From onium salts

Unlike the three fundamental mechanisms of olefin formation already outlined, the a -ji (or ylid) mechanism is only applicable to elimination from onium salts . The base abstracts an alpha hydrogen from the leaving group to form an ylid which subsequently acts as an internal base and abstracts the beta hydrogen atom, viz. 

Figure 2.1. " Acid produced from onium salt decomposes t-BOCPHS. A proton is reproduced during the decomposition of ester. This is called chemical amplified resist because of its high efficiency by proton cycling.

Fig. 20. Proposed photochemical mechanisms for the generation of acid from sulfonium salt photolysis. Shown ate examples illustrating photon absorption by the onium salt (direct irradiation) as well as electron transfer sensitization, initiated by irradiation of an aromatic hydrocarbon.

The tetra alkyl amm onium salts of [B Hg] , formed by ion-exchange reactions, have proven to be useful synthetic reagents because of their thermal and air stabihties. The stmcture of the [B Hg] ion has been determined by an x-ray study (66) and shown to have the 2013 styx stmcture, C2 symmetry. Mechanisms for the formation of this ion have been proposed (67). Tetraborane(lO) can be easily obtained from salts of [B Hg] (eq. 9). 

BF4. In large rings the fate of the onium ions depends mostly on the structure and degree of unsaturation of the particular compound, and the onium salts range from completely stable to highly unstable. 

Since polymerization of epoxies proceeds in the presence of onium salts [17,33,34,29], it is most likely that the above-mentioned cationic mechanism proceeds in their presence, and that the onium salts prevent water from inhibiting the cationic polymerization. 

Spelling of amonium, imonium, and iminium indicates derivation from amine and imine onium salts. 

Saponification of esters. Aqueom sodium hydroxide method. To hydrolyse an ester of an alcohol, reflux 5-6 g. with 50 ml. of 20 per cent, sodium hydroxide solution for 1-2 hours or until the ester layer disappears. Distil the alkaline mixture and collect about 6 ml. of distillate. This will ccmtain any volatile alcohol formed in the saponification. If the alcohol does not separate, i.e., is water-soluble, satmate the distillate with solid potassium carbonate an upper layer of alcohol is then usually formed. (The alcohol may be subsequently identified as the 3 5-dinitrobenzoate see Section III,27J.) Cool the residual alkaline mixture, and acidify it with dilute sulphuric acid. If no crystalline acid is precipitated, the acid may frequently be isolated by ether extraction, or it may be distilled from the acidified solution and isolated from (or investigated in) the distillate. (The acid may be subsequently identified, e.g., as the S-benzyl-iso-thiiu onium salt see Section 111,85,4.)... 

A multiphase system consisting of a hydrocarbon solvent, a strong alkaline solution, and a quaternary onium salt, in the presence of a Pd/C catalyst with hydrogen that was bubbled at atmospheric pressure through the organic phase, allows the rapid displacement of chlorine from polyhalogenated benzenes. The onium salt, insoluble in both phases, is localized in the interfaces, coats the Pd/C catalyst, and constitutes the phase in which the reaction takes... 

The most common ligands are those derived from imidazole and benzimidazol (Scheme 54), followed by the (benz)thiazols. The free Wanzlick-Arduengo carbenes can be isolated and employed for the synthesis of the complexes, but often it is more convenient to prepare the carbenes in situ from the dimers or the corresponding onium salts, or to use carbene-transfer reactions.256-259... 

The synthetic principles underlying the preparation of the 1 1 carbene complexes (above) are also applicable to products with a 2 1 ligand/metal ratio . The carbene components may be introduced as free carbenes or their dimers, or generated in situ from the onium salts. A second major pathway uses anionic hetero-... 

In an early example, Mukaiyama and coworkers used hetaryl onium salts for nucleoside synthesis. The active hetaryl onium salt is generated in situ from the reaction of 2-chloro-3-ethylbenzoxazolium tetrafluoroborate 77 and the glycosyl acceptor. With benzimidazole as glycosyl acceptor, the resulting 2-(l-benzimida-zoyl)benzoxazolium tetrafluoroborate 78 was obtained. The reaction between the hetaryl onium salt 78 and hemiacetal donor 1 occurs at 60 °C to activate the hemiacetal and thereby reveal the glycosyl acceptor. This procedure led to the formation of nucleoside 80 with exclusive 1,2-trans selectivity [139]. The nucleoside 81 was similarly prepared. Alternatively, 2-fluoro-l-methylpyridinium tosylate 79 directly... 

This study will show an approach to materials which have many of the desirable features of the poly(phthaladehyde)-onium salt imaging system but do not suffer from the problem of spontaneous gaseous material evolution upon irradiation. The new polyformal-based imaging systems all have a "built-in" thermal activation requirement which allows for image self-development outside of the exposure tool. 

With a view to producing catalysts that can easily be removed from reaction products, typical phase-transfer catalysts such as onium salts, crown ethers, and cryptands have been immobilized on polymer supports. The use of such catalysts in liquid-liquid and liquid-solid two-phase systems has been described as triphase catalysis (Regen, 1975, 1977). Cinquini et al. (1976) have compared the activities of catalysts consisting of ligands bound to chloromethylated polystyrene cross-linked with 2 or 4% divinylbenzene and having different densities of catalytic sites ([126], [127], [ 132]—[ 135]) in the... 

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