Carboxonium salts

Direct alkylation or acylation of the oxygen of THF by exchange or addition occurs with the use of trialkyl oxonium salts, carboxonium salts, super-acid esters or anhydrides, acyhum salts, and sometimes carbenium salts. 

My last kinetic work was aimed at determining the kp+ of a range of monomers by what I believed to be a reliable method. For kinetic and electrochemical reasons I chose nitrobenzene as the solvent, and I chose carbenium and carboxonium salts as initiators so as to achieve a clean and fast initiation. The rate-constants were adequately reproducible, but it turned out that they were not the kp+. The project was flawed because I had been unaware of the reversible cationation of the solvent by the carbenium ions. A careful analysis of the kinetic, analytical and thermochemical results gave a new insight into the reaction mechanisms in nitrobenzene, but the main objective had eluded me. 

Since some carbenium and carboxonium salts are useful initiators, it is appropriate to analyse the AH4 of reaction (IV) ... 

From carboxonium salts, alkoxymethyleneiminium salts and related compounds (including amide acetals) 561... 

AlAough carboxonium salts (76 equation 47) are easily available and of high alkylation power, they have found little application for alkylation of amide groups. 

Dialkylcarboxonium ions possess a very high alkylation power. If they are reacted with amines or amine derivatives, the substitution of the alkoxy group by the amino compound competes with the dealkylation of the carboxonium ion. Therefore only a few examples for the preparation of iminium salts are known, e.g. (102 equation 61), from carboxonium salts. 

Smith et al. have isolated a series of cyclic carboxonium salts (such as 119-121) by acylation of alkenes. 

The action of trialkyloxonium salts on aliphatic ethers leads to exchange of alkyl groups. Sulfoxides,212 nitriles,213 and disulfides214 can be alkylated smoothly by oxonium salts. Saturated aldehydes and ketones can be alkylated only if, like pinacolin and camphor, they contain a tertiary alkyl group neighboring the carbonyl group. Then for instance, the carboxonium salts obtained can be readily converted by alkali alkoxides into acetals and orthoesters which are difficult to obtain in other ways. 

Tertiary phosphines can be quaternized by sulfonic esters317 or carboxonium salts.318 More important for synthesis is that they can be quaternized even by alcohols in the presence of mineral acid 319,320... 

Intermediates analogous to the carboxonium salt formulated in the preceding example are also formed during Friedel-Crafts acylations and these can undergo decarbonylation, so that reaction of an aromatic compound with an acid chloride may result in alkylation instead of acylation for example,... 

Exclusive alkylation of normally unfavored positions in (po/y-N)-heterocyclics can be achieved by using acyl (acetyl, benzoyl, carbethoxy) as N-protective groups. - E Trimethyloxonium fiuoroborate in nitromethane dropped rapidly under anhydrous conditions into a cooled soln. of l-acetyl-1,2,4-triazole in the same solvent, methanol added to hydrolyze the acyl cation to the protonated salt, and the aq. soln. of the latter neutralized with Na-carbonate -> 4-methyl-l,2,4-triazole. Y 88%. F. e., also alkylation with carboxonium salts, s. R. A. Olofson and R. V. Kendall, J. Org. Chem. 55, 2246 (1970). 

Without additional reagents Carboxonium salts cyclic s. 17, 993 Quinones/perchloric acid Phenalenium salts s. 19, 996... 

Cyclic carboxonium salts. A soln. of 2-elhoxy-l,3-dioxolane in methylene chloride vigorously stirred and treated with BFg-etherate, stirring continued for some time 1,3-dioxolenium fluoroborate. Y 80.7%. F. e. and reactions s. H. Meerwein et al., A. 632, 38 (1960). 

Carboxonium salts, N-alkylation with - 26,454 1-Carboxyacetals, cyclic... 

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