Triphenylmethyl 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. 

A simple example of such abstraction of proton is the formation of triphenylmethyl carbanion by NaNH2 in presence of liquid ammonia. 

Charge delocalization of benzyl type carbanions is clearly monitored by the ortho and para carbon shifts [506 507]. Delocalizaton includes a second phenyl ring in benzhydryl anions, as indicated by an increased deshielding of the carbanionic carbon [507] (Table 4.77). Additional deshielding introduced by a third phenyl ring, however, is only about 10 ppm (Table 4.77) due to steric hindrance of coplanarity in the triphenylmethyl carbanion. 

The crystal structure of the red triphenylmethylsodium TMEDA complex (compound XVI in Fig. 3), published by Weiss and Koster (41), resembles that of the red triphenylmethyllithium-TMEDA complex (42) and can be described as a n complex between a triphenylmethyl carbanion with an sp2-hybridized central carbon atom and a sodium cation coordinated to the bidentade ligand TMEDA. The sodium atom has close contacts to several carbon atoms of the triphenylmethyl ligand, which possesses twisted phenyl groups. An additional short distance exists between sodium and a p-C (phenyl) atom of a neighboring n system. 

Triphenylmethyl carbanion is a strong enough base to convert an ester entirely into its enolate, Reaction of the enolate with a second molecule of ester then gives the keto-ester in good yield. 

Under suitable conditions the carbanions can be oxidized. Thus, triphenylmethyl carbanion is oxidized to triphenylmethyl radical slowly by air. The triphenylmethyl radical so obtained can be reduced back to the carbanion by shaking with sodium amalgam. 

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. 

When the lithium cation is unable to associate with the carbanion, as is the case for the Li (12-crown-4) complexed lithium diphenylmethane carbanion (56) or Li+ (12-crown-4) triphenylmethyl carbanion (57), the entire aromatic carbanions are relatively planar. The planarity of (56) and (57) is indicative of... 

Each Li atom is co-ordinated to two N atoms of the bidentate chelate and to one ir-carbanion. Each Li-N distance is 2.10 A. The Li atom is not located directly over any one carbon atom but has four close contacts to the carbanion 2.49 and 2.51 A to two C atoms on one phenyl group, one close contact of 2.54 A to one C atom of a second phenyl group, and one 2.23 A distance to the central carbon atom. The twist angles of the phenyl rings depend on their interaction with the Li atom, giving an overall propeller geometry to the triphenylmethyl carbanion. It is not possible to explain the stereochemistry by two-centre a or ionic interactions alone, and a delocalized bonding mechanism which involves all the 2s and 2p orbitals of the Li... 

The carbaction contains the cartx>n atom bearing a negative charge this atom has a lone electron pair. The carbanionic center has a planar or a pyramidal structure. For example, three C—C bonds in triphenylmethyl carbanion are arranged in one plane, and the trypticyl anion has the structure of a trihedral pyramid... 

Evidence of ester anion formation according to equation 7 has been reported in the literature. Other strongly basic catalysts such as the triphenylmethyl carbanion can be used. ... 

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