Triphenylmethylium cations

In spite of many publications in which equilibria of the type described in Scheme (19) are postulated, accepted or rejected on various grexmds, the cartenium-onium eqpilibrium has not been determined quantitatively. Only recently has this equilibrium been studied in a modd sydem, using tri enylmethylium cation as a model carbenium ion and various linear or cyclic ethars or acetals as nucl )philes At room temperature in CH2CI2, the equflibrium constant wasfound to be K = 1.6 mole 1 and Ke = 6.3 10 mole 1 for THF and 1,3-diox[Pg.17]

Attempts to measure kn for the following cyclic acetals and ethers were unsuccessful because of the too low value of kn (below 10 mole"" 1 s at 25 °Q 2-Trichk>ioinethyi l,3 x-olane, 1,3,5-trioxane, tetrahydropyran, and 1,4-dioxane. The non-planar and puckered structure of the last three compounds can hinder a sufficiently close approach to the central atom of the triphenylmethylium cation necessary for hydride transfer to proceed. ... 

It would be probably instructive to take two examples from Table 19 in order to put a rationale for these data. There is a competition, in every case, between addition, H , and transfer. The competition between addition and H transfer is discussed in Section 3.2.3 with triphenylmethylium cation taken as an example (no expulsion possible for structural reasons). 

Advantage can be taken of the ionizing and dissociating effects produced by a solvent to activate inert molecules and initiate a polymerization. For instance, triphenylmethyl chloride in pure sulfuric acid solution undergoes an instantaneous ionization and produces a triphenylmethylium cation with a characteristic red color ... 

Besides the carbocations mentioned above, numerous highly stable carboca-tions have been isolated as the salts of inorganic anions. Azulene analogues of triphenylmethylium ion [ll ]-[20 ], [21 j, [22 " ] and [23 j trisubsti-tuted cyclopropenylium ions [l" ] and [24 ]-p6 ] cyclopropyl-substituted tropylium ions [27 ]-[30 ] tropylium ions annelated with bicyclic frameworks [31 ]-[36 ] and [37 ]-[39 ] fulvene-substituted cyclopropenylium [40 ] and tropylium [41 ] ions a tropylium ion condensed with acenaphthylene [42 ] and a cation containing a 147t periphery [43 ] have... 

The glycosylations were carried out in dry dichloromethane in the presence of 0.1 equiv of triphenylmethylium perchlorate. It was assumed that the triphenylmethy-lium cation attacks the nitrogen of the thiocyanate group whereas the oxygen of the trityl ether attacks the anomeric carbon in a push-pull fashion. The trityl isothiocyanate formed was found to be unreactive under the reaction conditions. The disaccharide derivatives with (1-6), (1-4), (1-3) and (1-2) linkages have been obtained in good yields. 

The paper contains several innovations in the interpretation of polarographic reduction potentials, Eyi. One is the recognition that the Eyi obtained in the presence of base-electrolytes is not that of an isolated, solvent-solvated cation, but of one which is part of an ion-pair or of a higher aggregate. A practically useful innovation is to use the Em of the triphenylmethylium ion as the zero of the potential scale in all solvents. By means of this device one can compare a wide range of Ey2 differences in different solvents, and it is especially useful because that ion is stable in strongly acidic media in which the commonly used marker ferrocene decomposes. 

In view of the useful interpretations which can be given to the differences in E1/2 values, an obvious step is to select the E1/2 of one convenient ion as a zero or reference point. We follow Taft [14] and Breslow [15] in selecting E1/2 for the triphenylmethylium (trityl) cation, as this ion is easy to procure and is stable under acidic conditions. It seems useful to define a quantity Ef/2 which is 1/2 of the species in question minus EV2 of the triphenylmethylium ion measured under the same conditions of concentration, temperature, solvent, and nature and concentration of base electrolyte, and against the same reference electrode ... 

Trityl (triphenylmethylium) and tropylium cations are two commercially available carbenium ions. They react with alkenes by either direct addition or by hydride abstraction tropylium ions are usually less reactive than trityl. As shown in Eq. (39), trityl ions initiate styrene polymerization by direct addition. 

As we have mentioned in Chap. II, it is possible to prepare stable, fully ionised crystalline salts of carbocations provided the positive charge is sufficiently delocalised over the cationic moiety and the nucleophilicity of the anion is sufficiently low. Within this family of compounds, we will limit our attention to two major groups of salts with which most of the fundamental studies on cationic polymerisation of olefins has been carried out the triphenylmethylium (trityl) and the cycloheptatrienylium (tropylium) salts. It has been a common practice to assume that if these salts are fully ionised in the solid state, they will also be completely dissociated in solution. Thus, the equilibria... 

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