Carbanions, and

Olefin synthesis starts usually from carbonyl compounds and carbanions with relatively electropositive, redox-active substituents mostly containing phosphorus, sulfur, or silicon. The carbanions add to the carbonyl group and the oxy anion attacks the oxidizable atom Y in-tramolecularly. The oxide Y—O" is then eliminated and a new C—C bond is formed. Such reactions take place because the formation of a Y—0 bond is thermodynamically favored and because Y is able to expand its coordination sphere and to raise its oxidation number. 

NMR spectroscopy is ideal for detecting charged fluorinated intermediates and has been applied to the study of increasingly stable carbocation and carbanion species. Olah [164, 165] has generated stable fluorocarbocations m SbFj/SOjClF at low temperatures The relatively long-lived perfluoro-rerr-butyl anion has been prepared as both the cesium and tris(dimethylamino)sulfonium (TAS) salts by several groups [166, 167, 168], Chemical shifts of fluonnated carbocations and carbanions are listed m Table 23. 

Table 1. Some imide ions and carbanions used in salts to enhance polymer electrolyte conductivity and reduce crystallinity...

The reaction of A-acyliminium ions with nucleophilic carbon atoms (also called cationic x-amidoalkylation) is a highly useful method for the synthesis of both nitrogen heterocycles and open-chain nitrogen compounds. A variety of carbon nucleophiles can be used, such as aromatic compounds, alkcncs, alkyncs, carbcnoids, and carbanions derived from active methylene compounds and organometallics. 

Two other important properties of silicon-carbon bonds are that carbonium ions fl and carbanions (or metalloid equivalents) a to silicon are favoured over alternatives, i.e. that situations involving Si—C—C+ and Si—C are thermodynamically relatively good. 

The nucleophilic attack of strong bases (e.g. hydroxide ion, alkoxide ions and carbanions) on either the a-carbon111 or the sulfur atom of the sulfone group99,113 of the thiirane dioxides is the initial key step that is responsible for the subsequent ring opening and further reaction. The formation of a three-membered a-sulfonyl carbanion is not observed in these cases (equation 11). 

FIGURE 2.8 Energy levels for the benzyl cation, free radical, and carbanion. Since a is the energy of a p orbital (p. 35) the nonbonding orbital has no bonding energy. 

A free radical (often simply called a radical) may be defined as a species that contains one or more unpaired electrons. Note that this definition includes certain stable inorganic molecules such as NO and NO2, as well as many individual atoms, such as Na and Cl. As with carbocations and carbanions, simple alkyl radicals are very reactive. Their lifetimes are extremely short in solution, but they can be kept for relatively long periods frozen within the crystal lattices of other molecules. Many spectral measurements have been made on radicals trapped in this manner. Even under these conditions, the methyl radical decomposes with a half-life of 10-15 min in a methanol lattice at 77 K. Since the lifetime of a radical depends not only on its inherent stabihty, but also on the conditions under which it is generated, the terms persistent and stable are usually used for the different senses. A stable radical is inherently stable a persistent radical has a relatively long lifetime under the conditions at which it is generated, though it may not be very stable. 

Addition of a masked Grignard reagent to an aldehyde or ketone From aromatic aldehydes and carbanions Bimolecular reduction of aldehydes or ketones... 

The synthetic procedures for isolation of the salt appear to be rather simple. First, one prepares a solution in which the carbocation and carbanion coexist free from any combination reactions. Then, the hydrocarbon cation-anion salt is isolated after separation of the concomitant inorganic salt and evaporation of the solvent. For the purification of the crude salt recrystallization or reprecipitation with proper solvents is used. 

Sufficient stability of the hydrocarbon ions, as the salt or in the solution, is an obvious prerequisite for these procedures, and, in practice, selecting or designing the stable ions and choosing a proper solvent are tasks of primary importance. As an ordinary stability index for the ions, thermodynamic scales referred to the water molecule, i.e. p CR+ and pKa values, are chosen for the carbocation and carbanion, respectively. 

