Silica gel-assisted preparation of 2,4,7-trihalobenzo[b]thiophenes and 4,7-dihalobenzo[b]thiophenes

All 6 isomers of (bromo)(chloro)(iodo)benzo[b]thiophenes containing three different halogen atoms at the 2-, 4- and 7-positions were synthesized from the corresponding 2-(1-adamantylsulfanyl)-1,4-dihalo-3-(haloethynyl)benzene derivatives, through treatment with silica gel under thermal conditions. In contrast to our previous method using gold chloride catalyst, migration of adamantyl group did not occur in this method using silica gel. Moreover, by using this method, we synthesized 6 types of 4,7-dihalobenzo[b]thiophenes in 4 steps from 2-fluoro-3,6-dihalo-benzaldehydes in moderate to excellent overall yields, 42–82%.