Trityl anion

Charge delocalization can also be promoted by multiple aryl substitution at the metalated carbon atom. A lucid example is the Ph3C trityl anion. The central carbon atoms in [(TMEDA)LiCPh3]2 (192) and [(Et20)2LiCPh3] 37 (193) show almost ideal planar environment. The three phenyl groups cannot all be arranged in a coplanar fashion due to... 

In the triphenylmethyllithium-tetramethylethylenediamine complex 30 (52), the lithium atom is not located directly over the central carbon, as might have been expected, but rather has four close contacts to the trityl anion. Besides the jp -hybridized central carbon, these involve two carbon atoms of one nearly coplanar phenyl group, and one carbon atom of a somewhat twisted second ring. The third phenyl is nearly perpendicular, so that the negative charge is largely delocalized over only two of the phenyls. 

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. 

Finally, in nonpolar aprotic solvents such as benzene or cyclohexane trityl anion formation is possible only with activating agents more powerful than /-BuONa vide infra). 

As already shown by many of the above examples, physical methods, especially spectroscopic ones, are having a very strong impact on the development of physical organic chemistry, and it is only for lack of space that such important techniques as laser flash spectroscopy can only be mentioned here in passing. Having established itself as a highly efficient method for the study of carbenes and carbocations [63] laser flash spectroscopy has also been used to generate carbanions like the trityl anion recently, [64] and promises to become a useful method for the determination of carbanion reactivity as well. 

Scheme 11.8 Reactions of trityl anion and trityl cation.

For a long time the trityl anion has been used as a proton abstractor of weakly acidic substances. Its inertness toward the ketonic function has been regarded as steric in origin. It should be emphasized that the trityl anion is quite soft as demonstrated by the following reactions (51). 

But we can go further than this. The trityl anion (Structure 13) is much more stable than the benzyl anion, so that we can envision the possibility of extensive charge delocalization, which in part concentrates its effect into a small number of bonds. 

It will be appreciated that, a priori, it could not be known if the ionization shown in Equation II-G to produce the chloride anion and the trityl cation is correct or if the chloride cation and the trityl anion were being generated. The former was more in concert with other evidence, some of which was only established subsequently. 

NaH stirred ca. 45 min. at 65-70° under Ng in excess dimethyl sulfoxide until Hg-evolution is complete, allowed to react with 1 equivalent of ethyl triphenyl-phosphonium bromide, then with 0.85 equivalent of benzophenone 1,1-di-phenyl-l-propene. Y 97.5%.—The reactivity of the methylsulfinyl carbanion (formed by reaction of NaH with dimethyl sulfoxide), which is even more basic than the trityl anion, is sufficient to convert phosphonium salts into ylides thereby permitting a simple and convenient modification of the Wittig reaction. E. J. Gorey and M. Ghaykowsky, Am. Soc. 8A, 866 (1962) / -diketones from carboxylic acid esters and ketones (s. Synth. Meth. 6, 737), sym. ) -diketones, s. J. J. Bloomfield, J. Org. Chem. 27, 2742 (1962). 

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

The intermediate N-acylpyridinium salt is highly stabilized by the electron donating ability of the dimethylamino group. The increased stability of the N-acylpyridinium ion has been postulated to lead to increased separation of the ion pair resulting in an easier attack by the nucleophile with general base catalysis provided by the loosely bound carboxylate anion. Dialkylamino-pyridines have been shown to be excellent catalysts for acylation (of amines, alcohols, phenols, enolates), tritylation, silylation, lactonization, phosphonylation, and carbomylation and as transfer agents of cyano, arylsulfonyl, and arylsulfinyl groups (lj-3 ). 

On the basis of these results we embarked on a systematic study on the synthesis of vinyl cations by intramolecular addition of transient silylium ions to C=C-triple bonds using alkynyl substituted disila alkanes 6 as precursors.(35-37) In a hydride transfer reaction with trityl cation the alkynes 6 are transformed into the reactive silylium ions 7. Under essentially nonHnucleophilic reaction conditions, i.e. in the presence of only weakly coordinating anions and using aromatic hydrocarbons as solvents, the preferred reaction channel for cations 7 is the intramolecular addition of the positively charged silicon atom to the C=C triple bond which results in the formation of vinyl cations 8-10 (Scheme 1). 

The initiation of the cationic polymerisation of alkenes is examined in detail by means of simple thermodynamic concepts. From a consideration of the kinetic requirements it is shown that the ideal initiator will yield a stable, singly charged anion and a cation with a high reactivity towards the monomer by simple, well defined reactions. It must also be adequately soluble in the solvent of choice and for the experimental method to be used. The calculations are applied to carbocation salts as initiators and a method of predicting their relative solubilities is described. From established and predicted data for a variety of carbocation salts the position of their ion molecule equilibria and their reactivity towards alkenes are examined by means of Born-Haber cycles. This treatment established the relative stabilities of a number of anions and the reason for dityl, but not trityl salts initiating the polymerisation of isobutene. 

The resolution of the above-mentioned conflicts is not yet clear, but this author thinks that a profitable approach can be made by investigating the nature of the different complexes that may be involved. This line of thought was catalysed partly by the papers from the Jena school concerning the formation of a variety of complexes between cations and molecules [8, 9]. Unfortunately, this extensive work is less valuable than it might be and difficult to use because no account was taken of the intervention of binary ionogenic equilibria (BIE) in the systems studied. These mostly comprise trityl and tropylium ions and a variety of composite anions of the MtX n+1, type, i.e., typical components of BIE [10]. 

The value of 0.05 M from equation (7) is consistent with values of A as < 1 and ( /fes) < 1 for reactions in water. For example, = 0.3 gives (Jdjki ) = 0.17 for the relative rate constants for addition of solvent to the carbocation-anion pair and free carbocation. By comparison, the three-fold smaller rate constant for addition of water to an intramolecular trityl carbocation-sulfonate ion pair compared with addition to the analogous substituted trityl carbocation o-sulfonyl methyl ester has been used to estimate a value of (kjk ) = 0.33. " ... 

The major obstacle of the hydride transfer reaction is the steric bulk of the trityl cation as the reagent of choice. Substrates that will allow the isolation of cations RsE, free from intramolecular and/or intermolecular interactions with solvent molecules or anions, need to have bulky substituents and therefore the hydride transfer reaction between the hydride and trityl cation is severely hampered or it is even impossible. Another drawback of this method is the limited availability of the starting hydrido compound, which for example, is not available for lead compounds, due to the high reactivity of lead(IV) hydrides. 

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