Ethers from trialkyloxonium salts

The starting trialkyloxonium salts can be rather simply prepared from the corresponding dialkyl ethers. However, only trimethyl-and triethyloxonium boroflu-orides have been used in the synthesis of nitronates to prepare O-methyl and O-ethyl nitronates, respectively. 

Pyridazine derivatives can also be obtained from dioxopiperazines (see p 423) by oxidation with trialkyloxonium salts forming bislactim ethers 27. On oxidation with DDQ, they are dehydrogenated providing 2,5-dialkoxypyridazines 28. 

Pyrazine derivatives can also be obtained from dioxopiperazines (cf. p. 486) by alkylation with trialkyloxonium salts providing bislactim ethers 27, which are dehydrogenated by 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) to give 2,5-dialkoxypyrazines 28. 

Tertiary oxonium ions that are active species in CROP of cyclic ethers are inherently stable (trialkyloxonium salts with stable counterions are commercially available and may be stored without special precautions for prolonged periods of time). Thus, if basic impurities are avoided in CROP of cyclic ethers, there is essentially no irreversible termination. Those polymerization are, however, not classified as living because reversibility of propagation and reversible chain transfer to polymer cause deviations from the ideal situation observed for, for example, anionic vinyl polymerization in which DP = [M]o/[I]o/ molecular weight distribution is close to Poisson distribution and the nature of end-groups may be strictly controlled. As will be discussed in subsequent sections, conditions of living polymerization may be more closely approached if polymerization proceeds by the AM mechanism. 

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