Boronic acids, with acyl halides

The acylation of unsaturated ketones constitutes one of the earliest routes to pyrylium salts (19CB1195). The reaction is better achieved with acyl halides than by anhydrides, and aliphatic are preferable to aromatic acid derivatives. The presence of a Lewis or Bronsted acid is usually necessary and iron(III) chloride, aluminum chloride, boron trifluoride and perchloric acid have found frequent application. It is considered that these interact with the acid derivative to generate the actual acylating agent. 

Palladium-catalysed carbonylation of halides, with carbon monoxide, can be used to prepare esters, amides and ketones by trapping the intermediate acylpalladium halide with alcohols,amines and organometal-lics, respectively. Boronic acids are probably the best organometallics for the preparation of ketones, but conditions must be adjusted to give the best selectivity between the acylation reaction and simple Suzuki coupling of the boronic acid with the starting halide. ... 

TRIOXAPENTADECANE (112-73-2) C12H26O3 Combustible liquid (flash point 245°F/118°C oc autoignition temp 590°F/310°C Fire Rating 1). Reacts violently with oxidizers, permanganates, peroxides and hydroperoxides, ammonium persulfate, bromine dioxide, sulfuric acid, nitric acid, perchloric acid, and other strong acids Lewis acids including aluminum chloride, boron trifluoride, iron(III) chloride, niobium pentachloride, and ytterbium(III) triflate. Incompatible with acyl halides. 

While the first two allow the isolation of the reaction products, the third does not. When a Lewis acid is mixed with an acyl halide, a donor-acceptor complex RCOX. .. MtX , or full ionisation by halide ion transfer, to give RCO MtX +i, or both, take place. The extent of ionisation depends mostly on the nature and strength of the Lewis acid used Thus, for example, acetyl halides react with stannic chloride and titanium tetrachloride to give mostly the coordination complex while with antimony pentachloride, pen-tafluoride and boron fluoride they give the conesponding acylium salts. Many of these... 

Crafts process is to generate carbonium ions from the alkyl or acyl halides. It would be expected, then, that a number of other combinations of starting materials and reagents which lead to carbonium ions should be capable of effecting acylation or alkylation. Indeed we find that olefins (p. 35), alcohols (p. 36), ethers (p. 36), and esters (p. 37) can be used as starting materials for aromatic alkylation reactions in the presence of such catalysts as boron trifluoride, sulfuric acid, or anhydrous hydrogen fluoride.69 Acylations can be carried out with acids (p. 37),64 acid halides (p. 230), and acid anhydrides (p. 37). The Fries reaction65 (in which phenolic esters are converted to hydroxy aromatic ketones by means of aluminum chloride) appears to be an example of a typical acylation reaction in which the ester itself acts as the source of an acyl carbonium ion ... 

Carboxylic acid anhydrides or halides normally require the presence of a Lewis acid (often boron trifluoride) for Friedel-Crafts acylation of furans, though trifluoroacetic anhydride will react alone. Aluminium-chloride-catalysed acetylation of furan proceeds 7 x lO times faster at the a-position than at the P-position. 3-Alkyl-furans substitute mainly at C-2 2,5-dialkyl-furans can be acylated at a P-position, but generally with more difficulty. 3-Bromofuran is efficiently acetylated at C-2 using aluminium chloride catalysis. ... 

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-... 

Metal triflates can be easily prepared from metal halides and triflic acid at -78 C. They show several unique properties compared with the corresponding metal halides. In an early study, Olah reported the use of boron-, aluminum-, and gallium triflates [M(OTf)J as effective Friedel-Crafts catalysts. In the benzoylation and acetylation of toluene and benzene with acyl chlorides, the relative reactivity is boron triflate > gallium triflate > aluminum triflate, in agreement with the relative acidity strength. 

Suh et al.65 have reported a formal total synthesis of Mansonone F and this is described in Scheme 12. 5-Methoxy-a-tetralone (127) was converted to 1-methyl-5-hydroxy-naphthalene (128) by the standard organic reactions. Treatment of (128) with phenyl boronic acid, paraformaldehyde and propionic acid followed by catalytic hydrogenation yielded compound (129). This on alkylation gave the compound (130) which on alkaline hydrolysis was converted to acid. The acid halide underwent intramolecular Friedal-Crafts acylation affording an intermediate (131) whose transformation to Mansonone F has been accomplished by Best and Wege.59... 

Triflates of aluminum, gallium and boron, which are readily available by the reaction of the corresponding chlorides with triflic acid, are effective Fnedel-Crafis catalysis for alkylation and acylation of aromatic compounds [119, 120] Thus alkylation of toluene with various alkyl halides m the presence of these catalysts proceeds rapidly at room temperature 111 methylene chloride or ni-tromethane Favorable properties of the triflates in comparison with the correspond mg fluorides or chlorides are considerably decreased volatility and higher catalytic activity [120]... 

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