Cation acetyl

The lack of reactivity of acyl cations such as the acetyl cation with deactivated aromatics or saturated hydrocarbons is therefore not un-... 

Apart from Bronsted acid activation, the acetyl cation (and other acyl ions) can also be activated by Lewis acids. Although the 1 1 CH3COX-AIX3 Friedel-Crafts complex is inactive for the isomerization of alkanes, a system with two (or more) equivalents of AIX3 was fonnd by Volpin to be extremely reactive, also bringing abont other electrophilic reactions. 

Volpin called the CH3COCI lAlCk complex an aprotic superacid. The results indicate that the acetyl cation is activated by further 0-complexation with a second molecule of AIX3 (the equivalent of protonation or protosolvation). 

The longer the chain of unbranched carbenium ions is, the more the calculated values deviate from those found experimentally in the direction of higher stability. However, the expected order of ion stability (primary < secondary < tertiary) remains intact. For cations, which are able to delocalize the positive charge due to conjugation in phenyl rings, the calculated stability is too small. The example of the acetyl cation shows that the reliability of the MINDO/3 method decreases, if charged species, especially those containing hetero atoms with free electron pairs, are calculated. 

Anisole under the same conditions gives only -methoxyacetophenone, even though the acetyl cation undoubtedly reacts faster with the methoxy group than with the benzene ring. Since no stabilizing car-bonium ion is split off the former reaction remains reversible and ineffective, while the nuclear substitution is pushed to completion by the removal of a proton. 

Simple acyl cations have been prepared in solution and even in the solid state. It has been found that acetyl cation CH3CO is as stable as ter-butyl cation. Their canonical forms are represented as ... 

The protonation of ketene, CH2 =C=0, in superacid solution leads exclusively to the acetyl cation, CH3. CO (Olah et al., 1972), but initial 0-protonation is not ruled out. Molecular orbital calculations confirm that the cations resulting from protonation of ketene at the carbon atom are more stable than the O-protonated ones, the a-C protonated cations being the least stable (Hopkinson, 1973). 

Simple acyl cations RCO+ have been prepared45 in solution and the solid state.40 The acetyl cation CH3CO is about as stable as the f-butyl cation (see, for example, Table 5.1). The 2,4,6-trimethylbenzoyl and 2,3,4,5,6-pentamethylbenzoyl cations are especially stable (for steric reasons) and are easily formed in 96% H2S04.47 These ions are stabilized by a canonical form containing a triple bond (G), though the positive charge is principally located on the carbon,48 so that F contributes more than G. 

The preparation of acetic acid represents a special case. Olah and coworkers as well as Hogeveen and coworkers have demonstrated that CO can react with methane under superacidic conditions, giving the acetyl cation and by subsequent quenching acetic acid or its derivatives (see Section 7.2.3). Monosubstituted methanes, such as methyl alcohol (or dimethyl ether), can be carbonylated to acetic acid.115 Similarly, methyl halides undergo acid-catalyzed carbonylation.115,116 Whereas the acid-catalyzed reactions can be considered as analogs of the Koch reaction, an efficient Rh-catalyzed carbonylation of methyl alcohol in the presence of iodine (thus in situ forming methyl iodide) was developed by Monsanto and became the dominant industrial process (see Section 7.2.4). 

Enolate additions to p-halovinyl sulfones (289a) show synthetic promise notable is the procedure, by Metcalf, for vinylation of a-amino acid derivatives, e.g. (289a — 317 — 318).220 Recently, Ban reported that alicyclic ketone enolates add to the acetyl cation equivalent (319) to afford 1,3-diketones (Scheme 98).22(" In general, (3-halovinyl sulfones are synthetically superior to the (3-halovinyl sulfoxides. 

Protonation and diprotonation of formic acid and acetic acid have also been studied theoretically (HF and MP2/6-312G levels).553,558 Both cations can be visualized as the donor-acceptor complex of H20 and diprotonated carbon monoxide (HCOH2+) and protonated acetyl cation (CHaCOH23"), respectively. Both dications 277 and 278 are characterized by a long C-OH2 bond and a shorter C-OH bond, suggesting that the latter retains substantial double-bond character. 

Yarkony and Schaefer268 have investigated the acetyl cation MeCO+, which is important in mass spectroscopy. The question of the structure of this species is an interesting one since there are several possible geometrical isomers. The authors investigated the four most stable configurations of C2H30+, structures (9)—(12)... 

Acetic anhydride serves as a source of acetyl cation. 

Scheme 1. Protosolvation of the acetyl cation and its reaction with isobutane.

As noted previously in Chapter 1, the electrophilic reactivities of acetyl salts increase dramatically as the acidity of the reaction medium increases. This was one of the observations that lead Olah and co-workers to first propose the concept of superelectrophilic activation, or protosolvation of the acetyl cation, in 1975.2 This seminal paper described the chemistry of acetyl hexafluoroantimonate (CHsCO+SbFg-) and the reaction with alkanes in various solvents. In aprotic solvents such as SO2, SO2CIF, AsF3, and CH2CI2, there was no reaction. However in HF-BF3, acetyl salts react with Ao-alkanes and efficient hydride abstraction is observed.27 This was interpreted by Olah as evidence for protonation of the acetyl... 

Acetic acid, protonated acetic, diprotonated acetic acid (89), acetyl cation, and the protioacteyl cation (46) were calculated at the MP2/6-31G //GIAO-MP2/tzp/dz level of theory nitric acid, nitronium cation, and the protionitronium cation (3) were calculated at the HF/6-31G // II//6-31G level of theory hydronium ion and the tetrahydridooxonium ion (90) were calculated at the MP2/6—31 G //GIAO-MP2/tzp/dz level of theory. 

There are several reports of activation of acyl cations by superacids, suggesting the involvement of gitonic superelectrophiles.61 As discussed in Chapter 2, hydride transfer from isobutane to the acetyl cation has been reported when the reaction is carried out in excess HF-BF3. At the same... 

Scheme 10. Electrophilic solvation of the acetyl cation and reactions with alkane.

The diprotonated species (179) is then observed slowly cleaving to the acetyl cation (181) in a process thought to involve dication (180). A number of /3-carbenium-acyl dications have also been studied by experiment and theory. Reaction of the 4-chloro-butanoyl cation (182) in superacidic HF-SbFs or HSOsF-SbFs leads to formation of the 2-butenoyl cation (185, eq 56).68... 

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