Carbenium ion salts

Aroyl and related salts (ArCO, AlkCO ) and stable carbenium ion salts (e.g. Ar3C ) of complex anions MtX . i can be synthesised essentially by two routes, silver salt double decomposition, and direct combination ... 

Table 7 Molar conductivities, ion pair dissociation constants, and thermodynamic parameters for the dissociation of carbenium ion salts in dichloromethane...

The term bare has been chosen here to define carbenium ions or radical cations which are formed without the corresponding anion. The difference in reactivity between these ecies and the free ions present in cationic systems involving fully dissociated entities such as carbenium ion salts, resides of course in the absence of all interactions involving the anion, namely ion pairing, coll se to give a covalent bond, etc. Most techniques available for the production of bare cations involve the expulsion of an electron from amono-mer molecule, i.e. ph3 cally-induced initiation. The present chapter deals with these techniques and their major contributions to the understanding of the chemistry and properties of active species in cationic polymerisation. 

Mechanism and kinetics of cationic poiymerization initiation. Unlike free-radical and anionic polymerization, initiation in cationic polymerization employs a true catalyst that is restored at the end of the polymerization and does not become incorporated into the terminated polymer chain. Initiation of cationic polymerization is brought about by addition of an electrophile to a monomer molecule. TVpical compounds used for cationic polymerization include protonic acids (e.g., H2SO4, H3PO4), Lewis acids (e.g., AICI3, BF3, TiCl4, SnCl4), and stable carbenium-ion salts (e.g., triphenylmethyl halides, tropylium halides) ... 

Treatment of PTM—Cl with AICI3 in CH2CI2, in the absence of CI2, gives the carbenium ion salt. After hydrolytic treatment the Cl—PTM recovered contains an insignificant proportion of PTM- (155). 

Oxonium ion salts can be generated by the interaction of carbenium ion salts and heterocyclic monomers, and this technique has been employed in the synthesis of 4,5- and 2,4,5-dioxolan-2-ylium salts according to equation (3). ... 

Stable carbenium ion salts such as the tropylium (a) and trityl ion (b) salts are especially convenient and facile initiators of vinyl ether polymerizations, and although not commercially feasible, have been studied extensively in many academic laboratories. 

In cationic polymerization typical catalysts are represented by strong electron acceptors including Lewis acids, Friedel-Crafts halides, Brdnsted acids and stable carbenium-ion salts. However, many of them are not sufficient to initiate the polymerization and, therefore, require small amounts of a co-catalyst, usually a proton donor such as alcohols. 

Propagation and chain transfer then occur as discussed above. The nature of termination reactions is obscure. The use of carbenium ion salts as polymerization initiators appears to be restricted to aromatic and vinyl ether monomers. The salts do not initiate polymerization of aliphatic olefins. 

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

Reaction of glycosylmethylamines with aryldiazonium salts gives a class of compounds which, by acid catalysis or unknown factors of enzymic catalysis, generate glycosylmethyldiazonium ions. These, in turn, lose nitrogen, to yield highly electrophilic carbenium ions, as illustrated for the ) -d-galactosyl derivative 38 (see Scheme 8). 

Figure 3 shows 13c MAS spectra of acetone-2-13c on various materials. Two isotropic peaks at 231 and 227 ppm were observed for acetone on ZnCl2 powder, and appreciable chemical shift anisotropy was reflected in the sideband patterns at 193 K. The 231 ppm peak was in complete agreement with the shift observed for acetone diffused into ZnY zeolite. A much greater shift, 245 ppm, was observed on AICI3 powder. For comparison, acetone has chemical shifts of 205 ppm in CDCI3 solution, 244 ppm in concentrated H2SO4 and 249 ppm in superacid solutions. The resonance structures 5 for acetone on metal halide salts underscore the similarity of the acetone complex to carbenium ions. The relative contributions of the two canonical forms rationalizes the dependence of the observed isotropic 13c shift on the Lewis acidity of the metal halide. 

To further demonstrate the stability and unreactivity of the isopropenylferrocene carbenium ion, a stable derivative of this ion was prepared and evaluated as an initiator for the cationic polymerization of styrene. The derivative prepared for this purpose was the tetrafluoroborate salt of the isopropenylferrocene carbenium ion. Even at 20°C a 26% yield (Mjj = 11,315, = 18,815) of... 

My last kinetic work was aimed at determining the kp+ of a range of monomers by what I believed to be a reliable method. For kinetic and electrochemical reasons I chose nitrobenzene as the solvent, and I chose carbenium and carboxonium salts as initiators so as to achieve a clean and fast initiation. The rate-constants were adequately reproducible, but it turned out that they were not the kp+. The project was flawed because I had been unaware of the reversible cationation of the solvent by the carbenium ions. A careful analysis of the kinetic, analytical and thermochemical results gave a new insight into the reaction mechanisms in nitrobenzene, but the main objective had eluded me. 

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 ... 

Acylium ions can be formed in superacid solutions from carboxylic acids and acyl halides (8). They are among the best characterized carbenium ions, and single-crystal X-ray structures of a number of them have been determined as BFf, SbFg, or TaClfi salts (135-139). Solid-state NMR characterization of these species on AlBr3 and other solid superacids was described earlier in this review. 

As described in Section III, practically all known conversions of 2-benzopyrylium salts are the result of their reactions with nucleophiles, and 2-benzopyrylium, similar to its monocyclic analog [82AHC(Suppl)], behaves chemically more like a carbenium ion, and not like an oxonium salt. This fact can be easily understood if one takes into account resonance formulas a-f, where an oxonium form f is added for the presence of an alkoxy (hydroxy) substituent in position 6. 

X-ray photoelectron (ESCA) spectra of carbenium ions have also been obtained in frozen superacid solutions, generally in SbF5-S02 solution or as isolated salts. Sulfur dioxide was subsequently removed by the usual freeze-thaw procedure. A thin layer of the viscous SbF5 solution was deposited on the precooled sample holder, in a dry nitrogen... 

Resonance, similar to that in pyrylium salts, was shown594,595 to exist between oxonium ion (299a) and carbenium ion (299b) forms in alkylated ketones, esters, and lactones that were obtained via alkylation with trimethyl- or triethyloxonium tetra-fluoroborates596 [Eq. (3.78)]. Ramsey and Taft597 used H NMR spectroscopy to investigate the nature of a series of secondary and tertiary carboxonium ions (300-302). 

Typical alkylation reactions are those of propane, isobutane, and n-butane by the ferf-butyl or sw-butyl ion. These systems are somewhat interconvertible by competing hydride transfer and rearrangement of the carbenium ions. The reactions were carried out using alkyl carbenium ion hexafluoroantimonate salts prepared from the corresponding halides and antimony pentafluoride in sulfuryl chloride fluoride solution and treating them in the same solvent with alkanes. The reagents were mixed at —78°C warmed up to — 20°C and quenched with ice water before analysis. The intermolecular hydride transfer between tertiary and secondary carbenium ions and alkanes is generally much faster than the alkylation reaction. Consequently, the alkylation products are also those derived from the new alkanes and carbenium ions formed in the hydride transfer reaction. 

---