Synthesis of Bridgehead Halides by Barton Halodecarboxylation

Abstract

Bridgehead carboxylic acids can be converted into their corresponding chlorides very efficiently under Barton halodecarboxylation conditions. Addition of the acid chloride to a suspension of the sodium salt of 1-hydroxypyridine-2(1H)- thione in boiling carbon tetrachloride under irradiation leads to excellent yields of the bridgehead chloride via the derived thiohydroxamic ester. In a useful modification for the synthesis of volatile halides, either 1,1,1-trichloro-2,2,2-trifluoroethane or trichlorofluoromethane can be employed as substitute solvents. It is found that the Barton procedure is applicable to the synthesis of labile bromides such as 1-bromobicyclo[3.1.1]heptane for which the usual Hunsdiecker reaction fails. For these, and other brominations , 2-bromo-2-chloro-1,1,1-trifluoroethane ('Halothane') is shown to function as an efficient solvent/bromine atom donor.