Salts onium

Onions, dehydrated Onium Onium salts On-line databases On-line sampling ONO802, [64318-79-2] Onsager model Onychomycosis Onyx... 

The solubHity properties of the PAG itself can play an important role in the overaH resist performance as weU (50). SolubHity differences between the neutral onium salt and the acidic photoproducts can be quite high and wHl affect the resist contrast. In fact onium salts can serve as dissolution inhibitors in novolac polymers, analogous to diazonaphthoquinones, even in the absence of any acid-sensitive chemical function (51). 

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.

J. CriveUo, in J. P. Eouiassier andj. E. Rabek, eds., Eadiation Curing in Polymer Science andTechnology (V9III) Photoinitiating Systems, Elsevier, New York, 1993, Chapt. 8, for a review on onium salts. 

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). 

When a diazonium salt is allowed to react with antimony pentachloride or an aryltetrachloroantimony compound, the onium salts [ArN2] [SbClg] or [ArN2] [Ar SbCl ], respectively, are formed. These can be decomposed ia an organic solvent by the addition of a powdered metal such as iron or ziac, with the formation of a diaryltrichloroantimony compound ... 

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). 

For practical appHcation in mixtures of water—organic solvent, only ammonium and phosphonium salts containing 15 or more C atoms are sufficiently lipophilic. In empirical catalyst comparisons crown ethers (hexaoxacyclooctodecanes) (1)—(3) were often as effective as the best onium salts. 

Other complexing agents sometimes advocated are cryptates, especially the compound dubbed [2.2.2] (Kryptofix 222) [23978-09-8] (see Chelating agents). Crown ethers were originally advocated for reactions in the presence of soHd reagents (Uquid-soHd PTC). It is now known, however, that onium salts are equally suitable in many cases. 

Often poly(ethylene glycol)s or derivatives thereof can be used instead of crowns or onium salts advantageously, although their catalytic activity frequently tends to be somewhat lower. The possible toxicity of crowns and cryptands and the price difference between these compounds and onium salts (100 1 to 10 1) are other important factors to be considered. Thus (1) [17455-13-9] (2) [14187-32-7] and (3) [16069-36-6] and cryptands are used more often in laboratory work, whereas onium salts are more important for industrial processes. 

Benzyltriethylammonium chloride [56-37-1] is the most widely used catalyst under strongly basic conditions. Methyltrioctylammonium chloride [5137-55-3] (Ahquat 336, Adogen 464) is probably the least expensive catalyst. Others of high activity and moderate price are tetra- -butylammonium chloride [1112-67-0] bromide [1643-19-2] hydrogen sulfate [32503-27-8], tetra- -butylphosphonium chloride [2304-30-5], and other phosphonium salts of a similar number of C atoms. Many other onium salts can also be utilized. 

In the chlorination of 2,4-dichlorophenol it has been found that traces of amine (23), onium salts (24), or triphenylphosphine oxide (25) are excellent catalysts to further chlorination by chlorine ia the ortho position with respect to the hydroxyl function. During chlorination (80°C, without solvent) these catalysts cause traces of 2,4,5-trichlorophenol ( 500 1000 ppm) to be transformed iato tetrachlorophenol. Thus these techniques leave no 2,4,5-trichlorophenol ia the final product, yielding a 2,4,6-trichlorophenol of outstanding quaUty. The possibiUty of chlorination usiag SO2CI2 ia the presence of Lewis catalysts has been discussed (26), but no mention is made of 2,4,5-trichlorophenol formation or content. 

To avoid the formation of this kind of by-product, a direct attack by chlorine ia the para position must be encouraged. This can be achieved by usiag catalysis based on a strong acid (27), or on a sulfur (28), an amine (29), or an onium salt (30). The yields can go as high as 98%. 

Alkylation, acylation, etc. at the heteroatom lead to onium salts. In small systems these are difficult to isolate, and very weakly nucleophilic counterions must be used, such as... 

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. 

The ylides are formed by deprotonation of the corresponding onium salts. ... 

In the absence of the carbonyl or similar stabilizing group, the onium salts are much less acidic. The pATp so of methyltriphenylphosphonium ion is estimated to be 22. Strong bases such as amide ion or the anion of DMSO are required to deprotonate alkylphos-phbnium salts ... 

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. 

The only example of this ring system to the best of our knowledge was formed by reaction of Mc3SiCH2SCH2Cl with quinoline to give the onium salt 693 which upon treatment with CsF in MeCN at room temperature afforded the thiazoloquinoline 694 (87JOC4423) (Scheme 118). 

Arenediazonium salts also react with stabilized phosphonium, arsonium, pyridinium, and sulfonium ylides (12.111) in acetonitrile, yielding via the azo-onium salt (12.112) the azo-onium ylide (12.113, yellow to red), and in some cases the for-mazane (12.114) (Froyen and Juvvik, 1992). 

The ate complexes are analogous to the onium salts formed when a Lewis base expands its valence, for example. 

Although crown ethers were often found to be as effective as the best onium salts in PTC, they have only found limited commercial application because of their high cost (10 to 100 times that of quats) and perceived toxicity. Cryptands are even more expensive than crown ethers. 

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