Triphenylmethyl salts

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. 

We now examine the theory recently put forward by Penczek [6]. Strictly, we would not need to concern ourselves with it, as the system to which it is said to apply does not involve initiation by perchloric acid, but by triphenylmethyl salts. None the less, it is useful to consider it briefly. Penczek believes that the propagating species in his systems is an oxy-carbenium ion stabilised by co-ordination of two oxygens from a polymer molecule, as shown in structure 6 ... 

Finally, highly reactive salts such as triphenylmethyl hexachloroantimonate provide very convenient scavenging agents for the removal of the last traces of nucleophilic impurities in polymerisation solvents. Penczek (35) has taken this further in developing a spectroscopic technique for the determination of trace quantities of residual water by reaction of the latter with known concentrations of triphenylmethyl salts, according to the following equilibria ... 

Treatment of 2/Z-chromenes which have at least one hydrogen atom at C-2 with acid gives a mixture of chroman and benzopyrylium salt (68T949). Reaction of a chromerte with triphenylmethyl salts gives essentially only the benzopyrylium salt (67AC(R)1045, 72CR(C)(274)650>. 

Initiation with Tropylium Ion. Tropylium hexachlorantimonate reacts with vinyl alkyl ethers in a manner very similar to the reactions of triphenylmethyl salts. Again, rapid initiation is followed by propagation without apparent termination. Termination can be demonstrated to be absent from experiments in which fresh samples of monomer are added to completed polymerizations, whereupon the measured reaction rates parallel those previously recorded (Table II). Molecular weights of the polymers from isobutyl vinyl ether are very similar to those obtained with triphenylmethyl salts as initiators and again give clear evidence for excessive monomer transfer. Gas chromatographic analysis of the reaction mixtures showed that cycloheptatriene (product of hydride abstraction) was not present which indicates clearly that initiation must arise via addition of the tropylium ion to the vinyl ether—i.e.,... 

As with triphenylmethyl salts, the hexachlorantimonate derivative of tropylium ion is by far the most convenient to handle and to use as a catalyst. In this case, thermal decomposition or hydrolysis of the tropylium salt is almost nonexistent under ordinary conditions, and the catalyst can be prepared conveniently, purified, and stored over long... 

Chemical oxidation of silyl enolates has been performed with a variety of inorganic and organic oxidants such as ozone, copper(II) salts, Pb(OAc)4, Ag20, hypervalent iodine compounds such as iodosobenzene in methanol, (NH4)2[Ce(N03)6], xenon difluoride, tetranitromethane, halogens, nitronium-, diazonium- and triphenylmethyl salts, chloranil and ddq. 

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. 

Led with and his co-workers [50] have also studied the mechanism of initiation by triphenylmethyl salts in some detail, by following the decay of the characteristic visible absorption maximum of the initiating cation. The kinetic equation which has been established for both methylene chloride and THF solvents is... 

In 1973 Penczek and Kubisa [137] published a detailed study of the interaction of triphenylmethyl hexachloroantimonate with 1,3-dioxolane. Polymerization occurs as a result of the interaction but the triphenylmethyl salt itself does not initiate the polymerization. Instead the sequence of reactions... 

Figure 10 TegClia ion in the triphenylmethyl salt with bond lengths (in A, standard error 0.005 A) and bond angles (averaged over chemically equivalent angles)...

In addition to the triphenylmethyl salts, other sources of carbenium ions are availaHe for use in one-electron oxidation reactions. Thus, the complex, [W(NO)2(P(OMe)3K n5-Cp)l is oxidized to the corresponding... 

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