5.7- Dicarbanion

Birch and others originally suggested that two electrons add to the aromatic ring affording a dicarbanion (34) which is subsequently protonated [Eq. (1)]. The preference for maximum separation of charge in dicarbanion... 

Reduction of a conjugated enone to a saturated ketone requires the addition of two electrons and two protons. As in the case of the Birch reduction of aromatic compounds, the exact order of these additions has been the subject of study and speculation. Barton proposed that two electrons add initially giving a dicarbanion of the structure (49) which then is protonated rapidly at the / -position by ammonia, forming the enolate salt (50) of the saturated ketone. Stork later suggested that the radical-anion (51), a one electron... 

Benzyl-type carbanions and their metallo compounds, derived from aromatic or hetero-aromatic precursors, bearing carbon- or hetero-substituents, are readily available with variable substitution patterns due to their mesomeric stabilization (see Section 1.3.2.2)2. Even dicarbanions are accessible without difficulty3,4. The equilibrium acidities of many aromatic hydrocarbons have been determined5-7. The acidities of a-hetero-substituted toluenes8 are similar to those of the corresponding allylic compounds and can usually be generated by the same methods. 

More recently it has become apparent that proton equilibria and hence pH can be equally important in aprotic and other non-aqueous solvents. For example, the addition of a proton donor, such as phenol or water, to dimethylformamide has a marked effect on the i-E curve for the reduction of a polynuclear aromatic hydrocarbon (Peover, 1967). In the absence of a proton donor the curve shows two one-electron reduction waves. The first electron addition is reversible and leads to the formation of the anion radical while the second wave is irreversible owing to rapid abstraction of protons from the solvent by the dicarbanion. 

Further examples of emissive cyclometallated gold(III) complexes are [Au(L)Cl] (L = tridentate carbanion of 4 -(4-methoxyphenyl)-6 -phenyl-2,2 -bipyridine) [53], as well as mono- and binuclear bis-cyclometallated gold(III) complexes, namely [Au (C N C )L ]" (C N C = tridentate dicarbanion of 2,6-diphenylpyridine L = depro-tonated 2-mercaptopyridine (2-pyS ), n = 0 L = PPh3 or 1-methylimidazole, n = 1) and [Au2(C N C )2(P P)](C104)2 (P P = dppm, dppe) respectively [54]. The crystal structures of the binuclear derivatives show intramolecular interplanar separations of 3.4 A between the [Au(C N C)] moieties, implying the presence of weak n-n interactions. The mononuclear complexes show absorption with vibronic structure at 380-405 nm (e > 10 cm ), attributed to metal-perturbed intraligand transition. 

Condensation of benzaldehydes with benzotriazole in the presence of thionyl chloride readily gives a,a-/fe(benzo-triazol-l-yl)toluenes 727 that can be considered as l,l -gm-dicarbanion equivalents. Thus, treatment of derivatives 727 with ketones and lithium metal suspended in THF at — 78 °C generates substituted propylene glycols 728 (Equation 18) <1998TL2289>. 

The styryl radical-anion is shown as a resonance hybrid of the forms wherein the anion and radical centers are alternately on the a- and ])-carbon atoms. The styryl radical-anion dimerizes to form the dicarbanion (XX)... 

That this reaction occurs is shown by electron spin resonance measurements, which indicate the complete disappearance of radicals in the system immediately after the addition of monomer. The dimerization occurs to form the styryl dicarbanion instead of - CH2CH4>CH4>CH2 -, since the former is much more stable. The styryl dianions so-formed are colored red (the same as styryl monocarbanions formed via initiators such as n-butyl-lithium). Anionic propagation occurs at both carbanion ends of the styryl dianion... 

Both Russell (5) and Barton (I) have examined the oxidation of dihydroanthracene in a solvent system consisting of 80% dimethyl sulfoxide and 20% tert-butyl alcohol and with potassium terf-butoxide as the base. In both studies, a large excess of base was used, so that there is a possibility of dicarbanion formation. In the present investigation, only catalytic amounts of base were used, which makes it unlikely that a... 

Probtom 10.23 Typical of mechanisms for reductions with active metals in protic solvents, two electrons are transferred from the metal atoms to the substrate to give the most stable dicarbanion, which then accepts two H s from the protic solvent molecules to give the product, (a) Give the structural formula for the dicarbanion formed from C Hj and (b) explain why it is preferentially formed. 

The synthesis of triblock copolymers B-A-B can be achieved by means of a bifunctional initiator a bifunctional a,u-dicarbanionic poly-A precursor is formed, and is used in a second step as the initiator for the polymerization of monomer B, with the same conditions to be observed as above. A number of efficient bifunctional initiators are commonly used, in polar solvent media. Recent work, carried out in several laboratories -- t aimed at the preparation of efficient bifunc-tional Initiators soluble in non polar solvents. Such systems... 

Compounds of the type CH3CO—CH2—CO— can give up two protons, if treated with 2 moles of a strong enough base, to give dicarbanions ... 

Electron transfer from polycyclic aromatic radical anions in polar solvents can also initiate propagation.120 168 169173 One of the early and best understood systems is naphthalene-sodium, a green solution of stable, solvated naphthalene radical anion.176 177 The electron transfer from the radical anion to the monomer yields a new radical anion [Eq. (13.33)]. The dominant reaction of the latter is its head-to-head dimerization to the stabile dimeric dicarbanion [Eq. (13.34)], which is the driving force for the electron transfer even when electron affinity of the monomer is less than that of the polycyclic molecule. Propagation proceeds at both ends of the chain ... 

---