Triphenylmethyl carbonium ion

This method is a modification of the method originally published by Kursanov and Vol pin.6 Tropylium salts have also been prepared by bromination-dehydrobromination of tropili-dene,6 and by the hydride-exchange reaction between tropilidene and triphenylmethyl carbonium ion.7... 

Hirschler and Hudson (36/6), however, favor the opinion that Bronsted sites are exclusively responsible for the activity of silica-alumina. In studying the adsorption of perylene and of triphenylmethane, they concluded that carbonium ions are not formed by a hydride abstraction mechanism as claimed by Leftin (362). Instead, triphenylmethane is oxidized by chemisorbed oxygen to triphenylcarbinol in a photo-catalyzed reaction, followed by reaction with a Bronsted acid giving water and a triphenylmethyl carbonium ion. After treatment with anhydrous ammonia, the organic compound was recovered by extraction as triphenylcarbinol. About thirteen molecules of ammonia per assumed Lewis site were required to poison the chemisorption of trityl ions. The authors explain the selective inhibition of certain catalyzed reactions by alkali by assuming that only certain of the acidic protons will ion-exchange with alkali ions. 

The rate of alkylation in the above sequence is actually dependent on the rate of dissociation of the alkyl halide and the nucleophilicity of the cyanide complex (12). The most stable carbonium ion derived from the alkyl halides in the above sequence—i.e., having the highest rate of dissociation—is the triphenylmethyl-carbonium ion, which showed widely different rates of reaction with various cyanide complexes having widely different nucleophilicities toward crabon (18) ... 

Fyfe et al. (355) were able to produce a very informative 13C CP/MAS NMR spectrum of the triphenylmethyl carbonium ion by using the tetra-fluoroborate counterion and by employing simultaneous 19F and H decoupling during spectral acquisition (see Fig. 81). The nonequivalence of the ortho and meta carbons is readily seen in the spectrum. 

Radical mechanisms account for the stoichiometry for reduction of triketohydrindane by N(5)-ethyldihydroflavin and reduction of triphenylmethyl carbonium ion species by dihydroflavin, (24). One-electron reduction of quinone by N(5)-ethyldihydroflavin also has been shown. These results are not surprising since the substrates and flavin support reasonably stable radical states. Radical species also can be established as intermediates in the oxidation of 9-hydroxyfluorene and methyl mandelate by Flox (Equations 28 and 29, respectively). The reactions of Equations 28 and 29 are facile when carried out in... 

A somewhat analogous ion-radical reduction of carbonium ions to free radicals has been shown to occur (Conant and Chow, 48) in the reduction of various triphenylmethyl carbonium ions with chromous or titanous ion. 

The sulfonium salt was shown to spontaneously oxidize highly stabilized free radicals such as the triphenylmethyl radical to form the corresponding triphenylmethyl carbonium ion.(20) It would also appear that the dimethoxybenzylic free radical (a Norrish Type I photocleavage product of 2,2-dimethoxy-2-phenyl-acetophenone) is similarily oxidized by the arylsulfonium salt ( ). 

Newman and Deno49 have observed that mono-, di-, and triphenylmethyl carbonium ions absorb in the same wavelength region, thus indicating incomplete coplanarity in the di- and triphenyl derivatives. Heilbronner and co-workers50 studied the absorption spectra of benzotropyiiura series. The absorption spectra of a number of tropyliimi salts have been recorded (Chapter 5). 

Pyrylium phosphonic acids (60a) have been prepared by the reaction of triethyl phosphite with pyrylium salts followed by oxidation of the product (either before or after hydrolysis) with the triphenylmethyl carbonium ion. This approach has been used to obtain the analogous thiopyrylium phos-phonates (60b) and the cyclopropene and tropylidene derivatives (61) and (62). The enamine phosphonates (63) are readily accessible from addition... 

Hydride abstraction from o-alkyl derivatives with -hydrogen atoms. Treatment of the ethyl derivative CsH5Fe(CO)2C2H5 with triphenylmethyl carbonium ion gives salts of the 7T-ethylene cation [CsH5Fe(CO)2C2H4]+ (XLIV R = H) (214)... 

The stability of the triphenylmethyl carbonium ion is well known and yet the above reaction proceeds at a greater rate than does the solvolysis of triphenylmethyl acetates. 

The selectivity relationship merely expresses the proportionality between intermolecular and intramolecular selectivities in electrophilic substitution, and it is not surprising that these quantities should be related. There are examples of related reactions in which connections between selectivity and reactivity have been demonstrated. For example, the ratio of the rates of reaction with the azide anion and water of the triphenylmethyl, diphenylmethyl and tert-butyl carbonium ions were 2-8x10 , 2-4x10 and 3-9 respectively the selectivities of the ions decrease as the reactivities increase. The existence, under very restricted and closely related conditions, of a relationship between reactivity and selectivity in the reactions mentioned above, does not permit the assumption that a similar relationship holds over the wide range of different electrophilic aromatic substitutions. In these substitution reactions a difficulty arises in defining the concept of reactivity it is not sufficient to assume that the reactivity of an electrophile is related... 

