Ion, onium

Oxonium, sulfonium and anuimnium ions will be described in this section  

The thesis of the nmst versatile trialkyloxonium ions, which can initiate all of the considered classes of heterocycles (cyclic acetals, ethers, sulfides, lactones, phosphates, amines) is simple in the case of BFJ and SbClg anions . 83) involves reactkrn of an ether with a-epk hlorohydrin and ttie correspondii metal halide, e.g.  

Trialkyloxonium salts with the more stable PFg or SbFg anions can be prepared, according to Olah, by using orthoformates as the alkylating agents )  

Recently, another method, which seems to be the most general one, was described  

The preparation of (C2Hs)30 SbFg and oxonium salts with higher alkyl groups can readily by achieved by a one-step nthesis. 

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Davidson W R, Sunner J and Kebarle P 1979 Flydrogen bonding of water to onium ions. Flydration of substituted pyridinium ions and related systems J. Am. Chem. Soc. 101 1675-80... 

Meot-Ner M 1984 Ionic hydrogen bond and ion solvation 2. Solvation of onium ions by 1-7 water molecules. Relations between monomolecular, specific and bulk hydration J. Am. Chem. Soc. 106 1265-72... 

Nonbonded electron pair donors (w-donors) are expectedly readily protonated (or coordinated) with superacids. Remarkably, this includes even xenon, long considered an inert gas. The protonation of some 7T-, (T- and -bases and their subsequent ionization to carbocations or onium ions is depicted as follows ... 

Onium Ions (with Laali, Wang, and Prakash), 1998. 

Cyclopentene derivatives with carboxylic acid side-chains can be stereoselectively hydroxy-lated by the iodolactonization procedure (E.J. Corey, 1969, 1970). To the trisubstituted cyclopentene described on p. 210 a large iodine cation is added stereoselectively to the less hindered -side of the 9,10 double bond. Lactone formation occurs on the intermediate iod-onium ion specifically at C-9ot. Later the iodine is reductively removed with tri-n-butyltin hydride. The cyclopentane ring now bears all oxygen and carbon substituents in the right stereochemistry, and the carbon chains can be built starting from the C-8 and C-12 substit""" ... 

At room temperature bismuthine rapidly decomposes into its elements. The rate of decomposition increases markedly at higher temperatures (8). Bismuthine decomposes when bubbled through silver nitrate or alkafl solutions but is unaffected by light, hydrogen sulfide, or 4 sulfuric acid solution. There is no evidence for the formation of BiH, though the phenyl derivative, (C H BU, is known. The existence of BiH would not be anticipated on the basis of the trend found with other Group 15 (V) "onium" ions. 

Onium ions of small and large heterocyclics are usually produced by electrophilic attack on a heteroatom. In three- and four-membered rings nucleophilic attack on an adjacent carbon follows immediately, in most cases, and ring opening stabilizes the molecule. In large rings the onium ion behaves as would its acyclic analog, except where aromaticity or transannular reactions come into play (each with its electronic and steric pre-conditions). A wide diversity of reactions is observed. 

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. 

When the reaction is performed in dioxane solution, an o onium ion is formed from the solvent and the chlorosulfite ester. The oxonium ion then undergoes substitution by chloride. l vo inversioRs are involved so that tiie result is overall retention. ... 

Similarly, ketones are prepared via acyl carb-onium ions ... 

The study of optical isomers has shown a similar development. First it was shown that the reduction potentials of several meso and racemic isomers were different (Elving et al., 1965 Feokstistov, 1968 Zavada et al., 1963) and later, studies have been made of the ratio of dljmeso compound isolated from electrolyses which form products capable of showing optical activity. Thus the conformation of the products from the pinacolization of ketones, the reduction of double bonds, the reduction of onium ions and the oxidation of carboxylic acids have been reported by several workers (reviewed by Feokstistov, 1968). Unfortunately, in many of these studies the electrolysis conditions were not controlled and it is therefore too early to draw definite conclusions about the stereochemistry of electrode processes and the possibilities for asymmetric syntheses. 

Scheme 11 Opposite regioselectivities of the electrophilic additions to enam-ines and their onium ions...

The cationic agents used for their antimicrobial activity all fall within the group known as the quaternary ammonium compounds which are variously described as QACs, quats or onium ions. These are organically substituted ammonium compounds as shown in... 

During the time of the Olin reports, the first examples of oligomeric boron-bridged (l-pyrazolyl)borate systems appeared from the laboratory of Trofimenko at DuPont Chemicals 24 He reported the synthesis of poly(l-pyrazolyl)borates (6) (Fig. 5) from the reactions of alkali metal borohydrides with the pyrazole ligand. The (l-pyrazolyl)borate ligand was obtained from two pyrazole units when bridged by a BR2 unit on one side and by a metal or onium ion on the other. Even though reports... 

What is retained nowadays of the initial mechanism (Scheme 1) is the occurrence of a cationic intermediate. But bromine bridging is not general, and its magnitude depends mainly on the double bond substituents (Ruasse, 1990). For example, when these are strongly electron-donating, i.e. able to stabilize a positive charge better than bromine, / -bromocarbocations are the bromination intermediates. The flexibility of transition state and intermediate stabilization puts bromination between hydration via carbocations and sulfenylation via onium ions. 

Considering the abundant evidence for carbene protonation, some quantitative estimate for the base strength of carbenes is clearly desirable. The conventional spectrometric or potentiometric methods of determining the pKa in solution are not applicable, with the exception of some onium ions 1 and their conjugate bases 2 (Section V.B). In favorable cases, equilibria of carbenes with the conjugate carbenium ions have been studied in the gas phase. Proton affinities of various carbenes can be obtained from their enthalpies of formation, and by ab initio computation (Section V.A). Kinetic data have been evaluated to obtain the pKa of carbenes in solution (Section V.B). 

In Recent Developments in Carbocation and Onium Ion Chemistry Laali, K. ACS Symposium Series American Chemical Society Washington, DC, 2007. 

Recent developments in carbocation and onium ion chemistry / Kenneth K. 

The remaining three chapters (Chapters 20-22) concentrate fully on onium ion chemistry. In Chapter 20, H.-J. Frohn and V. V. Bardin describe synthesis and multi-nuclear NMR studies of organoxenonium salts. Chapter 21 by R. S. Brown et al. focuses on the synthesis of chiral... 

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