In the course of the salt synthesis, it was found that a hydrocarbon [3-2], which was formed by an unfavourable cation-anion combination reaction, dissociates into the original carbocation and carbanion in a polar aprotic solvent (Okamoto et ai, 1985) (1). This was the first example of ionic dissociation of the carbon-carbon a bond in genuine hydrocarbons, although a few cases of heterolytic dissociation of carbon-carbon tr bonds had been reported by Arnett (Arnett et al., 1983 Troughton et al., 1984 Arnett and Molter, 1985) for compounds bearing cyano and nitro groups, e.g. [4-6] and [5-6] as in (2). 

Structural factors necessary for carbocation and carbanion stability are discussed briefly. The mechanistic problems of dissociation of the carbon-carbon a bond are then discussed, and finally the physicochemical nature of hydrocarbon salts and the related novel hydrocarbons. 

As thermodynamic stability indexes for the hydrocarbon ions, pA R+ and pA a values [(4) and (5)] have been widely applied for the carbocation and carbanion, respectively, in solution. Here K + stands for the equilibrium constant for the reaction (6) of a carbocation and a water molecule stands for the equilibrium constant for the reaction (7) of a hydrocarbon with a water molecule to give the conjugate carbanion. The equilibrium constants are given by (8) and (9) for dilute aqueous solutions. Obviously, the reference system for the pKn+ scale is the corresponding alcohol, and... 

Unlike such unstable intermediates, the first, rare example of reversible dissociation of a carbon-carbon a bond into a stable carbocation and carbanion was reported for a nitro-dicyano compound (20) prepared from trimethyl- and triphenyl-cyclopropenylium tetrafluoroborate ([4" ]BF4 and [5 JBFJ) with the potassium salt of p-substituted-phenylmalononitrile anions (Arnett et al., 1983 Troughton et al., 1984 Arnett and Molter, 1985). Other ionically dissociative malononitrile derivatives have been prepared from such carbocations as the tropylium [S ] (Arnett and Troughton, 1983) and the tris(p-methoxyphenyl)methylium [93 j (Arnett and Troughton, 1983) ions. 

In view of the observations of the ionic dissociation of nitro-cyano compounds, it is hardly surprising that even a hydrocarbon could dissociate ionically into a stable carbocation and carbanion, provided that the medium is polar enough to prevent the recombination reaction and to ensure equilibration. 

Another linear correlation between A// values and between AG values has been proposed to correlate the heats of heterolysis for the carbon-carbon <7 bond with p Cr+ values of the cations and values of the conjugate acids of the anions by Arnett et al. (1987a, 1990a). From the results of calorimetry for the coordination of resonance-stabilized carbo-cations and carbanions in sulfolane or acetonitrile, these workers demonstrated that (28) and (29), for secondary and tertitu cations, respectively, can be used for predicting heats of heterolysis of the carbon-carbon a bond. 

Winstein Robinson (1958) used this concept to account for the kinetics of the salt effects on solvolysis reactions. They considered that carbonium ions (cations) and carbanions could exist as contact ion-pairs, solvated ion-pairs and as free ions and that all these forms participated in the reactions and were in equilibrium with each other. These equilibria can be represented, thus ... 

Figure 7.8. X-ray structures of carbanion 11 and carbanion 12. ORTEP representations adapted from references 55 and 56.

These three types, radicals, carbocations and carbanions, by no means exhaust the possibilities of transient intermediates in which carbon is the active centre others include the electron-deficient species carbenes, R2C (p. 266), nitrenes, RN (p. 122) and also arynes (p. 174). 

Most of the reactions that have been considered to-date have involved the participation of polar reactants and intermediates, i.e. carboca-tions and carbanions, or related highly polarised species, involving the heterolytic fission, and formation, of covalent bonds ... 

As with carbocations (p. 104) and carbanions (p. 276), the question arises of whether simple radicals—of the type R3C—accommodate... 

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