Bromides, acyl, also

Sulfonylation. Under Friedel-Crafts reaction conditions, sulfonyl haUdes and sulfonic acid anhydrides sulfonylate aromatics (139), a reaction that can be considered the analogue of the related acylation with acyl haUdes and anhydrides. The products are sulfones. Sulfonyl chlorides are the most frequently used reagents, although the bromides and fluorides also react ... 

In the acylation of enamines derived from 3-substituted cyclohexanones, 6-acylated products were favored over 2-acylated products (398), thus revealing another selective enamine reaction sequence. The use of oxalyl bromide for the acylation of enamines has also been described (399). 

Polyphosphoric acid is a commonly used catalyst for this reaction however, in some cases a mixture of hydrogen bromide/acetic acid gives better results. Acylation of the S-phenyl-, V-(4-tolyl)- or S-(l-naphthyl)-substituted thiobenzenepyruvic acids 3a-c affords the corresponding dibenzo[A,/]thiepins in satisfactory yields, while reaction of the S-(4-methoxyphenyl) or S-(2-naphthyl) derivatives fails to provide any thiepin.60 The intramolecular Friedel-Crafts acylation of 2-(arylsulfanyl)benzeneacetic acids also yields the corrresponding dibenzothiepins in this case the use of hydrogen fluoride sometimes results in purer products.38 The applicability of this method is restricted to the synthesis of stable bisannulated thiepins. 

Aldehydes can be directly converted to acyl chlorides by treatment with chlorine however, the reaction operates only when the aldehyde does not contain an a hydrogen and even then it is not very useful. When there is an a hydrogen, a halogenation (12-4) occurs instead. Other sources of chlorine have also been used, among them S02Cl2 and r-BuOCl. The mechanisms are probably of the free-radical type. V-Bromosuccinimide, with AIBN (p. 912) as a catalyst, has been used to convert aldehydes to acyl bromides. 

Wilkinson s catalyst has also been reported to decarbonylate aromatic acyl halides at 180°C (ArCOX ArX). This reaction has been carried out with acyl iodides, bromides, and chlorides. Aliphatic acyl halides that lack an a hydrogen also give this reaction, but if an a hydrogen is present, elimination takes place instead (17-16). Aromatic acyl cyanides give aryl cyanides (ArCOCN—> ArCN). Aromatic acyl chlorides and cyanides can also be decarbonylated with palladium catalysts. °... 

Carboxylic acids can be converted to acyl chlorides and bromides by a combination of triphenylphosphine and a halogen source. Triphenylphosphine and carbon tetrachloride convert acids to the corresponding acyl chloride.100 Similarly, carboxylic acids react with the triphenyl phosphine-bromine adduct to give acyl bromides.101 Triphenviphosphine-iV-hromosuccinimide also generates acyl bromide in situ.102 All these reactions involve acyloxyphosphonium ions and are mechanistically analogous to the alcohol-to-halide conversions that are discussed in Section 3.1.2. 

The synthesis of azoniaspirocycles can also be carried out on solid support (Scheme 17) <2005JOC9622>. In the following example the resin-bound piperazine-tethered secondary amine 149 underwent an acylation to give amide 150. This was followed by a spontaneous intramolecular displacement of the bromide to yield the trisubstituted azoniaspiroundecane 151. 

Because of the particular structural features of compound 4, pointed out in Section I, the D-glucofuranosyluronic halide anomers not only have inverse thermodynamic stabilities with respect to those of D-glucopyranosyl halides but also show a different behavior towards alcohols. For instance, 2,5-di-O-acyl-a-D-gluco-furanosylurono-6,3-lactone halides, which are difficult to prepare, do not react with alcohols, inasmuch as an endo approach of the reagent is inhibited.14 The /3-bromides and -chlorides, however, just like /3-D-glucopyranosyl chlorides, are subject to alcoholysis, with formation of /3-D-glucofuranosidurono-6,3-lactones.16... 

As noted earlier, [HCr(CO)5] also converts alkyl halides to alkanes, but the reactivity of the acyl chloride is much higher, such that it was possible to selectively convert the acyl chloride to an aldehyde in one step, without interference from the alkyl bromide functionality. A second equivalent of [HCr(CO)5] further reduced the alkyl bromide (Eq. (27)). 

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