Oxonium ion salts

Reaction with preformed trialkyl oxonium salts. The easiest mode of initiation to interpret involves transfer of an alkyl group from a preformed trialkyl oxonium ion salt to a THF molecule ... 

Like the propagation reaction, this probably occurs by a nucleophilic attack by the THF oxygen atom on a carbon alpha to the oxygen of the trialkyl oxonium ion salt. Commonly the triethyl-oxonium ion with a tetrafluoroborate gegenion has been used (3, 24, 32). Meerwein (3) has also used the hexachlorantimonate, tetrachloroferrate, and tetra-chloroaluminate gegenions. 

The reaction used to prepare the trialkyl oxonium salts is that originally used by Meerwein (33). The reaction is carried out at dry ice temperatures. The intermediate product, which Meerwein calls an inner oxonium ion salt is not isolated. The preparation of Et30+BF4 is illustrated in equation 6. 

Trialkyloxonium salts can also be prepared by the reaction of secondary oxonium ion salts with diazoalkanes93 [Eq. (4.16)]. 

Organic Cation Salts.—Cationic salts can be generated in solvents of low polarity from a wide variety of organic substrates and either isolated or used in situ as polymerization initiators. Recent work has concentrated on the development of carbenium and oxonium ion salts. 

Oxonium ion salts can be generated by the interaction of carbenium ion salts and heterocyclic monomers, and this technique has been employed in the synthesis of 4,5- and 2,4,5-dioxolan-2-ylium salts according to equation (3). ... 

The nature of the counterion in an oxonium ion salt initiation affects the stability of the active centres during polymerization. Even relatively stable counterions such as SbCle undergo decomposition in polymerizations of cyclic ethers. ... 

Often the requisite THF oxonium ion is generated m situ by using a combination of reagents based on the Meerwein syntheses of trialkyl oxonium salts (150). These combinations include epichlorohydrin or a reactive haUde with a Lewis acid, a reactive hahde with a metal salt, or sometimes just a Lewis acid alone. The epoxide portion is often referred to as a promoter. 

HF is miscible with water in all proportions and the phase diagram (Fig. 17.4a) shows the presence of three compounds H2O.HF (mp — 35.5°), H2O.2HF (mp-75.5°) and H2O.4HF (mp — 100.4°, i.e. 17° below the mp of pure HF). Recent X-ray studies have confirmed earlier conjectures that these compounds are best formulated as H-bonded oxonium salts [HsOJF, [H30][HF2], and [H30][H3F4] with three very strong H bonds per oxonium ion and average O - - F distances of 246.7, 250.2... 

Oxonium ions are excellent alkylating agents, and ethers can be conveniently prepared by treating them with alcohols or phenols. Quaternary ammonium salts can sometimes also be used. ... 

This is an Sn2 process, since inversion is found at R. Another good leaving group is NTS2 ditosylamines react quite well with acetate ion in dipolar aprotic solvents RNTs2 4- OAc — ROAc. Ordinary primary amines have been converted to acetates and benzoates by the Katritzky pyrylium-pyridinium method (p. 447). Quaternary ammonium salts can be cleaved by heating with AcO in an aprotic solvent. Oxonium ions can also be used as substrates RsO -f R COO —> R COOR R2O. 

Oxidation of either alkyl or aryl sulphoxides to sulphones in 65-90% yields may be accomplished by treatment with a nitronium salt . In the case of aryl sulphoxides no nitration is observed (which is in contrast to the results of nitric acid oxidation). The reaction was shown to proceed through intermediate nitratosulphonium and nitritosulph-oxonium ions, as depicted in equation (7), which were studied by nmr spectroscopy. 

The case of the salt of a weak base and strong acid is treated in an analogous way. With decreasing strength of the base, the concentration of oxonium ions increases and the pH decreases. 

Further, because there had been so little work on the physical chemistry of the oxonium ions involved, it seemed appropriate to also study other aspects of their chemistry. These episodic and mainly practical researches are not presented here, but they included a direct demonstration by conductivity measurements of the solvation of the EtsO+ ion by Et20 and DCA and the effect of this on the dissociation constant of Et30+ salts 77. Also, because of the frequent use of theses salts as polymerisation initiators by others, we made a detailed study of their spontaneous decomposition in solution which had been known since their discovery by Meerwein 74, 110. ... 

