SYNTHESIS OF BRIDGEHEAD FLUORIDES BY FLUORODEIODINATION

Fluorodeiodination is found to be an attractive procedure for the synthesis of bridgehead fluorides. Thus, treatment of the corresponding iodide with xenon difluoride in dichloromethane at ambient temperature generally leads to high yields of the fluoride. Evidence suggests the intermediacy of the bridgehead cation in this reaction, and accordingly the substrates which are unfavorably disposed to fluorodeiodination are the bicyclo[n.1.1]alkyl iodides. In this context the isolation of a small quantity of methyl 4-fluorobicyclo[2.1.1]hexane-1-carboxylate (46, R = COOMe) is significant because it represents the first occasion on which the elusive 1-bicyclo[2.1.1]hexyl cation has been trapped. We have also demonstrated that synthesis of the iodides themselves can be accomplished efficiently both by Barton halodecarboxylation and by treatment of the carboxylic acid with lead tetraacetate and iodine.