Acyl bromide

In the following the reaction is outlined for an a-bromination. The reaction mechanism involves formation of the corresponding acyl bromide 3 by reaction of carboxylic acid 1 with phosphorus tribromide PBr3. The acyl bromide 3 is in equilibrium with the enol derivative 4, which further reacts with bromine to give the a -bromoacyl bromide 5 ... 

The a -brotnoacyl bromide 5 converts unreacted carboxylic acid 1 by an exchange reaction into the more reactive acyl bromide 3, which subsequently becomes a-brominated as formulated above ... 

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. 

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. 

Diazadienes have been used in organic synthesis for the preparation of various heterocyclic compounds. Alkylation of 1,3-diazadienes 207 and the benz-fused analog 210 at the nitrogen atom by aryl acyl bromides provided the iV-alkyl amidinium bromides 208 and 211, which underwent annulation to the 2,3-dihydro-imidazo[2,l-A]thiazole 209 and imidazo[2,l- ]benzothiazoles 212, respectively (Equations 92 and 93) <2001S741, 2002J(P1)741>. 

Two substitutions are occurring here H to Br, and Br to MeO. Looking at the order of reagents, the first substitution is H to Br. Br2 is electrophilic, so the a-C of the acyl bromide must be made nucleophilic. This is done by enolization. The substitution of Br with MeO occurs by a conventional addition-elimination reaction under acidic conditions. 

In addition to acyl chlorides and acyl bromides, there are a number of milder and more selective acylating agents which can readily prepared from carboxylic acids, hnidazolides, the /V-acyl derivatives of imidazole, are examples.108 Imidazolides are isolable substances and can be prepared directly from the carboxylic acid by reaction with... 

Aminotriazole (38) can be alkylated at N(4) with benzyl or acyl bromides in moderate to good yield to give the corresponding bromides (39) <898269,90S707>. These salts can be converted into the corresponding 5-triazolinethiones (40) and into 5-triazolinones (42) via (41) (Scheme 6). 

The relative polarity of two adjacent bound atoms depends in the first place on the quantization, in the second place on polarization properties. A different quantization can lead, for the same two atoms, to the relative polarity. . A+B".. as well as. . A B+.. For instance, C as compared with Br, is more positive in acyl bromide, RCBr, but more... 

Alkylation of a ring nitrogen on 1 with an acyl bromide leads to a cyclization reaction producing a tricyclic compound 2 (Equation 1) <2003CHE669>. 

The easy cleavage of the oxetane ring with acyl chlorides is well known and was reviewed previously (64HC(l9-2)983). More recently it has been observed that 2-oxaspiro[3.2]hexane was cleaved by acyl bromides to give 1-bromomethylcyclopropylcarbinol esters in high yield (68IZV670). 

Halide 32 was used first, in nitromethane-benzene solution in the presence of an equimolar proportion of mercuric cyanide. It was found83 that 32 gave a disaccharide at 0-6 of a protected derivative of 2-acetamido-2-deoxy-D-glucose in good yield, with a certain degree of stereoselectivity (a /3 ratio 7 3). Complete stereospecificity was attained, leading to the desired a-linked disaccharides, when the partially acylated bromide 33 was condensed with a number of different nucleophiles.66,83,84,93-95 A mechanism was postulated83 to explain the... 

These reactions are most important for the preparation of acyl fluorides.1061 Acyl chlorides and anhydrides can be converted to acyl fluorides by treatment with polyhydrogen fluoride-pyridine solution996 or with liquid HF at -10°C.1062 Formyl fluoride, which is a stable compound, was prepared by the latter procedure from the mixed anhydride of formic and acetic acids.1063 Acyl fluorides can also be obtained by reaction of acyl chlorides with KF in acetic acid1064 or with diethylaminosulfur trifluoride (DAST).1065 Carboxylic esters and anhydrides can be converted to acyl halides other than fluorides by the inorganic acid halides mentioned in 074, as well as with Ph3PX2 (X = Cl or Br), 066 but this is seldom done. Halide exchange can be carried out in a similar manner. When halide exchange is done, it is always acyl bromides and iodides that are made from chlorides, since chlorides are by far the most readily available.1067... 

The reaction is slow in the absence of phosphorus, whose function appears to be to form phosphorus tribromide, which then reacts with the acid to give the acyl bromide ... 

Formation of the acyl bromide speeds up the reaction because acid-catalyzed enolization of the acyl bromide occurs much more readily than enolization of the parent acid. Bromine probably reacts with the enol of the acyl bromide in the same way as it reacts with the enols of ketones (Section 17-2 A). 

The final step is the formation of the a-bromo acid by bromine exchange between the a-bromoacyl bromide and the parent acid the acyl bromide, which is necessary for continued reaction, is thus regenerated ... 

The product is weighed, and the theoretical amount of water required to convert it into bromoacetic acid added gradually (1-8 gms. for 20 gms. of the acyl bromide). The mixture solidifies to a white crystalline mass. This is purified by distilling under ordinary pressure from a small distilling flask provided with an air condenser, the portion distilling at 190°—210° being retained. 

A novel microwave-mediated three-component coupling of a-acyl bromides, pyridine and internal alkynes was carried out in the absence of a solvent on activated basic alumina to provide a collection of indolizines (Scheme 3.5)7. It was proposed that the reaction proceeded via in situ generation of a dipole from an N-acyl pyridinium salt, followed by a [3+2] cycloaddition reaction. A dedicated laboratory microwave system was... 

Phosphorus reacts with bromine to give phosphorus tribromide, and in the first step this converts the carboxylic acid into an acyl bromide. 

An acyl bromide can readily exist in the end form, and this tautomer is rapidly brominated at the a-carbon. The monobrominated compound is much less nucleophilic, so the reaction stops at this stage. This acyl intermediate compound can undergo bromide exchange with unreacted carboxylic acid via the anhydride, which allows the catalytic cycle to continue until the conversion is complete. 

RCHO - RCOBr.1 Reaction of aldehydes with NBS in the presence of AIBN provides acyl bromides in good yield. The products, without isolation, can be contorted into amides. 

In contrast to eq. 2.29, eq. 2.30 shows the oxidative conversion of aldehydes (62) to amides (63) via acyl bromides with NBS/AIBN/R2NH under refluxing conditions in CC14 [74]. The reaction comprises of the abstraction of the formyl hydrogen atom by the succinimidyl radical, bromine atom abstraction from NBS by the acyl radical, and lastly,... 

Acyl chlorides are simply transformed into acyl bromides or iodides by reaction with Me3SiBr or Me3Sil, respectively903. Nal is, however, a much simpler alternative904,905. 

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