Hunsdiecker reaction, Barton esters

Perhaloalkanes serve as bromination or iodination agents in the radical decarbox-ylative halogenation of carboxylic acids. In an interesting modification of the Hunsdiecker-Bodin reaction Barton and coworkers have applied iV-hydroxypyridine-2-thione esters as nonelectrophilic intermediates for the decarboxylative bromination and iodination of primary, secondary and tertiary aliphatic and alicyclic592, as well as aroma-... 

The last, but certainly not the least, is the Barton modification to the Hunsdiecker reaction.24-26 It involves decomposition of thiohydroxamate esters in halogen donor solvents such as CCU, BrCCh, CHI3, or CH2I2 promoted by a source of radical initiation, which could be radical initiator (e.g., 18—>20),24 thermal (e.g., 21—>22),25 or photolytic26 conditions. The Barton modification is highly compatible with most functional groups. For example, under photolytic conditions, acid 23 was converted to acid chloride 24, which, without isolation, was treated with the sodium salt of Z/-hydroxypyridine-2-thione (19) with bromotrichloromethane as solvent to give alkyl bromide 25 in 90% yield.26... 

An excellent alternative to the classical Hunsdiecker reaction and its variants, which totally avoids the use of heavy metal salts and potent electrophilic reagents, consists of the simple photolysis or thermolysis of Barton esters in refluxing bromotri-chloromethane for the bromides or tetrachloromethane for the chlorides [4], The analogous decarboxylative iodination can also be achieved using iodoform as the reagent in a benzene/cyclohexene solvent system (Scheme 5). For the cases of vinylic and aromatic acids, where the usual problems of chain efficiency are encountered, the addition of azobisisobutyronitrile (AIBN) is also required [10]. Nevertheless, since this method can operate on both electron-rich and electron-poor aromatic systems, and moreover does not suffer from the competitive electrophilic aromatic bromination found with electron rich aromatics under normal Hunsdiecker conditions, this route to synthetically useful aryl iodides and bromides should find widespread application. 

The difficulties inherent in the original Hunsdiecker reaction and its modifications stimulated the development of an additional halo-decarboxylation method that involves treatment of thiohydroxamic esters of carboxylic acids with BrCCls, ICH3 or CH2I2 in the presence of a radical initiator (Route 3, Barton reaction, Figure 10.23). 

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