One must treat a reaction mixture with the sodium salt RO-Na+ of an alcohol or phenol, and search for compounds of the type H-[0(CH2)20CH2] -0R, which would be formed from homologues of (VII) (Mainz theory), but not from secondary oxonium ions such as (V) or (VI) (Y = H) (Keele theory), which would form ROH. 

Silylium ions, which are not protected sterically or are not stabilized either electronically or by intramolecular interaction with a remote substituent do interact strongly with the solvent and/or the counteranion. The reaction of the transient silylium ion with solvents like ethers, nitriles and even aromatic hydrocarbons lead to oxonium, nitrilium and arenium ions with a tetrahedral environment for the silicon atom. These new cationic species can be clearly identified by their characteristic Si NMR chemical shifts. That is, the oxonium salt [Me3SiOEt2] TFPB is characterized by S Si = 66.9 in CD2CI2 solution at —70°C. " Similar chemical shifts are found for related silylated oxonium ions. Nitrilium ions formed by the reaction of intermediate trialkyl silylium ions with nitriles are identified by Si NMR chemical shifts S Si = 30—40 (see also Table VI for some examples). Trialkyl-substituted silylium ions generated in benzene solution yield silylated benzenium ions, which can be easily detected by a silicon NMR resonance at 8 Si = 90—100 (see Table VI). ... 

In nonpolar media, on the other hand, the newly formed oxonium ion will either quickly convert to the corresponding soluble ester, or it will precipitate, since monomeric or short-chain oligomeric oxonium salts have low solubility in such media. The soluble ester is structurally similar to the initiator and may add another THF molecule. The resulting oxonium ion will again revert to the ester or precipitate. In fact, precipitates are generally observed durina the eairly stages of polymerization in media of low polarity. They have been isolated and characterized as monomeric or short chain oligomeric oxonium salts (17). 

The reaction of pyridine with living polyTHF at -10eC has been examined in detail by gpc and H nmr, and the formation of polymer pyridinium salts (equation 6) has been shown to be rapid and quantitative.8. The salt is stable and does not exchange with any excess oxonium ions present in solution. Interestingly,... 

RIR+), which can be prepared in super-acid solutions (p. 312) and isolated as solid SbFfi salts, are also extremely reactive in nucleophilic substitution.345 Of the above types of compound, the most important in organic synthesis are tosylates, mesylates, oxonium ions, and triflates. The others have been used mostly for mechanistic purposes. 

Alkyl halides react with diazines less readily than with pyridines. All the diazines are, nevertheless, more reactive toward methyl iodide than predicted by their pKa values and the Bronsted relationship. The significant although modest rate enhancements found are considered to arise from interactions between the two lone pairs on the nitrogen atoms this interaction is largest in pyridazine. Use of oxonium ions can convert the diazines into diquatemary salts. Quinoxalines and phenazines similarly yield diquatemary salts under forcing conditions. 

Even more stable salts than those already mentioned can be prepared from oxonium, R30+, sulphonium, R3S+, and ammonium, R4N+ ions. Not surprisingly, however, these are in general too stable to initiate vinyl polymerisations though Et30+BF4 has been reported to polymerise alkyl vinyl ethers (96). Both oxonium and strained sulphonium ion salts are very efficient initiators of ring opening polymerisations as we shall see later (Section IV). 

Carbocation-oxonium ion equilibria are obvious complicating factors in studies of the kinetics of initiation of polymerisation and useful thermodynamic data for such equilibria involving Ph3C+ and a variety of linear and cyclic ethers have been reported by Slomkowski and Penczek (132). A dramatic increase in rates of initiation of polymerisation of THF induced by Ph3C+ salts is observed on addition of small amounts of epoxides such as propylene oxide (113a,b), which compete favourably with THF in the primary carbocation-oxonium ion equilibria and simplify the initiation reaction ... 

Both stable oxonium ion (118,119) and carbocation salts (23,54,151) have been used in the study of 1,3-dioxolan polymerisations in methylene chloride solvent. Some confusion still surrounds the complex mechanism involved though the work of Penczek and his co-workers (54) has gone far towards clarifying the situation. 

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