Carbocations salts

When using a cation source in conjunction with a Friedel-Crafts acid the concentration of growing centers is most often difficult to measure and remains unknown. By the use of stable carbocation salts (for instance trityl and tropyhum hexachloroantimonate) the uncertainty of the concentration of initiating cations is eliminated. Due to the highly reproducible rates, stable carbocation salts have been used in kinetic studies. Their use, however, is limited to cationicaHy fairly reactive monomers (eg, A/-vinylcarbazole, -methoxystyrene, alkyl vinyl ethers) since they are too stable and therefore ineffective initiators of less reactive monomers, such as isobutylene, styrene, and dienes. 

Another reaction that has been applied to the generation of highly functionalized polymers is cationic polymerization [12-15]. Catalysts for cationic polymerizations are aprotic acids, protic acids, or stable carbocation salts. In these processes, the catalyst generally reacts with a cocatalyst to form an active initiated species. Initiation takes place by protonation of the monomer (Fig. 2A). Monomers that possess cation stabilizing groups, such as electron rich olefins, are preferred as they more readily undergo the desired polymerization process... 

Carbocations, 12 161—162, 163 from alkanes, 12 187 Carbocation salts, stable, 14 269 Carbochlorination, zircon, 13 82 Carbocyclic azo dyes, 9 245, 251 Carbodiimide (CDI) method, for covalent ligand immobilization, 6 396t Carbohydrate chemistry, microwaves in, 16 547-548... 

As a measure of their thermodynamic stability, the pAfR+ values for the carbocation salts were determined spectrophotometrically in a buffer solution prepared in aqueous solution of acetonitrile. The KR+ scale is defined by the equilibrium constant for the reaction of a carbocation with water molecule (/CR+ = [R0H][H30+]/[R+]). Therefore, the larger p/CR+ index indicates higher stability for the carbocation. However, the neutralization of these cations was not completely reversible. This is attributable to instability of the neutralized products. The instability of the neutralized products should arise from production of unstable polyolefinic substructure by attack of the base at the aromatic core. 

Nuclear magnetic resonance spectra were obtained for S02C1F solutions at -80 °C. b The symbols, FA and FB, denote axial and equatorial fluorine atoms, respectively. c The anion parameters apply to all carbocation salts and to the Br(OTeF5)2+ salt of Sb(OTeF5)6" also see ref 73. d Predicted from pairwise additivity parameters as described in the Chemical Shifts and Coupling Constant Trends section.f See ref 84 and 85.e The l23Te satellites were not observed. 

The initiation of the cationic polymerisation of alkenes is examined in detail by means of simple thermodynamic concepts. From a consideration of the kinetic requirements it is shown that the ideal initiator will yield a stable, singly charged anion and a cation with a high reactivity towards the monomer by simple, well defined reactions. It must also be adequately soluble in the solvent of choice and for the experimental method to be used. The calculations are applied to carbocation salts as initiators and a method of predicting their relative solubilities is described. From established and predicted data for a variety of carbocation salts the position of their ion molecule equilibria and their reactivity towards alkenes are examined by means of Born-Haber cycles. This treatment established the relative stabilities of a number of anions and the reason for dityl, but not trityl salts initiating the polymerisation of isobutene. 

In summary, a good initiator will yield a stable, singly charged anion and a cation with a high reactivity (large kf) towards the alkenes concerned, via simple, well defined reactions. The remainder of this paper is concerned with finding, by appropriate calculations, some carbocation salts which best meet the specified requirements. 

As far as we know, the literature contains no quantitative information on the solubility of carbocation salts and the qualitative information available indicated that such salts are relatively insoluble in the solvents of interest. Our first problem was to predict how changes in the structure of the salt would affect the solubility. Although the ideal solubility equation [Equation (1)] cannot be applied rigorously to ionic solutes, our first step was to examine its utility. 

If one wants to use only a metal halide as the initiator for an alkene polymerisation, an analysis similar to ours can be made for the metal halide alone. In addition, there is a 1 2 equilibrium for the organic halide [35, 36] and a molecular aggregate <-> single molecule equilibrium associated with the metal halide. Thus, a solution of a carbocation salt is more exactly described by the series of linked equilibria summarised in Scheme 5. 

Polymerizations initiated by ionizing radiation or stable carbocation salts such as trityl or tropylium hexachloroantimonate are useful for evaluating the free-ion propagation rate constant. Ionizing radiation yields free ions (in the absence of ion pairs) whose concentrations... 

The hrst X-ray crystal structure of a carbocation salt was reported in 1965. Triphenylmethyl perchlorate (27) has a planar central carbon. The three phenyl rings are each twisted 30°, so that overall the cation has a propellor shape. Disordered perchlorate anions sit above and below the central carbon, with a C—Cl separation of 4.09 A. 

Preformed or in situ-prepared carbocation salts (tropylium, trityl, etc.) are also active in transforming alkenes to carbocations.119,138,140 Preformed carbocation salts are the simplest initiators in cationic polymerization and ideal if the cation is identical to the one derived from the momomer (e.g., fert-butyl cation in the polymerization of isobutylene). 

