Onium salts, syntheses

However, the vast majority of research has been devoted to synthesis involving electrophilic substitution on the aromatic ring of hydroquinone. Hence, phenylhydroquinone can be obtained by the reaction of phenyl dia onium salts (18) with hydroquinone (82). 

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... 

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... 

As for solvents, liquid ammonia or dimethylsulfoxide are most often used. There are some cases when tert-butanol is used as a solvent. In principle, ion-radical reactions need aprotic solvents of expressed polarity. This facilitates the formation of such polar forms as ion-radicals are. Meanwhile, the polarity of the solvent assists ion-pair dissociation. This enhances reactivity of organic ions and sometimes enhances it to an unnecessary degree. Certainly, a decrease in the permissible limit of the solvent s polarity widens the possibilities for ion-radical synthesis. Interphase catalysis is a useful method to circumvent the solvent restriction. Thus, 18-crown-6-ether assists anion-radical formation in the reaction between benzoquinone and potassium triethylgermyl in benzene (Bravo-Zhivotovskii et al. 1980). In the presence of tri(dodecyl)methylammonium chloride, fluorenylpi-nacoline forms the anion-radical on the action of calcium hydroxide octahydrate in benzene. The cation of the onium salts stabilizes the anion-radical (Cazianis and Screttas 1983). Surprisingly, the fluorenylpinacoline anion-radicals are stable even in the presence of water. 

Ylides in which the heteroatom is N, P, As, S, or Se are well known. Other ylides containing Sb, Bi, O, Te, I, or Br are also known, but they are rarely used as ligands since they are very unstable, and they will not be treated here. The synthesis of the ylides is achieved through several preparative methods, most of which have been comprehensively reviewed [2-11]. The most relevant of these requires two steps, and involves the reaction of a halide with an EZ nucleophile (NR3, PR3, ASR3, SR2, etc.) and subsequent dehydrohalogenation of the onium salt (method a) as represented in Scheme 2 [2-6]. This process has been reported in a wide variety of experimental conditions, using virtually all kinds of solvents and bases (provided that they are compatible). The desilylation of some a-SiMe3 onium salts (method b)... 

The recent development of several new classes of practical photoinitiators for cationic polymerization has now made it possible to utilize this chemistry in a number of ways to produce highly sensitive photoresists (142-144). The facile synthesis of onium salts (I-III)... 

SYNTHESIS AND ION-COUPLING REACTIONS OF TELECHELIC POLY(DIMETHYLSILOXANE) HAVING CYCLIC ONIUM SALT GROUPS... 

Ionic Photoacid Generators. Ease of synthesis, high thermal stability, and good quantum yield have made sulfonium and iodonium salts the most widely used onium salts. Figure 19 depicts some representative examples. 

Diisopropylethylamine is a useful base for this purpose, especially when further transformations of the onium salts are possible. a-Elimination reactions of this type have recently been employed for the synthesis of alkenyl(diaryl)-and alkenyl(triaryl)onium salts of Group 15 and Group 16 elements (Scheme 46)... 

Salt synthesis is a helpful development for solving this problem. Acid esters are foremost among the salts used for this purpose. Tertiary amines are reacted with acid esters to introduce an alkyl group onto the nitrogen atom and prepare the onium cation. The reacted acid then remains as the counter anion. This method is well covered in several other books on ionic liquids [1]. Several acid esters are known to be effective for direct preparation. Recently even TFSI methyl ester was reported as an effective reagent for the preparation of ionic liquid containing TFSI... 

The reactivities of alkyl halides are in the sequence RI > RBr > RCl and MeX > EtX > PrX. Benzyl hahde reactions with tin do not require catalysts (equation 2). For less reactive halides, the catalysts and promoters employed include metals (sodium, magnesium, zinc, or copper), Lewis bases (amines, triorganophosphines and -stibines, alcohols, or ethers), iodides, and onium salts (R4MX). The use of tin-sodimn alloys can result in tri- or tetraorganotin products. Electrochemical synthesis has also been reported, e.g. the formation of R2SnX2 from the oxidation of anodic tin by RX in benzene solution and the formation of ILtSn from RI (R = Me or NCCH2CH2) and cathodic tin. 

Scheme 8) are employed for the introduction of nitrogen alkylation into a peptide chain in order to suppress aggregation that interferes with chain assembly. Pentafluorophenyl esters of Fmoc-cysteine derivatives are employed in solid-phase synthesis to circumvent the enantiomerization that occurs during certain onium salt mediated couplings of the corresponding acids which have to be carried out in the presence of tertiary amines. The same phenomenon may apply to serine derivatives. Water-soluble tetrafluoro-4-sulfophenyl esters 22 are available. ... 

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