Acylium ions from acyl halides

Before you start, discuss as a group the mechanisms for forming acylium ions from acyl halides and carboxyhc anhydrides in Section 15-13. Then divide your group in two and analyze the outcome of the following two reactions. Use the NMR spectral data given to confirm your product assignments. (Hint D is formed via C.)... 

The acyl cation (acylium ion, oxocarbonium ion) is a resonance hybrid of the two main contributing structures 192 and 192a. The importance of structure 192a is indicated, for instance, by the high frequency carbonyl absorption (2200-2300 cm-1) observed in acyl cations generated from acyl halides and Lewis acids (Olah et al., 1962, 1963 Bethell and Gold, 1967). 

The electrophile is the acylium ion, R-C +, generated by Lewis acid-catalyzed ionization of a leaving group (path Dn) from acyl halides or acid anhydrides (shown in the previous section). The proton that is lost comes from the same carbon that the electrophile attacked. The reaction fails for deactivated rings (Ai wg, meta directors). After the electrophile adds it deactivates the ring toward further attack. No rearrangement of the electrophile occurs. 

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 ... 

Brown and Jensen395 suggested that the rate equation (194) for the reaction of benzene with excess benzoyl chloride could be interpreted according to the mechanisms given by the reactions (201) and (202), (203) and (204) and (205) and (206) which refer to nucleophilic attack of the aromatic upon the polarised acyl halide-catalyst complex, upon the free acylium ion, and upon an ion pair derived from the acyl halide-catalyst complex, viz. 

Acylium ions can be formed in superacid solutions from carboxylic acids and acyl halides (8). They are among the best characterized carbenium ions, and single-crystal X-ray structures of a number of them have been determined as BFf, SbFg, or TaClfi salts (135-139). Solid-state NMR characterization of these species on AlBr3 and other solid superacids was described earlier in this review. 

The acylium ion (RCO ), which may be generated from an acyl halide and a Lewis acid catalyst, also achieves some stabilization from oxygen lone pairs, and the carbocation remains localized on the carbonyl group. Consequently, Friedel-Crafts acylation is not accompanied by the rearrangements that affect alkylations (Scheme 4.7). 

Friedel-Crafts acylation of alkenes (Daizens-Nenitzescu reaction ) with unsaturated acylium ions generated from acid halides and Lewis acids constitutes a general synthesis of divinyl ketones. 

Acyl halides bearing a-hydrogen atoms, and acetyl chloride in particular, have limited stability in the presence of strong Lewis acids. Elimination of proton from the acylium ion leads to the ketene. This intermediate undergoes ready Friedel-Crafts acylation, acetyl chloride eventually forming the diacetylace-tylium ion, ° which is a poor acylating agent. For this reason, excessive reaction temperatures and times should be avoided in Friedel-Crafts acetylations. 

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-... 

Lewis acids often form strong interactions with electronegative atoms such as halides or oxygen. In the Friedel-Crafts acylation, which you will meet in Chapter 21, for example, AICI3 removes the chloride ion from an acyl chloride to give a species, the acylium ion, which is reactive enough to combine with benzene. 

In most Friedel-Crafts acylations the electrophile appears to be an acylium ion formed from an acyl halide in the following way ... 

Many electrophilic species are generated by the action of Lewis acid catalysts. For example, Friedel-Crafts acylation may occur through the involvement of the acylium ion (i.e., 4) often generated by Lewis acid-promoted hahde abstraction (Eq. 1.2) [7]. Similar Lewis acid-promoted reactions may be used to give carbocationic species from alkyl halides, carboxonium ions from acetals and related precursors, iminium ions from a-haloalkylamines, and others. 

Apart from the arene substrate and an alkyl halide or acyl halide/acid anhydride, Friedel-Crafts alkylations and acylations most often involve a Lewis acid promoter in the form of a metal halide, like AlCl. Several species may function as the active electrophile, and both acyl halide/metal halide complexes and acylium ions have been observed experimentally. The complex forming ability of these metal halides complicates all mechanistic evaluations, and Friedel-Crafts reactions have rarely been the subject of quantum chemical mechanistic studies. 

The electrophile in a Friedel-Crafts acylation is an acyl cation (also referred to as an acylium ion) and is formed on reaction of acyl chlorides with aluminum chloride in much the same way as alkyl cations are formed from alkyl halides. 

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