Anion-radical salts sulfoniums

The reactions discussed so far involved anion radicals. Now we want to turn to conversions of the cation radical type. One of these reactions is anisylation of the thiantrene cation-radical (Svanholm et al. 1975 Hammerich and Parker 1982) (Scheme 5-6). The reaction gives the sulfonium salt (anion ClOf) in 90% yield (route a). One-electron reduction of the thianthrene cation radical by anisole is the side reaction (route b). Route b leads to products with a 10% total yield. Addition of the dibenzodioxine cation radical accelerates the reaction 200 times. The cation radicals of thianthrene and dibenzodioxine are stable. Having been prepared separately, they are introduced into the reaction as perchlorate salts. 

In spite of the large number of dithiolene complexes synthesized, 50 of their derived compounds exhibit a metal-like behavior at a temperature close to RT and within a significant temperature range. These compounds, listed in Tables I-IV, together with the relevant references and some details about their electrical behavior are either NIOS salts or D-A adducts. For the NIOS salts of dmit and mnt complexes, the associated counterions are mostly closed-shell cations such as alkyl-ammonium or sulfonium, or alkaline metal cations. For the D-A adducts derived from dmit and mnt acceptor complexes, the associated donors are TTF-like molecules and Per, respectively. The dddt complexes deserve special mention as the derived metal-like conductors involve M(dddt)2 v - cation radicals (instead of anion-radicals as in dmit or mnt systems). 

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). 

Photoinduced polymerization can also be obtained through the dissociation of organic salts such as sulfonium, diazonium and similar salts. The photodissociation leads to several species, a radical cation, a neutral free radical and a closed-shell anion for example. The radical cation can then react further, e.g. through hydrogen abstraction from a substrate, ZH, to form another free radical Z. ... 

Triorganyl-sulfonium, -selenonium and -telluronium salts are reduced by carbon dioxide radical anions/solvated electrons produced in aqueous solution by radiolysis. The radical expulsion accompanying reduction occurred with the expected leaving group propensities, i.e. benzyl > secondary alkyl > primary alkyl > methyl > phenyl. Much higher product yields in the reduction of selenonium and telluronium compounds have been accounted for in terms of a chain reaction with carbon-centred radicals, with formate serving as the chain transfer agent.282... 

An intramolecular nucleophilic substitution of the dimethyl sulfide group by a cathodically generated radical anion is postulated in the formation of the in-dane derivative 133 from the sulfonium salt 134 (Eq. (203)). 

Generally, chiral tricoordinate centers are configurationally stable when they are derived from second-row elements. This is exemplified by sulfonium salts, sulfoxides and phosphines. In higher rows, stability is documented for arsines and stibines. In contrast, tricoordinate derivatives of carbon, oxygen, and nitrogen (first-row atoms) experience fast inversion and are configurationally unstable they must therefore be viewed as conformationally chiral (see Fig. 3, Section 3.b). Oxonium salts show very fast inversion, as do carbanions. Exceptions such as the cyclopropyl anion are known. Carbon radicals and carbenium ions are usually close to planarity and tend to be achiral independently of their substituents [21-23]. 

Initiators based on halonium and sulfonium salts are used commercially in various microlithographic processes and in the coating industry. Onium salts were developed commercially as photoinitiators due to the lower sensitivity of cationic polymerizations to oxygen compared to radical polymerizations. Aromatic halonium and sulfonium salts with complex anions such as SbF6, AsF6 and BF4- do not initiate cationic polymerizations spontaneously, but must be activated by UV irradiation. 

Cationic polymerization of diethyleneglycol divinyl ether and butanediol divinyl ether in the presence of oniiim salts was induced by y-irradiation. The mechanism for the initiation process involves the reduction of onium salts either by organic free radicals or solvated electrons depending on the reduction potentials of the onium salts. The reduction potentials of sulfonium salts was determined by polarography at the dropping mercury electrode. Only solvated electrons were capable of reducing the salts with reduction potentials lower than approximately -100 kJ/mol. The redox process liberates the non-nucleophilic anion from the reduced onium salt and leads to the formation of a Bronsted acid or a stabilized carbenium ion. These species are the true initiators of cationic polymerization in this system. The y-induced decomposition of onium salts in 2-ethoxyethyl ether was also followed by measuring the formation of protons. An ESR study of the structure of the radicals formed in the y-radiolysis of butanediol divinyl ether showed that only a-ether radicals were formed. 

In addition to iodonium, sulfonium and selenonium compounds, onium salts of bromine, chlorine, arsenic, and phosphoras are also stable and can act as sources of cation radicals as well as Bronsted acids, when irradiated with light. Performance of diaryl chloronium and diaryl bromonium salts was studied by Nickers and Abu. Also, aryl ammonium and aryl phosphonium, and an alkyl aryl sulfonium salt were investigated. It appears that the general behavior of these materials is similar to diphenyl iodonium and triphenyl sulfonium salts. These are formations of singlet and triplet states followed by cleavages of the carbon-onium atom bonds and in-cage and out of cage-escape reactivity. The anions of choice appear to be boron tetrafluoride, phosphorus hexafluoride, arsenic hexafluoride, and antimony hexafluoride. 

Few photoinitiators are available for cationic systems. The most widely used are diaryl iodonium salts such as diaryliodonium hexafluoro-antimonate, triaryl salts such as triphenyl sulfonium hexafluoro-phosphate, and mixed triphenyl sulfonium salts. These photoinitiators are decomposed by UV light by a homolytic cleavage to produce a radical anion and a radical cation. The latter abstracts hydrogen from surrounding molecules and generates a proton, which is the initiating species ... 

The formation of the quinoidal p-xylylene intermediate can be monitored by the appearance of a peak in the UV spectrum around 310 nm. This has been used to optimize reaction conditions for polymerizations involving unreactive sulfonium salts [48]. There has been some controversy over the precise nature of the polymer coupling reaction. The initial assumption was that the polymerization was a radical-promoted process [46]. The presence of radicals was very hard to prove, and the pendulum swung for a while toward an anionic mechanism [51]. However, careful work by Lahti and coworkers [53] showed that radical trapping reagents did indeed suppress the polymerization. As an example, the addition of TEMPO to the reaction mixture not only dramatically lowered the yields and molecular weights but also caused the disappearance of the spin label. The mechanism of radical initiation is unknown it may involve spontaneous coupling of two quinoidal p-xylylene intermediates to form a biradical. 

---