Fluorides, acyl from tetrafluoroborate salts

The transient nature of carbocations arises from their extreme reactivity with nucleophiles. The use of low nucleophilicity gegenions, particularly tetrafluoroborates (BF4 ) enabled Meerwein in the forties to prepare a series of oxonium and carbo-xonium ion salts, such as R30+BF4- and HC(OR)2+ respectively. These Meerwein salts are effective alkylating agents, and transfer alkyl cations in SN2 type reactions. However, simple alkyl cation salts (R+BF4 ) were not obtained in Meerwein s studies. The first acyl tetrafluoroborate, i. e. acetylium tetrafluoroborate was obtained by Seel16 in 1943 by reacting acetyl fluoride with boron trifluoride at low temperature. 

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-... 

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