Carbanions triphenylmethyl

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. 

Carbanions can, under suitable conditions, be oxidised thus the triphenylmethyl anion (85) is oxidised, fairly slowly, by air ... 

The ortho-ring junction that converts the triphenylmethyl structure into that of the ion LX increases the stability of the carbanion but decreases that of the carbonium ion. It will be recalled that this structural modification of the triphenylcarbonium ion had about the same effect as the introduction of one to two nitro groups. 

Another differential reaction is copolymerization. An equi-molar mixture of styrene and methyl methacrylate gives copolymers of different composition depending on the initiator. The radical chains started by benzoyl peroxide are 51 % polystyrene, the cationic chains from stannic chloride or boron trifluoride etherate are 100% polystyrene, and the anionic chains from sodium or potassium are more than 99 % polymethyl methacrylate.444 The radicals attack either monomer indiscriminately, the carbanions prefer methyl methacrylate and the carbonium ions prefer styrene. As can be seen from the data of Table XIV, the reactivity of a radical varies considerably with its structure, and it is worth considering whether this variability would be enough to make a radical derived from sodium or potassium give 99 % polymethyl methacrylate.446 If so, the alkali metal intitiated polymerization would not need to be a carbanionic chain reaction. However, the polymer initiated by triphenylmethyl sodium is also about 99% polymethyl methacrylate, whereas tert-butyl peroxide and >-chlorobenzoyl peroxide give 49 to 51 % styrene in the initial polymer.445... 

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. 

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. 

As you might expect, all the equilibria are now unfavourable, and this reaction does not go well under the normal equilibrating conditions (EtO- in EtOH), It can be made to go in reasonable yield if a stronger base is used, Traditionally, triphenylmethyl sodium is chosen. This is made from PhjCCl and sodium metal and is a very conjugated carbanion. 

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. 

Triphenylmethanol, prepared in the experiment in Chapter 31, has played an interesting part in the history of organic chemistry. It was converted to the first stable carbocation and the first stable free radical. In this experiment triphenylmethanol is easily converted to the triphenylmethyl (trityl) carbocation, carbanion, and radical. Each of these is stabilized by ten contributing resonance forms and consequently is unusually stable. Because of their long conjugated systems, these forms absorb radiation in the visible region of the spectrum and thus can be detected visually. 

Chapter 32 Reactions of Triphenylmethyl Carbocation, Carbanion, and Radical... 

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. 

However, the acidities of the benzyltriphenylphosphonium cation (pATa = 17.4) and of diethyl benzylphosphonate (pATa = 27.6) are sufficiently different to throw into question the likelihood of deprotonation of diethyl benzylphosphonate by the radical anion of 1. On the basis of a detailed study of the processes associated with the reduction of 1 (discussed later), it is likely that the effective base may well be the much more basic triphenylmethyl-derived carbanion, 4 in Scheme 20 (pATa of PI13CH = 30.6). This may possibly be produced by adventitious protonation of the fuchsone radical anion. Alternatively, the active EGB may be the dianion formed by disproportionation. 

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... 

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