Triphenylmethylium salts

Y. V. Vozney and N. K. Kochetkov, Tritylation of secondary hydroxyl groups of sugars by triphenylmethylium salts, Carbohydr. Res., 54 (1977) 300-303. 

As it was already discussed, these salts are easily formed by hydride transfer from the corresponding dioxolanes and triphenylmethylium salts but react differently with nucleophiles — exclusively by addition. Strong nucleophiles mostly form the kinetic product by substitution at C-2, weaker ones yield the thermodynamic products throu ring opening at 0-1—C-5 (0-3—C-4). These paths of the ambident reactivity, described in detail by Pittman and by Perst are shown below ... 

Dioxolenium salts initiate polyn izatkin of THF with rate constants comparable to that of projagation and provide a class of convenient initiators that can be prepared with a variety of anions, since the starting triphenylmethylium salts are available with SbFj, AsFj, CIOJ, BFJ, and PFg anions. A laboratory-scale preparatk>n, identification, properties and H-NMR spectra of unsubstituted diox-olenium salts are described in Refs. 71 and 72. 

The AH0 and ASq values of triphenylmethylium salts are also close to those found for poly-THF and polyoxepane ion-pairs, being equal to - 2.2 kcal mole and - 25.0 e. u., respectively (measured for AsF and SbFg anions in CH2CI2 at 25... 

Heublein a.o. ° have studied stability of various complex anions in decomposition of triphenylmethylium salts, assuming that the rate constant of collapse of ion pair can be used as a measure of anion stability ... 

Three significant hydrocarbon ions, [S" ], [9 ] and [10 ], should be mentioned as additional examples of the formation of stable salts of the hydrocarbon ions. The triphenylmethylium ion [8" ], as a prototypical aromatic carbocation, reacts with the chloride ion to give the chloride, a... 

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... 

Of particular interest are quantitative syntheses of extremely deliquescent and reactive alkyliminium salts by milling of solid imines with oxonium salts (178), triphenylmethylium tetrafluoroborate (180), or triphenylmethyl chloride (183) (dry atmosphere in these cases). The sensitive salts 179, 181, and 184 are formed in pure form without any waste by an easy experimental technique [10]. Precisely weighed samples of 178 (ca. 2 mmol), 180 (ca. 1 mmol), or 183 (1.00 mmol) were placed in a ball-mill under argon together with the precise equivalent of 54a,f,g, or 54f, or 182, respectively. The Teflon gasket was closed... 

Stability of triphenylmethylium (trityl) salts is due to the resonance stabilization and the lack of /3-protons which could undergo elimination. Several trityl salts are commercially available, they may also be easily prepared by direct mixing of components or by the silver salt method, e.g. [17,18] ... 

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... 

Salts of (XXVII) and substituted derivatives have been prepared by reaction of bicyclo[6.3.0]undecapentaenes with triphenylmethylium... 

Some carbenium salts that form sterically hindered addition product, that is, triphenylmethylium (trityl) salts may initiate through hydride ion abstraction as shown in Scheme 9 (route Oxocarbenium ions thus formed serve as actual... 

For some initiators, however, an alternative route of initiation is possible. For triphenylmethylium (tritylium) salts, the preinitiation step is highly reversible (Scheme 13). ... 

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