Acyl hypobromite, Hunsdiecker reaction

Problem 16.20 Suggest a typical free-radical mechanism for the Hunsdiecker reaction which requires the initial formation of an acyl hypobromite. RC—OBr, from the Ag salt and Br,. ... 

The Hunsdiecker reaction, a bromodecarboxylation classically involving treatment of the silver salt of a carboxylic acid with molecular bromine, results in the transient generation of an acyl hypobromite, followed by O-Br homolysis. It works with protected silver aldonates, as shown in Scheme 7.5. 

The first step of the Hunsdiecker reaction is quite straightforward. The reaction between silver carboxylate 1 and bromine gives rise to insoluble silver bromide along with acyl hypobromite 3. The unstable acyl hypobromite 3 undergoes a homolytic cleavage of the O-Br bond to provide carboxyl radical 4. Carboxyl radical 4 then decomposes via radical decarboxylation to release carbon dioxide and alkyl radical 5, which subsequently reacts with another molecule of acyl hypobromite 3 to deliver alkyl bromide 2, along with regeneration of carboxyl radical 4. Because of the radical pathway, chirality is often lost for the chiral carbon atom immediately adjacent to the carboxylic acid. 

In a similar vein, the silver (Ag+) salts of carboxylic adds undergo decarboxylation in the presence of bromine (Br2) to produce silver bromide and bromoalkane (the Hunsdiecker reaction). The Hunsdiecker reaction also appears to involve radicals as shown in Scheme 9.101, where,in the first step,silver bromide precipitates from the reaction mixture with formation of an acyl hypobromite (RC(D2Br). Then, homolysis of the oxygen-bromine bond generates bromine atoms and the carboxyl radical, seen here as the same radical generated in Scheme 9.100. Following loss of carbon dioxide (CO2), it is held that the alkyl radical is captured by the bromine atom (Br ) to produce alkylbromide (1-bromoethane [CH3CH2Br]). 

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