Triphenylmethane carbanion

On treatment with a strong base such as sodium hydride or sodium amide, dimethyl sulfoxide yields a proton to form the methylsulfinyl carbanion (dimsyl ion), a strongly basic reagent. Reaction of dimsyl ion with triphenylalkylphosphonium halides provides a convenient route to ylides (see Chapter 11, Section III), and with triphenylmethane the reagent affords a high concentration of triphenylmethyl carbanion. Of immediate interest, however, is the nucleophilic reaction of dimsyl ion with aldehydes, ketones, and particularly esters (//). The reaction of dimsyl ion with nonenolizable ketones and... 

Methylsulfinyl carbanion (dimsyl ion) is prepared from 0.10 mole of sodium hydride in 50 ml of dimethyl sulfoxide under a nitrogen atmosphere as described in Chapter 10, Section III. The solution is diluted by the addition of 50 ml of dry THF and a small amount (1-10 mg) of triphenylmethane is added to act as an indicator. (The red color produced by triphenylmethyl carbanion is discharged when the dimsylsodium is consumed.) Acetylene (purified as described in Chapter 14, Section I) is introduced into the system with stirring through a gas inlet tube until the formation of sodium acetylide is complete, as indicated by disappearance of the red color. The gas inlet tube is replaced by a dropping funnel and a solution of 0.10 mole of the substrate in 20 ml of dry THF is added with stirring at room temperature over a period of about 1 hour. In the case of ethynylation of carbonyl compounds (given below), the solution is then cautiously treated with 6 g (0.11 mole) of ammonium chloride. The reaction mixture is then diluted with 500 ml of water, and the aqueous solution is extracted three times with 150-ml portions of ether. The ether solution is dried (sodium sulfate), the ether is removed (rotary evaporator), and the residue is fractionally distilled under reduced pressure to yield the ethynyl alcohol. 

Mechanisms depending on carbanionic propagating centers for these polymerizations are indicated by various pieces of evidence (1) the nature of the catalysts which are effective, (2) the intense colors that often develop during polymerization, (3) the prompt cessation of sodium-catalyzed polymerization upon the introduction of carbon dioxide and the failure of -butylcatechol to cause inhibition, (4) the conversion of triphenylmethane to triphenylmethylsodium in the zone of polymerization of isoprene under the influence of metallic sodium, (5) the structures of the diene polymers obtained (see Chap. VI), which differ. both from the radical and the cationic polymers, and (6)... 

The stable carbanions may belong in a special category since their stability is in most cases due to resonance, and the resonance has geometrical requirements that might or might not be the same as those of the bond hybridization of an ordinary carbanion. The central hydrogen of triptycene has none of the acidity of the central hydrogen of triphenylmethane.364... 

Treatment of tri-(2-thienyl)methane with BunLi in the presence of TMEDA in THF at — 78°C gives exclusively the tri-(2-thienyl)methyllithium (393) (96%) without any nuclear lithiation (92CL703). This lithiation is faster than that of triphenylmethane. Treatment of (393) with primary alkyl halides leads to alkylation at the carbanion center, forming (394). However, secondary alkyl halides give mixture of (394) and (395). 

The triphenylmethyl carbanion, the trityl anion, can be generated by the reaction of triphenylmethane with the very powerful base, n-butyllithium. The reaction generates the blood-red lithium triphenylmethide and butane. The triphenylmethyl anion reacts much as a Grignard reagent does. In the present experiment it reacts with carbon dioxide to give triphenylacetic acid after acidification. Avoid an excess of n-butyllithium on reaction with carbon dioxide, it gives the vile-smelling pentanoic acid. 

The above carbanions are stabilized by delocalization of charge onto an unsaturated group (allylic, propargylic or benzylic). In a less effective type of stabilization, the anionic charge is stabilized in an orbital having increased s character (e.g., sp rather than sp hybridization), e.g., in the cleavage of triphenylmethane ... 

Triphenylmethane (pK = 31) is not sufficiently acidic it is not metallated by Mg metal in EtjO. On the other hand, C—H bonds that are strongly activated by carbanion-stabilizing groups can be metallated directly, as illustrated by the reaction of... 

In the above case the low k /k ratio indicates that the complex between acid and carbanion survives for a significant time prior to breakdown by diffusion. A further case involves tautomerism in the triphenylmethane system (Eqn. 62) in deuterium-containing solvents [45]. 

