Alkoxycarbenium ions under acidic conditions

Generation of alkoxycarbenium ions is often carried out by treatment of the corresponding acetals or their derivatives with a Lewis acid in the presence of a carbon nucleophile. This type of reaction serves as an efficient method for carbon-carbon bond formation under acidic conditions. Mechanistic studies of Lewis-acid promoted reactions of acetals using NMR spectroscopy showed the presence of Lewis acid-acetal complexes in the solution, but alkoxycarbenium ions were never detected. The absence of signals resulting from alkoxycarbenium ions in the NMR spectra, however, does not necessarily rule out their intermediacy in the reactions with nucleophiles. Therefore, it was imperative to accomplish the reactions of spectroscopically characterized, nonsta-bilized alkoxycarbenium ions with carbon nucleophiles. The cation-pool method made this possible and opened a new chapter in the chemistry of alkoxycarbenium ions. 

When alcohols are added to the reaction mixture, unsymmetrical ether products may be obtained. Starting with a mixture of aldehydes can also give rise to the formation of unsymmetrical ethers. These ether products are formed under conditions different from those used in the formation of ethers directly from alcohols. Thus, it is postulated that the reaction sequence that leads from the carbonyl substrate to the ether involves the intermediate formation of hemiacetals, acetals, or their protonated forms and alkoxycarbenium ions, which are intercepted and reduced to the final ether products by the organosilicon hydrides present in the reaction mix. The probable mechanistic scheme that is followed when Brpnsted acids are present is outlined in Scheme 2.311-327 328... 

Problems have been observed when attempting to carry out reactions with either diacid chlorides or half-ester half-acid chlorides when the two carbonyl functions are separated by either two or three carbon atoms. Rearrangement reactions occur with those compounds and so Friedel-Crafts acylation reactions may yield mixtures of products. Optically active methyl 3-methylglutarate was shown to racemize easily Suggested explanations of these effects include the involvement of alkyl diacyloxonium and acyl-oxy-alkoxycarbenium ions. NMR studies have shown that the half methyl ester-half acid chloride from phthalic acid forms the acyloxy-alkoxycarbenium ion very easilyj and that the related ions derived from succinic and glutaric acids can also be generated under stable ion conditions. ... 

This reaction was first reported by Kriewitz in 1899. It is the condensation between formaldehyde and olefins to form unsaturated primary alcohols under conditions of high temperature (e.g., 170°C). In this reaction, water or acetic acid is usually applied as the solvent, and an acid is added as a catalyst, such as sulfuric acid. Under these conditions, formaldehyde and some ketones function as an enophile. The double bond in the new unsaturated alcohol is adjacent to its original position in the starting olefin," with the exception of camphene, which gives an allylic alcohol. For the acylic olefins in this condensation, the isobutylene-type olefins are much more reactive than the monosubstituted olefins or 1,2-disubstituted olefins. The further extension of this reaction involving an intramolecular condensation of an a-alkoxycarbenium ion with a double bond to form a hydropyran derivative is known as the Prins-Kriewitz cyclization. ... 

By reacting 1,4-butane diol with paraformaldehyde in the presence of sulfuric acid at 150-180" , the seven-membered 1,3-dioxepane is prepared. With other aldehydes, homologous 2-alkyl-substituted 1,3-dioxepanes have also been prepared using a cationic ion exchange resin instead of sulfuric acid. This latter technique was introduced by Astle [42]. The dioxepanes have been converted to polymers in methylene dichloride or in 1,2-dichloro-ethylene. The initiator used was boron trifluoride etherate the reaction temperatures ranged from -10° to +10°. The reactions were carried out under anhydrous conditions by techniques suitable for reaction kinetics studies. The work indicated that, at least for this class of compounds, the polymerization propagation step involves linear alkoxycarbenium ions [47]. 

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See also in sourсe #XX -- [ ]

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