Oxonium exchange reaction

As illustrated in Scheme 2, [12] four possible reactions produce the cis unit (P-form). They are (1) the oxonium exchange reaction at a penultimate unit (upper left), (2) the propagation by an oxacarbenium ion (upper right), (3) the propagation by a covalent species (lower left), [16] and (4) the inter- or intramolecular transacetalization via macrocyclic or linear trialkyloxonium ions (lower right). The common features of the polymerization of the monomers 3 and 6-8 described above cannot be accounted for either by the propagation involving oxacarbenium ions or covalent species or by the transacetalization. 

The oxonium exchange reactions may occur with the polymer ether linkages as well as with cyclic tetramers that form, as shown above. The concentrations of the oxonium ions of the ether group on the polymer and on the cyclic tetramers, however, are very small. Polymerizations with PF5, on the other hand, or with (C2H5)30PF6 either in bulk or in methylene chloride solutions, yield no significant amounts of cyclic oligomers. ... 

Heteronuclear /x-oxo complexes have been prepared by oxo/halide exchange reactions, then the reaction of metal halide complexes with the gold oxonium species gives complexes of the Jy [Rh2(dien)2 0(AuPPh3)2 (BF4)2 (476) (dien = COD, NBD) or [Pt(COD)2 OAu(PR3) 2] 2725 J "... 

Another important synthesis method is transalkylation reactions with other oxonium ions.95 However, these reactions are reversible to a certain extent. The equilibrium between two oxonium salts and the corresponding ethers in solution is established by both differences in solubility and stability of the oxonium salts (Scheme 4.4). By an exchange reaction of this type, the trimethyloxonium salt 31 can be... 

Reaction of pyridine-N-oxides with triethyloxonium salt in methylene chloride or chloroform gives directly AT-alkoxy pyridinium salts with high yield. Quinolinium salts can also be prepared from the corresponding N-oxides [46], The synthetic procedure does not require the anion exchange reaction since triethyl oxonium salt possesses a non-nucleophilic counter anion, i.e. PFg ... 

When Lewis acid complexes with active hydrogen compounds initiate the polymerizations, the complexes acts as protonic acids. On the other hand, etherates initiate by forming oxonium ions and may involve alkyl exchange reactions with the monomer ... 

We have recently shown that metal-exchanged zeolites give rise to carbocationic reactions, through the interactions with alkylhalides (metal cation acts as Lewis acid sites, coordinating with the alkylhalide to form a metal-halide species and an alkyl-aluminumsilyl oxonium ion bonded to the zeolite structure, which acts as an adsorbed carbocation (scheme 2). We were able to show that they can catalyze Friedel-Crafts reactions (9) and isobutane/2-butene alkylation (70), with a superior performance than a protic zeolite catalyst. 

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,... 

A number of other heterocycllcs have been similarly studied and shown by H nmr, to produce quaternary ammonium salts with living polyTHF.2- Moreover, their rates of reaction are a direct function of their basicities, the following order of reactivities being observed ethyl pyridine > pyridine > isoquinoline > quinoline > acridine. Aliphatic tertiary amines also react in the same way the order of reactivities was found to be triethylame > tributylamlne > dlethylanillne. In all cases studied, the quaternary ammonium salt once formed did not exchange with any excess oxonium lone. 

The preparation and properties of the oxonium salts have been considered here in some detail because of their great importance in the polymerization of cyclic ethers by Friedel Crafts catalysts. From the point of view of this review the most important reactions of these salts are those with ethers and alcohols with ethers they exchange alkyl groups in an equilibrium process,... 

Minkwitz et al.604 have prepared the hexafluorometalates of cations 308. The reaction of a,a-dichloromethyl methyl ether in HF-Lewis acid solution at —78°C leads to the formation of chloro cation 308-C1, whereas at -65°C fluoro derivative 308-F is isolated as a result of chlorine-fluorine exchange [Eq. (3.80)]. Interestingly, the chlorine atom and the methyl group are trans in the hexafluoroantimonate salt of cation 308-C1, whereas the fluorine atom and the methyl group are cis in cation 308-F. The arrangement of the C—O—C—H atoms is nearly planar with F/Cl—C—O—C torsion angles of 2.84° (308-F) and 179.0° (308-C1). The C Obond distances (1.224 and 1.479 A for cation 308-F, and 1.252 and 1.517 A for cation 308-C1) reveal dominant oxonium ion character. 

Scheme 2 Reaction of an alkylchloride with a metal-exchanged zeolite. Formation of alkyl-aluminumsilyl oxonium ion.

The reactivity of both species may be quite different. Because the protonation, at least when it involves proton exchange, is fast, the ratedetermining step in initiation may be the reaction of protonated monomer with the next monomer molecule to form tertiary oxonium ion. 

The proportions of oxonium and carbenium ions in the chain growth, as well as their relative reactivities, may markedly depend on the polymerization coixlitions and vary with monomer structure. The high values of rate constants of the reaction between carbenium cations and ethers or acetals make questionable Okada s interpretation of the 1,3-dioxane-triethyloxonium salts system It is impossible to observe, at room temperature, the individual ROCHl species with excess of such nucleophiles as diethyl ether. We would rather suggest that the signal at 11—13 ppm 6 ( H-NMR) or 175—180 ppm 6( C-NMR) is due to some exchanging protonated species. 

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