Oxonium salts, cyclic

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],... 

The substitution of a CH unit in benzene by 0+ (the oxonia group) gives rise to the pyrylium cation (3). Since this ring still possesses 6 ir-electrons, it may be expected to exhibit aromatic properties. As the oxygen is primarily tricovalent, the pyrylium ring may be formally regarded as a cyclic oxonium ion. However its enhanced stability relative to aliphatic and alicyclic oxonium salts is doubtless due to its aromatic nature. 

When tellurium tetrachloride is condensed with diacetylacetone, CH3.CO.CH2.CO.CH2.CO.CH3, the triketone is transformed to di-methylpyrone, and the cyclic compound combines with tellurium tetrachloride and hydrogen chloride to form the oxonium salt dimethyl-pyrone tellurichloride,... 

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,... 

Remote hydroxylation. Dehalogenation of ot-bromo ketosteroids by AgSbF6 generates by a series of hydride shifts cyclic oxonium salts, which are converted on hydrolysis into hydroxylated steroids. 

Non-deprotonated amides are weak nucleophiles and are only alkylated by trialkyl -oxonium salts or dimethyl sulfate at oxygen or by some carbocations at nitrogen [16, 83]. Alkylation with primary or secondary alkyl halides under basic reaction conditions is usually rather difficult, because of the low nucleophilicity and high basicity of deprotonated amides. Non-cyclic amides are extremely difficult to N-alkylate, and few examples of such reactions (mainly methylations, benzylations, or allyla-tions) have been reported (Scheme 6.21). 4-Halobutyramides, on the other hand, can often be cyclized to pyrrolidinones in high yield by treatment with bases (see Scheme 1.8) [84—86]. 

The oligomer formation during the polymerization of oxetanes has been reininves-tigated by two groups recently. Drey fuss and Dreyfuss (21) found that with oxonium salts or with ethyl trifluoromethanesulfonate as initiator, oxetane forms not only cyclic tetramer and polymer as reported by Rose, but also cyclic trimer. The amount and type of oligomer obtained depends on counter ion, temperature and solvent. Polymeri-... 

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). 

Cationic polymerizaton of vinyl and cyclic monomers can be initiated by various initiators such as protonic and Lewis acids, carbocations, and trialkyl-oxonium salts [1]. General reactions for the initiation of the polymerization may be represented as shown below (Scheme 1). 

Polymerization of cyclic sulfides and amines can be initiated not only by the usual cationic and cationogenic compounds like Lewis and protonic acids, carbenium and oxonium salts and esters of strong acids, but also by alkyl halides, which are active enough to induce polymerization of some azetidines 9 10). 

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. 

Several methods are available for the preparation of acyclic and cyclic tertiary oxonium salts.1 The action of metal or non-metal halide ether complexes on epoxides is generally applicable. For eaxample, triethyloxonium fluoroborate can be prepared as follows 217... 

Mootz, D. and Steffen, M., Crystal-structures of acid hydrates and oxonium salts.l7. Tetrafluoroboric acid methanol (1-2), cyclic molecules by hydrogen-bonds between ions, Angew. Chem.-Int. Ed. Engl. 20 (2), 196 (1981). 

Most commonly initiation proceeds as direct addition of initiator to monomer molecule (route 1). Cationic polymerization of cyclic ethers may be initiated by both Bronsted and Lewis acids. Most commonly used initiators include strong protic acids such as trifluoromethanesulfonic (triflic) acid (also its anhydride or esters), fiuorosulfonic acid, perchloric acid, or heteropolyacids, oxonium salts such as triethyloxonium (e.g., EtsC A ), carbenium (e.g., Ph3C A ), or carboxonium (e.g., CeHsCO A ) salts where A should be stable, weakly nucleophilic counterion (e.g., BF4, PFg, and SbFg) or Lewis acids (most commonly used is BF3 -Et20). Several other initiation systems have been used (e.g., rare earth triflates) but the advantages over typically used simple and easily available initiators have not always been shown. 

Most of the salts capable of polymerising the vinyl monomers already described are also efficient initiators of cyclic monomer polymerisations, but in addition appropriate salts of more stable species such as oxonium, sulphonium and diazonium ions... 

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 ... 

The initiation mechanism for cationic polymerization of cyclic ethers, vinyl amines, and alkoxy styrenes has been investigated by A. Ledwith. He used stable cations, like tropylium or triphenylmethyl cations with stable anions, like SbCl6, and distinguished between three initiation reactions cation additions, hydride abstraction, and electron transfer. One of the typical examples of cationic polymerization, in which the propagating species is the oxonium ion, is the polymerization of tetra-hydrofuran. P. and M. P. Dreyfuss studied this polymerization with the triethyloxonium salts of various counterions and established an order of... 

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