Triphenylmethyl cations

If triphenylmethyl chloride in ether is treated with sodium, a yellow colour is produced due to the presence of the anionic spiecies PhsC". Alternatively, if triphenylmethyl chloride is treated with silver perchlorate in a solvent such as THF, the triphenylmethyl cation is obtained. More conveniently, triphenylmethyl salts, PhsC X", can be obtained as orange-red crystalline solids from the action of the appropriate strong acid on triphenylcarbinol in ethanoic or propanoic anhydride solution. The perchlorate, fluoroborate and hexafluoro-phosphate salts are most commonly used for hydride ion abstraction from organic compounds (e.g. cycloheptatriene gives tropylium salts). The salts are rather easily hydrolysed to triphenylcarbinol. 

Photochromism Based on Dissociation Processes. Both heterolytic and homolytic dissociation processes can result in the generation of a photochromic system. An example of an heterolytic process is the reversible formation of triphenylmethyl cation, by photolysis of... 

The N+ relationship, as discussed above, is a systematization of experimental facts. The equation of Scheme 7-4 has been applied to nearly 800 rate constants of over 30 electrophiles with about 80 anionic, neutral, and even cationic nucleophiles covering a range of measured rate constants between 10-8 and 109s 1 (Ritchie, 1978). Only about a dozen rate constants deviated from the predicted values by more than a factor of 10, and about fifty by factors in the range 5-10. It is therefore, very likely that this correlation is not purely accidental. Other workers have shown it to be valid for other systems, e.g., for ferrocenyl-stabilized cations (Bunton et al., 1980), for coordinated cyclic 7r-hydrocarbons (Alovosus and Sweigart, 1985), and for selectivities of diarylcarbenes towards alkenes (Mayr, 1990 Mayr et al., 1990). On the other hand, McClelland et al. (1986) found that the N+ relationship is not applicable to additions of less stable triphenylmethyl cations. 

B. Triphenylmethyl (Trityl) Cation The highly stable triphenylmethyl cation (85) is found to undergo a wide variety of photoreactions depending on such variables as the nature of the solvent, the pH of the medium, and the presence or absence of... 

In weaker acid systems, other reactions involving the triplet state supervene to the exclusion of dimerization. Photolysis of 85 in 3-3% sulfuric acid, 96-5% acetic acid, and 0-2% water gave as products tri-phenylmethane (93), 9-phenylfluorene (94), 6is-9-phenylfluorenyl peroxide (95) and benzophenone (96). When benzene was present, tetra-phenylmethane (97) was also formed in addition to the other products. When the triphenylmethyl cation is irradiated in 3-3% H2SO4, 80 1% HOAc, 16-4% toluene, and 0-2% H2O, the products observed were... 

In an indirect method of achieving this conversion, the silyl enol ether of a simple ketone is treated with or with triphenylmethyl cation (for another indirect... 

Scheme 2.25 shows some examples of additions of enolate equivalents. A range of Lewis acid catalysts has been used in addition to TiCl4 and SnCl4. Entry 1 shows uses of a lanthanide catalyst. Entry 2 employs LiC104 as the catalyst. The reaction in Entry 3 includes a chiral auxiliary that controls the stereoselectivity the chiral auxiliary is released by a cyclization using (V-methylhydroxylamine. Entries 4 and 5 use the triphenylmethyl cation as a catalyst and Entries 6 and 7 use trimethylsilyl triflate and an enantioselective catalyst, respectively. 

Thus a number of canonical structures is possible. Like triphenylmethyl cation, diphenylmethyl cation is also very stable. In some cases the carbocations are so stable that their salts have been... 

Let us take the case of phenols and nitro-phenols. The phenoxide ion C6 H50 is colourless, but p nitro phenol is yellow, because the -N02 group in /(-position produces a yellow ion—p.N02-C6H40 . Similarly triphenylmethyl cation is colourless or pale yellow or pale yellow in acidic solution, but the corresponding ion containing two or three OH groups in para position produces deep red colour. 

Hydride Ion Transfer between Triphenylmethyl Cation and Tetrahydrofuran, 1,3-Dioxolan and 1,3-Dioxepan, Kabir-ud-Din and P.H. Plesch, Journal of the Chemical Society, Perkin II, 1978, 937-938. 

