Acyl cations acetyl cation

In 1943, Seel observed14 the first stable acyl cation. Acetyl fluoride with boron trifluoride gave a complex (decomposition point 20°C) that was characterized as the acetyl tetrafluoroborate salt 331 [Eq. (3.94)]. 

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. 

Friedel-Crafts acylation involves electrophilic attack by acyl cation (CHsCO ) on the ring, and the ring s electronic character should indicate its susceptibility to attack. Compare electrostatic potential maps of ferrocene and acetylferrocene. Which molecule contains the most electron-rich ring Which acylation reaction should be faster Does an acetyl substituent enhance or diminish ring reactivity What should be the major product when ferrocene is combined with one equivalent of acetic anhydride ... 

DNA sequencing and. 1113 Electrospray ionization (ESI) mass spectrometry, 417-418 Electrostatic potential map, 37 acetaldehyde, 688 acetamide, 791,922 acetate ion. 43. 53, 56, 757 acetic acid. 53. 55 acetic acid dimer, 755 acetic anhydride, 791 acetone, 55, 56. 78 acetone anion, 56 acetyl azide, 830 acetyl chloride, 791 acetylene. 262 acetylide anion, 271 acid anhydride, 791 acid chloride, 791 acyl cation, 558 adenine, 1104 alanine, 1017 alanine zwitterion, 1017 alcohol. 75 alkene, 74, 147 alkyl halide, 75 alkyne. 74... 

Alkylation of an anionic acylmetallate is the second step in the classic Fischer synthesis of carbene complexes. The same type of reaction can be performed with stable neutral acyl complexes, producing cationic carbene compounds. Compound 17 has been prepared from a ruthenium acetyl complex by direct methylation and by protonation with subsequent methylenation (45) ... 

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 ... 

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 importance of acyl cations in acetylation is indicated in several studies (Gore, 1955). Friedel-Crafts catalysts alter the rate of acylation but do not modify the selectivity of the reaction. For example, the antimony pentachloride-catalyzed benzoylation of toluene proceeds 1.3 x 103 more rapidly than the aluminum chloride-catalyzed reaction. 

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... 

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... 

Acyl cations are formed in the reversible addition of carbon monoxide to carbonium ions (Balaban and Nenitzescu, 1959 Koch and Haaf, 1958a, b 1961). The formation of the acetyl cation 193 from carbon monoxide and methane in SbF6 has been directly observed by XH n.m.r. 

The catalysts were examined in the acetylation of toluene, one of the difficult acylations because of the difficulty in the formation of an intermediate acetyl cation (MeCO ) from alkyl chain acid anhydrides and hydrides. The reactivities increase as the length of the hydrocarbon chain of the acylating agent is lengthened [210], which shows acetylation to be the most difficult acylation. The heterogeneous acetylation of toluene using solid acidic promoters has not been reported in detail. In fact, to complete acetylation with the anhydride is quite difficult under reflux condihons [211]. 

Similar studies were also performed with acyl cations. Whereas no exchange was observed in the acetyl cation, exchange did take place in the... 

Metal-exchanged KIO clays were studied as catalysts in the acylation of benzo crown ethers 36 with AC (Scheme 4.25). The best catalyst is SnKlO, which affords product 37 in 90% yield after 1 h. The activity of SnKlO is about three times higher than that of clayfen. Product 37 is accompanied by a minor by-product ( 5%) due to the opening of the polyether ring followed by O-acetylation. The cafalysf acfivify is lowered upon recycling. The redox mechanism depicfed in Scheme 4.26 can represenf an alternative pathway for the formation of an acyl cation fhaf can accounf for the high activity of SnKlO. 

Although the mechanism for the formation of acetylcyclohexane is known, an alternative mechanism is proposed here for the acylation of cyclohexene with acetyl chloride. In this reaction, after the acyl cation adds to double bond, the elimination of the a-proton gives an a,)0-unsaturated ketone, whereas the attack of chloride produces a 0-chloroketone. Since acetyl is an electron-withdrawing group, it is plausible that the positive charge migrates to form a more stable carbocation, which electrophilically alkylates with benzene. 

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). 

Friedel-Crafts acylation was studied and kinetic and spectroscopic evidence was reported for acylium ions as the key reactive intermediate. The acetyl cation was detectable (absorption band at 2308 cm ) in low concentration by in situ IR studies, during the acetylation of toluene. The 4-tert-butylbenzoyl cation was also observed as a weak band at 2283 cm during the course of an acylation of mesitylene. 

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