Thiohydroxamate esters, Hunsdiecker reaction

Decarboxylativehalogenation (12,417). The Hunsdiecker reaction is not useful for aromatic acids, but decarboxylative halogenation of these acids can be effected in useful yield by radical bromination or iodination of the thiohydroxamic esters, as reported earlier for aliphatic acids.1 Thus when the esters 2 are heated at 100° in the presence of AIBN, carbon dioxide is evolved and the resulting radical is trapped by BrCCl3 to provide bromoarenes (3). Decarboxylative iodination is effected with iodoform or methylene iodide as the iodine donor. 

Dauben et al. found that the CCI3 radical produced by sonolysis of carbon tetrachloride can be used in a decarboxylation-halogenation sequence (Scheme 3.5) [43]. Sonication of a thiohydroxamic ester at 33 °C for 10 - 50 min in carbon tetrachloride leads to the corresponding chloride in high yield. In the presence of bromotrichloromethane or iodoform, bromides and iodides are formed in yields > 80 %. This reaction can be successfully applied to primary, secondary, or tertiary esters and offers an interesting variant to the usual Hunsdiecker procedure. 

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... 

This fast, easily performed variant to the Hunsdiecker procedure, applies successfully to primary, secondary, or tertiary esters, even unsaturated ones (p. 353). From citronellic acid, despite the presence of a double bond in a position permitting cyclization, the latter reaction was not detected. Mechanistically, the sonochemical reaction differs from the photochemical analogue, during which the initiation occurs from the cleavage of the thiohydroxamic ester itself. The initiation probably takes place in the bubble then the halogen atom couples with the alkyl radical in the solution, possibly by a chain mechanism. 

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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