Radical initiation, 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 following reaction, known as the Hunsdiecker reaction, proceeds by a radical chain mechanism. Show the steps in this mechanism. (Hint The weakest bond in the compound, the oxygen-bromine bond, is broken in the initiation step.)... 

Only unrearranged cyclopropyl products were reported for photochemical chlori-nation and vapor phase nitration of cyclopropane. The Hunsdiecker reaction of silver cyclopropanecarboxylate and the thermal decomposition of cyclopropanoyl peroxide also gave exclusively unrearranged product as did the di-t-butyl peroxide initiated decarbonylation of 1-methyl and 1-phenylcyclopropanecarboxaldehyde. In general one can predict that when a good radical scavenger, solvent or substrate, is present in the reaction, unrearranged product will result (i.e. see Tables 11 and 13). 

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... 

Microwave Reactions. Side chain bromination of mono and dimethyl heteroaromatic and aromatic compounds by a solid phase A-bromosuccinimide reaction without radical initiator under microwave conditions was developed. The stereoselective S3mthesis of ( )- 8-arylvinyl bromides by microwave-induced Hunsdiecker-type reaction has also appeared. ... 

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). 

The Hunsdiecker Reaction. The classical Hunsdiecker reaction is somewhat restricted due to the relatively harsh conditions required. In the Barton version, alkyl radicals generated from O-acyl thiohydroxamates, under either thermal or photolytic conditions, are efficiently trapped either by CI3C-X (X=C1 or Br CbC is the chain carrier) or by IodoformThe method is applicable to sensitive substrates for which the classical methods are unsuitable. thus allowing the preparation of a wide range of alkyl chlorides, bromides, and iodides by the one-carbon degradation of a carboxylic acid. Similar reactions of aromatic acid derivatives tend to require an additional radical initiator (e.g. Azo-bisisobutyronitrile), if high yields (55-85%) are to be obtained. ... 

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. 

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