Acetylium ion

The formation of acyl halide-Lewis acid complexes have been observed by several methods. For example, both 1 1 and 1 2 complexes of acetyl chloride, with AICI3 can be observed by NMR spectroscopy. The existence of acylium ions has been demonstrated by X-ray diffraction studies on crystalline salts. For example, crystal structure determinations have been reported for /i-methylphenylacylium and acetylium ions as SbFg salts. There is also a good deal of evidence from NMR measurements which demonstrates that acylium ions can exist in nonnucleophilic solvents. " The positive charge on acylium ions is delocalized onto the oxygen atom. This delocalization is demonstrated in particular by the short O—C bond lengths in acylium ions, which imply a major contribution from the structure having a triple bond ... 

A similar reaction occurs between alkenes and acylium ions, as in the reaction between 2-methylpropene, and the acetylium ion leads regiospecifically to (3,y-enones.54 A concerted mechanism has been suggested to account for this regiochemical preference. 

The acetylium ion is the strongest acid species in the acetic anhydride system, and acetyl fluoborate, which is a good conductor in liquid sulfur dioxide, is a strong acid in acetic anhydride. Acetyl fluoborate, chloride, thiocyanate, and benzene sulfonate may all be titrated as acids in acetic anhydride using sodium acetate, a strong base in that system. 08 The neutralization reaction is ... 

Fig. 5. 75.4-MHz l3C cross-polarization spectra of the acetylium ion 1 on TaCl5 acquired at 298 K. The nonspinning spectrum shows a broad and axially symmetric powder pattern. The MAS spectrum shows that ca. 80% of the starting material (acetyl-f-13C chloride) was ionized to form the acetylium ion (<5lso = 153 ppm), and the rest formed the donor-acceptor complex with TaCls ( lso = 189 ppm). The nonspinning spectrum requires 20 times more scans to acquire than the MAS spectrum. The principal components of the l3C shift tensor of 1 were measured from both spectra, and the results are in very good agreement.

We have prepared a number of acylium ions on metal halide powders and measured the principal components of their chemical shift tensors (43-45). Most of these cations have isotropic l3C shifts of 154 1 ppm. Often insensitivity to substituents results from opposite and offsetting variations in the principal components. The acetylium ion has an axially symmetric chemical shift tensor because of its C3 rotation axis. When the symmetry is reduced from C3v to C2v or lower, a nonzero 27 value may be observed. The sensitivity of chemical shift tensors to symmetry is a powerful means of probing molecular structure and temperature-dependent molecular dynamics. Multiple orders of spinning sidebands may offend those who seek solution-like NMR spectra of solids, but discarding most of the information inherent in the chemical shift is a considerable concession to aesthetics. 

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... 

Although in the reaction of acetyl chloride with 0-naphthol (in nitromethane) unionised halide was thought to be implicated, Satchell108 has shown that in the case of acetyl bromide (in acetonitrile) there is strong evidence that reaction proceeds via free and ion-paired acetylium ions, viz. 

When a stream of oxygen containing 15% ozone was passed through a solution of isobutane in HSC F-SbFs-SOiClF solution held at —78°C, the colorless solution immediately turned brown in color. 1H and 13C NMR spectra of the resultant solution were consistent with the formation of the dimethylmethylcar-boxonium ion in 45% yield together with trace amounts of acetylium ion (CH3CO+). Further oxidation products (i.e., acetylium ion and C02) were reported to be observed in a number of reactions studied. Such secondary oxidation products, however, are not induced by ozone. Similar treatment of isopentane, 2,3-dimethylbutane, and 2,2,3-trimethylbutane resulted in formation of related carboxonium ions as the major products (Table 5.37). 

Straight-chain alkanes also efficiently react with ozone in Magic Acid at —78°C in SO2CIF solution. Ethane gave protonated acetaldehyde as the major reaction product together with some acetylium ion (Scheme 5.62). Reaction of methane, however, is rather complex and involves oxidative oligocondensation to terf-butyl cation, which reacts with ozone to give methylated acetone (Scheme 5.63). 

Relative rates and isomer distributions of the catalyzed acetylation of five-membered heterocycles are not affected by change in catalyst (tin tetrachloride or iodine).130 This is an indication that the electrophilic species involved in the reaction is the acetylium ion CH3CO+ rather than the oxonium complex. 

Other disubstituted carbonium ions such as R2C+ and R2C2+ as well as monosubstituted cations, R2C==C+ and RC+ have not been suggested as relevant reaction intermediates and only for some of them mass spectral evidence has been reported. Although acetylium ions RC=C+ may be reasonably thought to be formed in the decomposition of dia-zonium precursors (Robson and Tedder, 1963) no clear evidence of their existence is as yet available. 

For example, acetylation of benzene by use of CHgCOCl with SbClg as a catalyst is due to the intermediate formation of the acetylium ion, which has been shown to exist in stable ionic compounds such as [CHgCOl+ESbClg]- or [CHgCO]+[BFJ-. 

As was already shown, the catalyst in this reaction serves to polarize the MeCO-Cl bond to form an intermediate tentatively ascribed the structure of the acetylium salt [MeCO] [AlCl4] . It turns out that the acetylium ion can actually be prepared as a stable salt, for example as [MeCO] [SbCy . Since the reactive species is already present in this salt, [MeCO] [SbCy serves as a mild and very active acetylating agent, even for unreactive aromatic compounds. Moreover, the reactions of this salt do not necessitate the presence of any acid catalyst. Hence, reagents of this type can also be utilized for unstable substrates. 

The commonest system for bringing about acetolysis, which will be regarded as being the cleavage of an 0-R bond with concomitant formation of an acetic ester, is acetic anhydride-sulfuric acid. Other systems that have been used to a lesser extent are acetic anhydride-perchloric acid, acetic anhydride-zinc chloride, or trifluoroacetio anhydride-acetic acid. All of these species give rise to the acetylium ion, nCH3CO]+, which is the attacking species in all of these reactions. 

A considerable amount of research has been concerned with the nature of the electrophiles that are involved in Friedel-Crafts acylation reactions. We will summarize the main points. Acyl halides and carboxylic acid anhydrides have been known, for many years, to form stable complexes with a variety of acid catalysts. A well-defined product is formed between acetyl fluoride and boron trifluoride at low temperatures. Analytical and conductivity data characterized the material as acetylium tetrafluoroborate, and this was further confirmed by IR measurements. In the system acetyl chloride-aluminum chloride the acetylium ion can be differentiated from the donor-acceptor complex involving the carbonyl group by means of their IR carbonyl stetching frequencies. A number of other acyl fluorides have been shown to form well-defined acylium salts by interaction with a number of metal fluorides. Acylium salts can also be prepared from acyl chlorides by means of metathetical reactions involving anhydrous salts such as silver hexafluoroantimonate. As well as characterization by means of IR spectroscopy, acylium salts have been studied in non-nucleophilic solvents by NMR spectroscopy. The NMR data for the ben-... 

Other derivatives of carboxylic acids and some unusual catalyst systems have found favor. Diarylboryl hexachloroantimonates activate acyl chlorides, carboxylic anhydrides and acyl enolates. A number of metal oxides have been successfully employed. It is worth noting at this point that the chloroacetylium and bromoacetylium ions, which can be prepared in either Freon 113 or sulfur dioxide, are more stable than the acetylium ion and have been shown to give high yields of ketones at low temperatures. ... 

As an example, acetyl derivatives usually fragment under electron impact to produce acetylium ions, CH3CO+, which, in turn, react with the neutral parent ... 

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