Fluorides, acyl salts

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-... 

Besides acetophenone, this reaction was also applied to p-chloro- andp-methoxyacetophenone, and even to an aliphatic ketone, acetone (although the yield was stated to be only half as large as that obtained from mesityl oxide, i.e., less than 30%, Dorofeenko and co-workers reported a 45% yield of 2,4,6-trimethylpyrylium perchlorate from acetone, acetic anhydride, and perchloric acid), and is the standard method for preparing pyrylium salts with identical substituents in positions 2 and 4. The acylating agent may be an anhydride in the presence of anhydrous or hydrated ferric chloride, or of boron fluoride, or the acid chloride with ferric chloride.Schneider and co-workers ... 

The synthesis of 3-aryltetrahydroisoquinolines was accomplished by electrophilic aromatic substitution of polysubstituted phenols and phenyl ethers with Lewis acid-generated tosyliminium ions of 2-tosyl-3-methoxytetrahydroisoquinoline derivatives <00SL801>. In addition isoquinoline was reported to react with N-tosylated (R)- or (S)-amino acid fluorides to afford optically active dihydroimidazoisoquinolinones. The reaction proceeds via acylation followed by attack of the tosylamino group at Cl of the intermediate 2-tosylaminoacylisoquinolinium salt <00TL5479>. 

Diisopropylethylamine (8) is a very hindered strong base that is appropriate for use in couplings of acyl azides and is the base of choice for initiating o/inium salt-mediated reactions and neutralizing the acid liberated during the aminolysis of Fmoc-amino acid fluorides, whether prepared beforehand... 

The conversion of acyl chlorides, sulphonyl chlorides and phosphoryl chlorides into the corresponding fluorides using potassium fluoride in the presence of a quaternary ammonium salt has been recorded [44-47]. Optimum yields are obtained when the dry potassium fluoride contains ca. 1% water. 

Pd salts and CC14 in the presence of K2CO3,362 or (3) RuH PPluV363 Primary alcohols RCH2OH can be directly oxidized to acyl fluorides RCOF with cesium fluoroxysulfate.364 Lactones can be prepared by oxidizing diols in which at least one OH is primary.365... 

Compounds which tend to eliminate hydrogen fluoride in the presence of a base are 1.2-difluoroalkancs, oqa -difluoro ethers, a,a -difluoro sulfides, /1-fluorocarbony] compounds, 1,1-diaryl- and 1,2-diarylpolyfluoroalkanes, 277-acyl fluorides and polyfluoroalkylammonium salts. Polyfluoroalkancs (see Table 2) are relatively labile towards basic reagents. If two elimination pathways are possible, such as in 1,1,2.3,3,3-hexafluoropropanc (1), hydrogen is eliminated from the carbon bearing fewer fluorines.11,2... 

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)]. 

Monocationic acyl ions are readily prepared as persistent species in solutions of low nucleophile strength.68 These acyl ions have been thoroughly characterized by IR and NMR spectroscopy, and several acyl ion salts have been characterized by X-ray crystallography. The monocationic acyl ions are often prepared in situ from carboxylic acids, esters, or anhydrides, by the action of a strong Brpnsted acid, or the ions can be prepared from ionization of an appropriate acid halide with a strong Lewis acid. Both methods have been used to prepare acyl-centered dications, some of which can be considered distonic superelectrophiles. As described previously, dicarboxylic acids cleave to the bis-acyl ions in superacid (FSChH-SbFs) provided that the acyl cations are separated by at least three methylene units (eq 54).55 The first bis-acyl dications were reported by Olah and Comisarow, being prepared by the reactions of dicarboxylic acid fluorides with superacidic SbFs (eq 72).69... 

Condensation of an MSMA isonitrile with an aroylfluoride gives a salt which undergoes loss of trimethylsilyl fluoride to form a nitrile ylid. This transient species reacts with DMAD to form 2-aroyl pyrrole in high yield.162 Substitution of acyl chloride for acyl fluoride in the reaction affords only poor yields of adducts.465... 

The preparation of the fiuoroalkyl perfluorovinyl ethers, e.g. II, starts from the corresponding acyl fluorides which are decarboxylated by heating up to 300 C in the presence of activators, such as zinc(II) oxide and silica gel, or in the presence of salt-forming agents of the sodium carbonate type." ... 

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