Ethers reaction with oxonium salts

Trifluoromethanesulphonic acid forms a very stable crystalline hydrate [205, 218] and reacts vigorously with alcohols, ethers and ketones. Oxonium salts are formed and further reaction may occur on heating (Figure 8.73). 

Oxonium salts can be obtained directly by reaction of B 2 H 2 2 with cyclic and open ethers and BF3 (added as etherate) [55],... 

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). ... 

With acyclic ethers. In a study of THF polymerization using PFjT gegenions Dreyfuss (25) show that in the presence of dialkyl ethers chain transfer occurs and continues to occur after equilibrium is reached. The ultimate conversion to polymer is not affected but the intrinsic viscosity of the polymer decreases with time (Fig. 6). The reaction involved is essentially the reverse of the initiation reaction with trialkyl oxonium salts (equation 5). In the case of transfer the dialkyl ether reacts with the propagating oxonium ion to give a trialkyl oxonium ion which has one long chain alkyl and two short alkyls derived from the ether. 

At lower temperatures the oxonium salt or the alkyl hydrogen sulfate may react by an SN displacement mechanism with excess alcohol in the reaction mixture, thereby forming a dialkyl ether. Although each step in the reaction is reversible, ether formation can be enhanced by distilling away the ether as fast as it forms. Diethyl ether is made commercially by this process ... 

The current work on Friedel-Crafts polymerization of cyclic ethers may be considered to date from about 1940 when Meerwein and his associates prepared a series of tertiary oxonium salts and applied them to the polymerization of tetrahydrofuran. These salts, of the general form R30+... M X4i, are easily prepared from the corresponding metal halide in a reaction with an epoxide (preferably epichlorohydrin) in ether solution. According to Meerwein et al. (3) this reaction takes place in the following steps ... 

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,... 

Since methyl dioxolane can amount to as much as ten percent of the dioxane, a termination of this kind could account for the low yield from regenerated ions but this scheme puts a lot of weight on the presence of dioxolane, a product which could, after all, arise by direct rearrangement of oxide to acetaldehyde. Furthermore, the zero order dependence on monomer does not necessarily rule out direct reaction between oxonium ion and monomer for it will be recalled that Meerwein observed that the inner oxonium salt complexed very strongly with a molecule of ether. If reaction were to occur between an ion and a monomer molecule held in this way, the rate could be independent of the monomer concentration. 

That ethylene oxide can react directly with oxonium ions is shown by its reaction with triethyl oxonium borofluoride but this reaction, because of its complexity, does not shed much light on the polymerization. The sole product is dioxane and the vinyl ether colors appear only after all epoxide has reacted. Initial rates suggest first order in catalyst but the first order plots of monomer disappearance are flat S-curves suggesting a rate which first falls off, then tends towards zero order. The first step in the reaction is most probably a simple bimolecular alkylation of the epoxide by oxonium salt but the subsequent steps are obscure. The simplest path would be... 

The analogous reaction, the alkylation of methyl (trimethylsilyl)methyl ether with alkyl iodide/AgBF4, and the reaction in Eq. (4.15) have been employed to prepare tertiary [(trimethylsilyl)methyl]oxonium salts. 

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... 

In many cases, the main step in the syntheses of trialkyloxonium salts is the alkylation of a dialkyl ether with a reactive intermediate oxonium ion formed in situ. Thus, the most widely used method for preparing trialkyloxonium tetrafluoroborates by the reaction of epichlorohydrin and BF3 is based on the intermediacy of the inner oxonium salt 3291 95 97 [Eq. (4.19)]. 

Here we can again make a distinction between (i) oligomerization occurs by the same mechanism as polymerization and (ii) oligomerization follows a different mechanism. The first kind has been proposed by Rose (19) for the tetramer formation of oxetane up to the fourth monomer added there is no difference between polymerization and tetramer formation. For polymerization the next step is addition of a fifth monomer molecule. If the next step is reaction between the end-standing hydroxy group with the growing chain, a cyclic secondary oxonium salt is formed which leads to tetramer by a proton transfer to another ether function (see p. 109). 

In recent years the alkylation of lactams has been achieved using tertiary oxonium salts, particularly triethyloxonium fluoroborate.19,20 This reaction proceeds via cation formation (cf. 5). Treatment of the salt with base leads to the lactim ether. 

No comparison between trialkyloxonium fluoroborates and dialkyl sulfates has been made, but analysis of available data shows that oxonium salts are more generally applicable reagents for the preparation of lactim ethers. The alkylation of 3-carbethoxypiperid-2-one (7)25 and morpholin-3-one (8)32 with dimethyl sulfate failed, but with triethyloxonium fluoroborate these compounds gave 2-ethoxy-3-carbethoxy-3,4,5,6-tetrahydropyridine (9) and 3-ethoxy-3,4-dehydro-morpholine (10) in excellent yield. The selective character of triethyloxonium fluoroborate is shown in its reaction with 3,3-diethyl-5-methylpiperidine-2,4-dione (11 ).25 Reaction of 11 with the calculated quantity of dimethyl sulfate resulted in alkylation of the carbonyl group in position 4 with formation of 12, but reaction of 11 with triethyloxonium fluoroborate gave the lactim ether (13). 

Oxonium salts, for example, [(Et20) H]+MCU from the reaction of MC13 with HCI in ether are viscous oils. 

An oxonium salt may be readily prepared from the reaction of AICI3 with HCl in diethyl ether (equation 18). This is a viscous oil similar to quaternary ammonium salt ionic liquids. 

Heterocyclics containing P-atoms are usually strong nucleophiles. For example the basicity of 2-methoxy-ODP (pKa = 3.1) is much higher than that of cyclic ethers or sulfides. Therefore, a wide range of initiators, i.e., carbenium or oxonium salts, Lewis and protonic acids, and relatively nonreactive alkyl halides and organoalu-minum compounds have been used. Usually, reactions were carried out in a N2 atmosphere, although no special precautions (e.g. vacuum) were used to avoid contamination with water. 

Reaction of ethers and oxonium salts with nitrogen compounds... 

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