It is known that tropylium may be prepared from tropylidene via hydride abstraction by PhgC or MegC carbonium ions therefore, it is very likely that here too the dehydrogenation is a hydride transfer from the 1,5-dione to an acceptor. A similar dehydrogenation of chromanones to chromones, with triphenylmethyl perchlorate was reported. A study of the electrooxidation of 1,5-diones on a rotating platinum electrode showed that 1,5-diaryl-substituted diones afford pyrylium salts in these conditions and that the half-wave potentials correlate with yields in chemical dehydrogenations. 

Triphenylcarbinol in sulfuric acid solution has a spectrum indicating the presence of the same carbonium ion responsible for the conductivity of triphenylmethyl chloride in liquid sulfur dioxide.171 In confirmation of this the freezing point depression is four times that of substances dissolving to give only one mole of particles per mole of dissolved substance.171-173... 

As befits their status as compounds well-known to be in equilibrium with carbonium ions in suitable solvents, triphenylmethyl halides and related compounds give particularly unambiguous evidence of reaction involving ionic intermediates. In polar solvents they give... 

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. 

The site of reaction on an unsaturated organometallic molecule is not restricted to the most probable position of the metallic atom or cation or to a position corresponding to any one resonance structure of the anion. This has been discussed in a previous section with reference to the special case of reaction with a proton. Although the multiple reactivity is particularly noticeable in the case of derivatives of carbonyl compounds, it is not entirely lacking even in the case of the derivatives of unsaturated hydrocarbons. Triphenylmethyl sodium reacts with triphenylsilyl chloride to give not only the substance related to hexaphenylethane but also a substance related to Chichi-babin s hydrocarbon.401 It will be recalled that both the triphenyl-carbonium ion and triphenylmethyl radical did the same sort of thing. 

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... 

Polarography of Carbonium Ions in Acid Solution, Part I, The Triphenylmethyl Ion in Sulphuric Acid, P.H. Plesch and I. Sestakova, Journal of the Chemical Society, (B), 1970, 87-92. 

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. 

The interpretation of the results of tracer experiments of this sort is sometimes complicated by, sO-exchange reactions. l80 from the solvent may be incorporated into the unreacted ester as hydrolysis proceeds (see below, p. 105), or into either or both of the products. The exchange reaction is significant with alcohols, such as triphenylmethyl alcohol, which give rise to relatively stable carbonium ions under acidic conditions (see, for example, refs. 67, 85), viz-... 

A number of years ago triphenylmethyl cation, Ph3C, formed in situ by dissociation of triphenylmethyl chloride, was shown [73] to initiate the polymerization of 2-ethylhexyl vinyl ether in m-cresol solvent. More recently certain stable carbonium ion salts, notably hexachloroantimonate (SbCls) salts of cycloheptatrienyl (tropylium, C7H7) and triphenylmethyl cations have been shown [74, 50] to be very efficient initiators of the cationic polymerization of many reactive monomers [27, 29, 75]. Since the discovery of the effectiveness of the SbClg salt, triphenylmethyl salts with different anions have also been used [76—78]. The most detailed kinetic studies using these initiators have been carried out on alkyl vinyl ethers [27, 30] and A-vinylcarbazole [39] in homogeneous solution in methylene chloride. 

In several ways this reaction resembles the molecular rearrangements involving transitory carbonium ions. The reaction is acid-catalyzed60 (see the Schmidt reaction). In the presence of triphenylmethyl free radicals no mixed products are formed.61 The migrating group never leaves the field of the electronically deficient atoms concerned since the Curtius reaction with (—)o-(2-methyl-6-nitrophenyl)-benzoic acid (LXXV) produces an optically active amine62 (LXXVI). 

The 9-phenylxanthyl radical is a resonance-stabilized triphenylmethyl analog. The corresponding carbonium ion and carbanion are also stabilized and can be prepared in sulfolane, so that A//het can be directly measured.The data for benzyl and t-butyl are obtained by measuring the reduction and oxidation potentials of the radicals in acetonitrile. The results show that C6H5CH2 and (CH3)3C are much harder than the 9-phenylxanthyl radical (the latter is just one of several studied with similar properties ). The solution hardnesses are then responsible for the difficulty in forming the ions in the benzyl and t-butyl cases, and the stability of the ions in the resonance-stabilized cases. The effect of the small hardness in the latter cases also is evident in the small bond energy for homolytic dissociation. 

The idea of carbonium ions is quite old in organic chemistry. Olah has traced the early history (15). In 1902 Von Baeyer wrote of carbonium salts in explaining the deep color formed when triphenylmethyl chloride was dissolved in sulfuric acid. Carbonium ions as reaction intermediates were proposed by Meerwein in 1922, and much used by Ingold, Hughes, and others in England soon thereafter. F. C. Whitmore in the USA from 1932 on showed how carbonium ions as reaction intermediated could explain the acid-catalyzed reactions of alkylation, polymerization, and isomerization. His studies were summarized in a review article in 1948 ( ). More recently, of course, there have been many spectroscopic studies of stable carbonium ions formed in highly acidic solutions at low or moderate temperatures, as, for example, in the works of N. C. Deno and G. A. Olah. 

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