Onium salts, from amines

Stable salts can be synthesized that have very low melting points for electrochemical and other applications. The neutralization method introduced in this chapter is an easy way to prepare pure salts and to use them in learning about the structure-property relationships of salts. It has been shown that excellent onium salts can be obtained from the results of neutralized amines. Besides serving as sources of qua-temized onium salts, neutralized amines have excellent properties. Some of these are unique and not retained by ordinary onium salts. More vigorous studies on neutralized amines can be expected to be published in the near future. 

Mukaiyama developed l-alkyl-2-halopyridinium salts, which are useful reagents for the preparation of carboxylic esters and lactones in the presence of tertiary amines [22]. After generation of activated onium salts from the corresponding co-hydroxycarboxylic acids, spontaneous lactonization smoothly proceeds to produce various macrolactones. Bartlett first used this lactonization method for the... 

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

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. 

Apart from the reactions of diazonium salts, a number of other reactions are known in which the C-N bond is broken. The best known of these is the Hofmann elimination of quaternary ammonium hydroxides (Scheme 2.37). An amine is converted by methylation with methyl iodide to the quaternary ammonium salt ( exhaustive methylation ). The iodide, on treatment with moist silver oxide, forms the quaternary ammonium hydroxide which undergoes a bimolecular elimination to form an alkene. The bimolecular elimination of onium salts yields the least alkylated alkene. This substitution pattern is determined by the ease with which a hydrogen atom can be attacked by the base. 

All other TSOS are generally obtained from halogen, alcohol or amine derived onium salts. However, purifications of salt mixtures are often tricky and preparation methods often need to be carefully optimized. In some cases, when compatible with other functional group, quatemarisation can be performed at a later stage in the process. 

Apart from ammonium derived hydroxide, few onium salts have been used as supported bases. However, an analog of Hunig s base was prepared and employed as a catalyst in Knoevenagel condensations [68], Increase in the linker length was found critical to observe a good activity, as cation effect on amine basicity can be detrimental for short alkyl chains. Finally, using a biphasic liquid-liquid mixture, no significant decrease in activity has been noticed after five cycles (Fig. 25). 

Scheme 7.29. A representation of two potential pathways for proton loss from a carbocation intermediate on the Sn1-E1 reaction surface. Both pathways are followed. It is supposed that proton loss via the pathway labeled (a), which results in the most highly substituted alkene (the Saytzeff product), occurs preferentially because that is the product formed in highest yield. The alkene resulting from pathway (b), the Hofmann product, is also formed. A. W. Hofmann (1818-1895) was a German chemist who was professor of chemistry at the Royal College of Chemistry in London (1845-1864) and then accepted a post as professor at the University of Berlin. Most of Hofmann s work dealt with amines (Chapter 10). Hofmann found, in contrast to Saytzeff, that the least highly substituted alkene is formed when the elimination is carried out on amine quaternary salts (so-called onium salts). This is, in part, presumably due to the close association between the base and the positively charged onium salt as well as to the removal of the proton in the rate-determining step (cf. the E2 reaction). (Note the 82 18 ratio of products shown here should be considered identical, within experimental error, to the 79 21 ratio of Table 7.9.)...

The sulfur and nitrogen series ressemble each other in so far as both amines and sulfides or the corresponding onium salts can be induced to isomerize. Benefitting from a greater driving force, S -ylides rearrange under very mild conditions. 

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