Acylium ions reaction with ethers

Ether cleavage can also be effected by reaction with acetic anhydride and Lewis acids such as BF3, FeCl3, and MgBr2.97 Mechanistic investigations point to acylium ions generated from the anhydride and Lewis acid as the reactive electrophile. 

Not only water and alcohols, but also other oxygen compounds, are able to react covalently with acylium ions. In the case of hydroxy compounds the product is stabilized by loss of the proton from the hydroxyl group, but certain ethers give an analogous reaction in which the product is stabilized by loss of a carbonium ion.288 Using acetyl chloride with silver perchlorate in nitromethane as the source of acetyl... 

For the purposes of this review, we include probe molecules that can be either directly adsorbed or formed in situ. Examples of the latter case are carbenium ions and related electrophilic species. We will also consider several important heteroatom-substituted carbenium ions and heteroatom analogs of carbenium ions. Acylium ions are the intermediates in Friedel-Crafts acylation reactions (96). The most simple, stable acylium ion is the acetylium ion, 1, and others are formally derived by replacing the methyl group with other R groups. Oxonium ions, formed by alkylation of an ether, resemble carbenium ions but are in fact onium ions in terms of their structures. Their stabilization requires strongly acidic media, and like carbenium ions, oxonium ions have been proposed as intermediates in a... 

In the course of acyloxycarbocation investigations112 it has been noted that the reactions of both aldehydes and ketones follow an unusual course or are strongly accelerated if either acylium ions RCO+ are present in the reaction mixtures or conditions to generate them in situ arise. These observations are explained by a transformation of carbonyl compounds into the highly reactive acyloxycarbocations 163 which easily react with weak nucleophiles such as vinyl ethers, vinyl esters, etc. Hence the electrophilic catalysis by acyl cations in carbonyl reactions takes place regardless of the origin of the latter. This catalysis was used in the reaction of ketones with nitriles. 

With the help of a combination of selective dissolution and chromatographic separation, several of the cyclic ethers have been separated and isolated <2003OL3745>. Though not of synthetic value, gas-phase cyclization reactions of acylium ions with nitriles, forming 1,3,5-oxadiazinium ions 375 by double nitrile addition followed by cyclization, have been reported (Scheme 75) <2001MI445>. Similarly, the gas-phase reactions of acylium and thioacylium ions with isocyanates (18 Y = 0) and isothiocyanates (18 Y = S) have been reported to result in oxadiazinium (16 X = 0) and thiadiazinium (16 X = S) salts, respectively (see Scheme 2) <2005JAM1602>. 

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