Intermolecular Nucleophihc Substitution with Heteroatom Nucleophiles. A patent issued in 1965 claims substitution for fluoride on fluorobenzene-Cr(CO)3 in dimethyl sulfoxide (DMSO) by a long list of nucleophiles including alkoxides (from simple alcohols, cholesterol, ethylene glycol, pinacol, and dihydroxyacetone), carboxylates, amines, and carbanions (from triphenylmethane, indene, cyclohexanone, acetone, cyclopentadiene, phenylacetylene, acetic acid, and propiolic acid). In the reaction of methoxide with halobenzene-Cr(CO)3, the fiuorobenzene complex is ca. 2000 times more reactive than the chlorobenzene complex. The difference is taken as evidence for a rate-limiting attack on the arene ligand followed by fast loss of halide the concentration of the cyclohexadienyl anion complex does not build up. In the reaction of fluorobenzene-Cr(CO)3 with amine nucleophiles, the coordinated aniline product appears rapidly at 25 °C, and a careful mechanistic study suggests that the loss of halide is now rate limiting. 

Oxidation of carbanions by molecular oxygen is also an important reaction. Equation (3.10) shows the oxidation of the anion of triphenylmethane by molecular oxygen in a solution composed of 80% DMF and 20% t-butyl alcohol. Addition of water to the reaction mixture allowed isolation of triphenylmethyl hydroperoxide in high yields (e.g., 87%). When the reaction was carried out in 80% DMSO-20%... 

It is claimed that many types of reactions can be performed more easily with multiple polymers. To avoid undesirable side reactions two reacting species can be used with each attached to a different polymer. Such polymer-bound reactants can coexist in the same reaction vessel without interacting. An example is the preparation of benzoyl acetonitrile by Patchomik and coworkers. Molecules of triphenylmethane lithium, attached to polystyrene supports, were combined with also immobilized o-nitrophenol. The < -nitrophenols were prereacted with benzoyl chloride. The two species were combined and acetonitrile molecules containing acidic hydrogens were introduced into the reaction mixture. This resulted in hydrogens being abstracted from the introduced molecules and formation of short-lived carbanions ... 

Triphenylmethide (19) is formed by the reaction of triphenylmethane (PhaCH) with sodium metal, as seen in Section 22.1. It is an unusual but effective base in this reaction because it is a relatively non-nucleophilic base (see Section 22.3). To explain the reaction with 60 and formation of product 61, a mechanism requires that the base first remove the acidic a-proton on C2 from the ester to form enolate anion 62. As with enolate anions derived from ketones and aldehydes, there are two resonance forms, and the carbanion form (62A) is the more nucleophilic. Therefore, resonance contribution 62A will lead to the... 

Direct reaction between sodium or potassium and alkyl and aryl halides is complicated by exchange and coupling reactions, which can lead to mixtures of products. These complications can be reduced by rapid stirring and the use of finely divided metal or amalgams. Phenylsodium can be made in this way PhCl -h Na->PhNa-h NaCl. Acidic hydrocarbons react with alkali metals in ether solvents. Cyclopentadiene, for example, affords sodium cyclopentadienide in tetrahydrofuran (p. 279). Triphenylmethylpotassium is obtained as a deep red solution from triphenylmethane and potassium in 1,2-dimethoxyethane. These carbanions, in which the negative charge is delocalized over several carbon atoms, do not attack ethers, in contrast to the simple alkyl or aryl carbanions present in methylsodium or phenylpotassium. 

Photoinitiation is not the only access to this chemistry, e.g., cathodic induced reduction or the use of alkali metals or other inorganic reducing reagents are also possible, but irradiation often is advantageous for preparative purposes. Since this is a chain process, the use of low-power lamps or a low quantum yield initiation step are not necessarily a limitation. Due to the requirement of a fast cleavage at the radical anion stage, aryl halides are by far the most used reagents, in particular iodides and, to a lower extent, bromides. Nucleophiles are carbanions from sufficiently acidic hydrocarbons, e.g., 1, 3-diphenylindane, fluorene or triphenylmethane [35-37] or, more commonly enolates from ketones [38], esters [39], MA -dialkylamides [40], nitriles [41]. C-C bond formation is obtained also with phenoxide or naphthoxide anions [42,43]. A few representative examples of synthetic applications of the S l... 

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