Trialkyloxonium salts

Meerwein s reagent, triethvloxonium tetrafluoroborate, is a powerful ethylating agent that converts alcohols into ethyl ethers at neutral pH. Show the reaction of Meerwein s Teagent with cyclohexanol, and account for the fact that trialkyloxonium salts are much more reactive alkylating agents than alkyl iodides. 

ESTERIFICATION OF CARBOXYLIC ACIDS WITH TRIALKYLOXONIUM SALTS ... 

The great utility of the trialkyloxonium salts is illustrated by the fact that high yields of esters are obtained using reagent which has been stored for up to 6 months under the submitters conditions (Notes 1 and 12). Thus either trimethyl- or triethyloxonium fluoborate can be prepared in quantity, stored, and used for esterification as required. 

Other examples of esterification with trialkyloxonium salts have been reported.7,8 The present procedure offers the advantages that the reactive carboxylate ion is generated in sitv and that a low-boiling, nonaqueous solvent is employed, whereby the experimental procedure is considerably simplified. A related method has been reported which utilizes a hindered amine wdth dimethyl sulfate [Sulfuric acid, dimethyl csterj as the alkylating agent.9 The present procedure is carried out under somewhat milder conditions and avoids the use of highly toxic reagents. 

The only other esterification method which rivals the present procedure in convenience, mildness of conditions, selectivity, and yield js the preparation of methyl esters wdth diazomethane [Methane, diazo-] 10 Esterification with trialkyloxonium salts, however, allows... 

Since nitrilium salts can be prepared by treatment of nitriles with trialkyloxonium salts (see 16-8), this is a method for the conversion of nitriles to secondary amines. 

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. 

Oximes such as cyclohexanone oxime and benzaldehyde oxime were catalytically transformed to the corresponding lactams in Af,Af-dhnethylformamide solutions of alkylating reagents such as trialkyloxonium salts. 0-Alkyl-Af,Af-dimethylfonnamidinium salt was... 

The trialkyloxonium salts are powerful alkylating agents. Trimethyl- and triethyloxonium tetrafluoroborates, in particular, have been widely employed for methylation and ethylation of sensitive or weakly nucleophilic functional groups. Alkylations of over 50 such functional groups have been reported in the literature. Examples include amides,4,7,13 16 lac-... 

Tosylhydrazones, with methyllithium to give olefins, 51, 69 Trialkyloxonium salts, as alkylating agents, 51,144... 

A-Alkoxypyridinium salts and their benzo analogues can be formed by treatment of the iV-oxides with alkyl halides, dialkyl sulfates (e.g. Scheme 111) <730SC(5)269), alkyl arenesul-fonates, trialkyloxonium salts (66CB1769), and trialkyl phosphites and their thio analogues (71JGU524). 

Several 1-alkoxyisoindoles have been prepared by alkylation of phthalimidines with trialkyloxonium salts (73TL19H). 

The implication is that where hydride ion abstraction is indicated, the probable true initiator is the add HSbCl6 or HPFS. Dreyfuss et al. further point out that one cannot make the apriori assumption that the reactivity of the dialkyl oxonium ion (THF HSbCle) is comparable to the reactivity of the trialkyl oxonium ion (i.e. the propagating species). In fact, it may be slower (46). Thus any assumptions about the rate of initiation, or about the number of active centers formed, or any attempts to correlate the degree of polymerization of the resulting polymer with the amount of carbonium ion salt or aryl diazonium salt initiators charged should be made with extreme caution. Again, for theoretical studies of the polymerization, the use of preformed trialkyloxonium salts is to be preferred. 

Corresponding thermal data with trialkyloxonium salts would be useful. 

Trialkyloxonium salts are generally prepared by the alkylation of dialkyl ethers.1 90 Alkyl halides can be used as alkylating agents in the presence of strong halide acceptors such as BF3, PF5, SbCl3, and SbF5 [Eq. (4.13)].91... 

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

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)]. 

In contrast to trialkyloxonium salts, the triphenyloxonium salt 34 can be obtained by the reaction of benzenediazonium tetrafluoroborate with diphenyl ether in poor (2%) yield [Eq. (4.20)] and is extremely inert toward nucleophiles. This fact is clearly demonstrated by the remarkable stability of triphenyloxonium halides (Cl, Br, ... 

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