Triphenylmethyl carbocation

Because of the high stability of the triphenylmethyl carbocation, the reductive ether cleavage of trityl ethers with EtySiH/trimethylsilyl triflate (TMSOTf) is highly successful. This reaction even occurs in the presence of highly reactive sugar ketals, leaving the ketals intact (Eq. 126).269... 

The triphenylmethyl carbocation shows its stability because the positive charge on the carbon is distributed uniformly over a number of structures. 

Probably the first isolation of a triphenylmethyl carbocation salt was by Gomberg and Cone (68) who successfully prepared the perchlorate from the corresponding chloride. A direct synthesis from the carbinol was achieved at about the same time (69), and more recently the preparation of the perchlorate and tetrafluoroborate have been much improved (70). Anderson (7/) succeeded in recording the characteristic visible absorption spectrum of the ion in concentrated acids, and Fairbrother and Wright (72) observed the same absorption when triphenylmethyl bromide was ionised in benzene in the presence of stannic bromide. 

Trityl ethers are easily cleaved by mild pro tic acids such as aqueous acetic or trifluoroacetic acid owing to the stability of the triphenylmethyl carbocation. They are also labile in the presence of Lewis acids such as ZnBr2—MeOH, FeCl3 or BF3 Et20.2 Trityl ethers can be cleaved selectively in the presence of TBDMS ether and isopropylidene acetals by brief exposure to formic acid.22 Catalytic hydrogenation has also been used to effect (9-detritylation. 

The introduction and cleavage of the trityl ether proceeds through a very well-stabilised triphenylmethyl carbocation. In the case of trityl ether bond formation, the reaction is performed under anhydrous conditions and the carbocation, which is formed by an SN1 mechanism, reacts with an alcohol. In the case of cleavage, the triphenylmethyl carbocation ion is formed by treatment with acid, which is then trapped by water or a nucleophilic solvent to give trityl alcohol or other derivatives, respectively. Trityl ethers have also been used to protect thiols. 

Most carbocations are quite unstable and have only a fleeting existence as intermediates in reactions such as the SN1 substitution. However, some, such as the triphenylmethyl carbocation, are stable enough that they can exist in significant concentrations in solution or even can be isolated as salts. 

When triphenylmethanol is dissolved in concentrated sulfuric acid, a solution with an intense yellow color is formed. The yellow species is the triphenylmethyl carbocation, formed by the following reaction ... 

If the anion is not very nucleophilic, solid salts containing the triphenylmethyl carbocation can actually be isolated. Thus, the tetrafluoroborate salt, Ph3C+ BF4 , can... 

Prelab Exercise Write all of the resonance structures of the triphenylmethyl carbocation. 

The reactions of triphenylmethanol are dominated by the ease with which it dissociates to form the relatively stable triphenylmethyl carbocation. When colorless triphenylmethanol is dissolved in concentrated sulfuric acid, an orange-yellow solution results that gives a fourfold depression of the melting point of sulfuric acid, meaning that four moles of ions are produced. If the triphenylmethanol simply were protonated only two moles of ions would result. 

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

This hydrocarbon is more basic than most hydrocarbons and will lose a proton to the triphenylmethyl carbocation to give triphenylmethane and the cycloheptatrienide carbocation, the tropilium ion. To demonstrate that hydride ion transfer has taken place, isolate triphenylmethane. 

Is the propeller-shaped triphenylmethyl carbocation a chiral species ... 

Definitive work using a range of triphenylmethyl carbocation salts with different anions has shown the importance of the tightness of the growing ion pair and... 

Triphenylcarbinol (PhsCOH), 281 Triphenylmethyl carbocation, 106, 146, 281 Triphenylphosphine (PPhs)... 

The triphenylmethyl carbocation, [CPhslfPFe], abstracts a hydride from a methyl group in OsMe2 (PR3) and... 

Cozens and co-workers found that the xanthyl cation was spontaneously generated by adsorption of 9-xanthenoI within several acidic zeolites [33]. The cation was characterized spectroscopically and was found to be stable over a long period of time. Diffuse reflectance spectra of the zeolite composites exhibited absorption bands similar to those for the xanthyl cation in solution [10]. Fluorescence spectra also corresponded to cation solution spectra. The 9-phenylxanthyl, 9-phenylfluorenyl, and triphenylmethyl carbocations are similarly formed and readily detectable on dry montmorillonite clay minerals [34]. 

Carbenes were the latecomers to the company of stable examples of organic reaction intermediates. The 100 anniversary of Gomberg s discovery of the stable triphenylmethyl radical was celebrated in 2000, and Gomberg and others were familiar with the triphenylmethyl carbocation by 1910. Stable anions derived from hydrocarbons like the fluorenyl anion were being investigated by Conant and McEwan in the decade 1930-40, but the first isolable species which might be considered to be carbenes were not reported until 1988 by Bertrand... 

The requisite tricarbonyl(ii -cyclohexadienylium)iron cation 2 is prepared from 1,3-cyclohexadiene and FefCOlj followed by treatment with triphenylmethyl carbocation tetrafluoroborate as shown. Subsequent oxidative cyclization of iron complex 3 with commercial manganese dioxide affords the iron-complexed dihydrocarbazol-3-one 4. 

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