A-bromocarboxylic acid bromid

The mechanism in Figure 12.7 implies that the stoichiometric use of phosphorus tribromide in the second variant of the Hell-Volhard-Zelinsky bromination leads to the selective formation of the a-bromocarboxylic acid bromide (Formula B in Figure 12.8). Often these compounds are hydrolyzed to give the bromocarboxylic acids C. More importantly, a-bromocarboxylic acid bromides B can also undergo alcoholysis and in this way provide access to a-brominated esters D. 

Fig. 12.8. Bromination of carboxylic acids in the presence of stoichiometric amounts of phosphorus tribromide ("Hell-Volhard-Zelinsky reaction II"). The actual reaction product is an a-bromocarboxylic acid bromide (B), which often undergoes further in situ reaction to afford an a-bromocarboxylic acid (C) or an a-bromocarboxylic acid ester (D).

Hell-Volhard-Zelinskii reaction The reaction in which a carboxylic acid is treated with Br2 and PBr3 (often formed in situ from red phosphorus and bromine) to give the a-bromocarboxylic acid bromide. 

H-l,4-Benzodiazepin-2,5(lH,4H)-diones a-Bromocarboxylic acid amides from a-bromocarboxylic acid bromides N-Debenzylation... 

McKillop and Ford synthesized a range of primary and secondary alkyl nitrates in excellent yields by treating alkyl bromides with mercury (I) nitrate in 1,2-dimethoxyethane at reflux (Equation 3.9). This method has been used to synthesize substituted nitrate esters from both a-bromocarboxylic acid and a-bromoketone substrates. Unlike metathesis with silver salts, which are widely known to promote SnI reactions, this method is not useful for the synthesis of nitrate esters from tertiary alkyl halides. 

Diorgano tellurium compounds have been reacted with methyl iodide (Vol. IX, p 1076), ethyl iodide ethyl iodoacetate, cyclohexyl iodide , methyl bromide, allyl bromide , benzyl bromide , bromoacetone , bromomethyl phenyl ketone , a-bromocarboxylic acids , a-bromocarboxylic acid esters , methyl chloride, and benzyl chloride . 

Dialkyl, diaryl, unsymmetrical dialkyl, alkyl aryl, and unsymmetrical diaryl telluriums served as the tellurium-containing starting materials (Vol. IX, p. 1075). The reactions with methyl iodide and low molecular mass organic bromides proceed at 20° in organic solvents such as chloroform and methanol or in mixtures of the neat reagents.With aryl telluriums several days are required for completion of the reaction. Diaryl telluriums with ortho-substituents react only with difficulty. Bis[2,4,6-trimethylpheny 1] tellurium did not combine with methyl iodide. The reactions with ethyl iodide and a-bromocarboxylic acids are accelerated by gentle heating. 

A number of syntheses of (V-hydroxy-a-amino acids and derivatives thereof have been reported. a-Bromocarboxylic acids or their r-butyl esters can be treated with hydroxylamine or 0-alkylated hydroxylamines to give the corresponding hydroxylamine derivatives. yV-Benzyloxy-t-alanine has been obtained by reaction of (/ )-a-bromopropionic acid with O-benzylhydroxylamine. But due to bromide exchange the optical yield was low. Anchimeric assistance of a suitable attached thio group can bring... 

Racemisation via Sn2 displacement has been used for the DKR of a a-bromoester by enzymatic hydrolysis the product, a a-bromoacid, was less reactive in the Sn2 process. Investigations of the bromide source and the lipase employed led to an optimised system where an immobilised phosphonium bromide was used, together with cross-linked enzyme crystals from Candida rugosa lipase (CLEC-CRL), to afford the corresponding a-bromocarboxylic acid (Scheme 3.37). This procedure could be extended to a-chloroesters. 

Magnesium bromide a-Bromocarboxylic acid amides from aziridinones... 

N-Bromosuccinimidelhydrogen bromide a-Bromocarboxylic acid dilorides from carboxylic acid chlorides... 

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