Probably the first isolation of a triphenylmethyl carbocation salt was by Gomberg and Cone (68) who successfully prepared the perchlorate from the corresponding chloride. A direct synthesis from the carbinol was achieved at about the same time (69), and more recently the preparation of the perchlorate and tetrafluoroborate have been much improved (70). Anderson (7/) succeeded in recording the characteristic visible absorption spectrum of the ion in concentrated acids, and Fairbrother and Wright (72) observed the same absorption when triphenylmethyl bromide was ionised in benzene in the presence of stannic bromide. 

CH2CHR. For a fast and efficient initiation the latter species should be at least as energetically favoured as A+, and in addition Arrhenius factors must also be favourable. Those olefins susceptible to initiation by stable salts are therefore, relatively few since only a limited number of stable carbocation salts are available. 

Probably the most rewarding experiments involving stable carbocation salts have been those using the monomers alkyl vinyl ethers (79-81X N-vinylcarbazole (82), p-methoxystyrene (21,67), cyclopentadiene (86), o-divinylbenzene (92), and to a lesser extent indene (34). 

Table 8. Rate constants for propagation (fcp), in the polymerisation of alkyl vinyl ethers by stable carbocation salts in CH2C12...

Both stable oxonium ion (118,119) and carbocation salts (23,54,151) have been used in the study of 1,3-dioxolan polymerisations in methylene chloride solvent. Some confusion still surrounds the complex mechanism involved though the work of Penczek and his co-workers (54) has gone far towards clarifying the situation. 

A book containing authoritative reviews of many aspects of carbocation chemistry has been published.1 These reviews are up-to-date to the end of 1991, as the book is the result of a symposium held in honour of George Olah in 1992 it features many of the world s leading carbocation chemists. At least one book review of it has also appeared.2 Olah has an introductory chapter concerning his decades-long search for stable long-lived carbocations in superacid media,3 and other review chapters include ones on carbocations at surfaces and interfaces,4 on the X-ray structural analyses that have been performed on many carbocation salts and related compounds in recent years,5 and on natural product chemistry in superacids.6 Other review chapters will be referred to below, as appropriate. 

Because a carbocation is sp2 hybridized, with trigonal planar geometry and an empty p orbital, this ion has a cycle of three p orbitals. (Remember that it is not the number of orbitals that determines whether a compound is aromatic or not, but rather the number of electrons in the pi MOs.) The cyclopropenyl carbocation has two electrons in its three pi MOs, so it fits Hiickel s rule and should be aromatic. In fact, cyclopropenyl carbocations are significantly more stable than other carbocations, even though they have considerable angle strain. For example, most carbocations react rapidly with water, a weak nucleophile. In contrast, tri-ferf-butyl-cyclopropenyl perchlorate, a carbocation salt, is stable enough to be recrystallized from water. 

In contrast to K, which strongly depends on the substitution pattern of the carbocations, the dissociation constant Kd, which could be obtained for all systems shown in Fig. 1, was found to be almost constant, (1.9-2.9) x 10-4 mol L 1 for CH2CI2 at -70° C, independent of the stabilization of the carbocations. Accordingly, similar Kd values have been reported for other carbocation salts with complex counter ions [41],... 

According to Eq. (17), AGd° is negative if AG 0 < AG. A value of -50 kJ mol-1 was estimated for AGa° (-70° C) above, so that carbocation salts can only add to olefins to form covalent products if AG > -50 kJ mol-1. As AG depends both on the structure of R+ and on the Lewis acidity of MCI, we can conclude that the thermodynamic driving force for Case d increases with decreasing stabilization of R+ and decreasing chloride-ion affinity of MC1 . Because the magnitude of AG can be estimated from the relative chloride affinities of carbocations and Lewis acidic metal halides in Scheme 7 (Section II.G), one can derive which carbocationic salts might add to alkenes with formation of covalent products. 

Until recently, knowledge about absolute and relative rates of reaction of alkenes with carbocations was very limited and came almost exclusively from studies of carbocationic polymerizations [119-125]. The situation changed, when it became obvious that reactions of carbocations with alkenes do not necessarily yield polymers, but terminate at the 1 1 product stage under appropriately selected conditions (see Section III.A). Three main sources for kinetic data are now available Relative alkene and carbo-cation reactivities from competition experiments, absolute rates for reactions of stable carbocation salts with alkenes, and absolute rates for the reactions of Laser-photolytically generated carbocations with alkenes. All three sets of data are in perfect mutual agreement, i.e., each of these sets of data is supported by two independent data sets. 

The consistency of the rate constants determined for the reaction of methylene-cyclopentane with stable carbocation salts in CH2C12 and with... 

The best known stable carbocation salts are trijrfjenylmethylium salts, (C6Hs)3C A, initiating polymerization of certain olefins by direct additfon ... 

Definitive work using a range of triphenylmethyl carbocation salts with different anions has shown the importance of the tightness of the growing ion pair and... 

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