Figure 14. Schematic presentation of correction of the proton chemical shifts for the triphenylmethyl cation and anion.

One of the most stable carbocation structures is the employing all three rings. Trityl chloride ionizes read-triphenylmethyl cation (trityl cation). In this struc- ily, and can capture an available nucleophile, ture, the positive charge is stabilized by resonance... 

Carbocations are a class of reactive intermediates that have been studied for 100 years, since the colored solution formed when triphenylmethanol was dissolved in sulfuric acid was characterized as containing the triphenylmethyl cation. In the early literature, cations such as Ph3C and the tert-butyl cation were referred to as carbonium ions. Following suggestions of Olah, such cations where the positive carbon has a coordination number of 3 are now termed carbenium ions with carbonium ions reserved for cases such as nonclassical ions where the coordination number is 5 or greater. Carbocation is the generic name for an ion with a positive charge on carbon. 

Rate constants for the reactions of carbocations with added nucleophiles are obtained in LFP experiments as the slopes of linear plots of first-order rate constants for cation decay against the concentrations of added nucleophile. One of the first detailed studies using LFP showed that rate constants for the parent triphenylmethyl cation did not adhere to the simple Ritchie N+ relation of Eq. 13, but that the slope of a plot of log Nu versus N+ was significantly < 1 This finding has been verified... 

The study of carbocations has now passed its centenary since the observation and assignment of the triphenylmethyl cation. Their existence as reactive intermediates in a number of important organic and biological reactions is well established. In some respects, the field is quite mature. Exhaustive studies of solvolysis and electrophilic addition and substitution reactions have been performed, and the role of carbocations, where they are intermediates, is delineated. The stable ion observations have provided important information about their structure, and the rapid rates of their intramolecular rearrangements. Modem computational methods, often in combination with stable ion experiments, provide details of the stmcture of the cations with reasonable precision. The controversial issue of nonclassical ions has more or less been resolved. A significant amount of reactivity data also now exists, in particular reactivity data for carbocations obtained using time-resolved methods under conditions where the cation is normally found as a reactive intermediate. Having said this, there is still an enormous amount of activity in the field. 

Pyrans and thiins are also easily aromatized, e.g. (483) + S2Cl2 — 1-benzothiinium ion. 2H-Thiins are aromatized by hydride acceptors such as triphenylmethyl cations to give thiinium salts, and similar conversions produce pyrylium salts from pyrans. 

Initiation of tetrahydrofuran polymerisation by Ph C SbClg has been studied speetrophotometrieally by Ledwith and co-workers (20). Decay of the characteristic triphenylmethyl cation absorption, in methylene chloride was represented by the expression... 

More recently, salts of the triphenylmethyl cation Ph3C+ have been shown to be effective in converting the diones into pyrylium salts (61BSF538). The cation may be generated in situ, for example from triphenylmethyl chloride and antimony(V) chloride (69T1209). The r-butyl cation has been used for the same purpose (67T4001). Earlier workers considered that the synthesis involved dehydration of the diketone to a 4//-pyran followed by rapid oxidation to the pyrylium salt. In view of the successful application of Ph3C, the reaction... 

Oxidation of the ring system in thiins has also been achieved with hydride acceptors such as triphenylmethyl cations, and results in high yields of thiopyrylium salts. This is far more efficient than the disproportionation reactions discussed above, as the only byproduct is triphenylmethane. The reaction will be discussed in the section on synthesis. 

Additional phenyl substituents stabilize carbocations even more. Triphenylmethyl cation is particularly stable. Its perchlorate salt is ionic and stable enough to be isolated and stored indefinitely. 

The relationship between charge density and the NMR chemical shifts is, however, only qualitative and should be used with caution. Other factors, such as neighboring anisotropic effects of the substituents, should also be considered. The cationic center of triphenylmethyl cation (<5I3C 211.2 ppm), for example, is much shielded from that of tricyclopropylmethyl cation (270.9 ppm), which may erroneously lead to the conclusion that a phenyl is more stabilizing than the cyclopropyl group. 

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