Selective bromination of polyenes by 2,4,4,6-tetrabromocyclohexa-2,5-dienone

2,4,4,6-Tetrabromocyclohexa-2,5-dienone (TBCO) liberates bromonium ion when treated with polyenes to form brominated products. The results of the reaction of TBCO with simple olefins are presented. The analogous bromo-ketones, 4-bromo-2,4,6-trichloro- and 2,4,6-tribromo-4-methyl-cyclohexa-2,5-dienone, (4) and (5) respectively, afford the same products (3a), (6), and (7) when treated with geranyl cyanide (1; R = CN). The evidence suggests that formation of the dibromide (3a) may be due to sequential reactions. TBCO in the presence of cetyltrimethylammonium bromide serves as an excellent reagent for selective bromination of polyenes under very mild conditions.