler and Liebig first discovered the cyanide-catalyzed coupling of benzaldehyde that became known as the benzoin condensation , they laid the foundations for a wide field of growing organic chemistry [1]. In 1903, Lapworth proposed a mechanistical model with an intermediate carbanion formed in a hydrogen cyanide addition to the benzaldehyde substrate and subsequent deprotonation [2]. In the intermediate active aldehyde , the former carbonyl carbon atom exhibits an inverted, nucleophilic reactivity, which exemplifies the Umpo-lung concept of Seebach [3]. In 1943, Ukai et al. reported that thiazolium salts also surprisingly catalyze the benzoin condensation [4], an observation which attracted even more attention when Mizuhara et al. found, in 1954, that the thiazolium unit of the coenzyme thiamine (vitamin Bi) (1, Fig. 9.1) is essential for its activity in enzyme biocatalysis [5]. Subsequently, the biochemistry of thiamine-dependent enzymes has been extensively studied, and this has resulted in widespread applications of the enzymes as synthetic tools [6]. 

We will discuss the preferred geometries and the MO descriptions of carbon radicals and the corresponding carbenium ions or carbanions in two parts. In the first part, we will examine carbon radicals, carbenium ions, and carbanions with three substituents on the carbon atom. The second part treats the analogous species with a divalent central C atom. Things like alkynyl radicals and cations are not really important players in organic chemistry and won t be discussed. Alkynyl anions, however, are extremely important, but will be covered later. 

Argiiello, J.E., Penenory, A.B. and Rossi, R.A. (2000) Quantum yields of the initiation step and chain propagation turnovers in SRN1 reactions photostimulated reaction of l-iodo-2-methyl-2-phenyl propane with carbanions in DMSO. Journal of Organic Chemistry, 65, 7175-7182. 

Marshall, L.J., Roydhouse, M.D., Slawin, A.M.Z. and Walton, J.C. (2007) Effect of chain length on radical to carbanion cyclocoupling of bromoaryl alkyl-linked oxazolines 1,3-areneotropic migration of oxazolines. Journal of Organic Chemistry, 72, 898-911. 

Barolo, S.M., Lukach, A.E. and Rossi, R.A. (2003) Syntheses of 2-substituted indoles and fused indoles by photostimulated reactions of o-iodoanilines with carbanions by the SRN1 mechanism. Journal of Organic Chemistry, 68, 2807—2811. 

The concepts of electron and ligand transfer can be applied to the oxidation and reduction of organic substrates by metal complexes,61-64 since one-equivalent changes in the oxidation states of metals in inorganic redox reactions also have analogies in organic chemistry. Thus, the interconversion of the series of species carbonium ion (R+), free radical (R ), and carbanion (R-) results from one-equivalent changes, namely,... 

Gomberg s proposal was the first suggestion that carbon is not always tetravalent Tile acceptance of the triphenylmethyl radical by the scientific community helped facilitate the development of mechanistic organic chemistry with its trivalent carbocat-ions, carbanions, and radicals. 

The carbon-metal bond in such compounds can range from an almost completely ionic bond to one that is predominantly covalent. Benzyl-sodium, for example, may be dissolved in ether to yield a conducting solution on the other hand, the lithium-carbon bond in the colorless ethyliithium is quite nonpolar. The chemistry of such compounds, be they ionic or covalent, is best understood by considering them as sources of the highly basic carbanions that would be formed by removal of the metal ion thus the chemistry of benzylsodium is the chemistry of the CeH CH ion, whereas the chemistry of ethyliithium is the chemistry of the ethide ion, C2H Such ions will attack acidic hydrogens to form the parent hydrocarbons, will attack the more positive end of a double bond, and can carry out a number of nucleophilic displacements these reactions are discussed in texts on organic chemistry. 

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