CATION RADICALS IN THE BROMINATION OF BENZODIPYRAN DERIVATIVES

Article abstract of DOI:10.1016/S0040-4020(01)86189-5

Bromine attacks the free aromatic sites in 2,3,4,7,8,9-hexahydro-2,2,7,7-tetramethylbenzo<1,2-b:4,5-b'>dipyran and its angular isomer 1,2,3,8,9,10-hexahydro-3,3,8,8-tetramethylbenzo<1,2-b:4,3-b'>dipyran without clear evidence for the intermediacy of cation radicals.When the nuclear sites are methylated bromine affords a cation radical tribromide (8) and then monohalogenates each aromatic methyl group giving 5,10-bis-(bromomethyl)-2,3,4,7,8,9-hexahydro-2,2,7,7-tetramethylbenzo<1,2-b:4,5-b'>-dipyran (10).In contrast, N-bromosuccinimide dehydrogenates and brominates the two pyran rings giving 3,8-dibromo-2,7-dihydrobenzo-2,2,5,7,7,10-hexamethyl<1,2-b:4,5-b'>dipyran (14).The angular isomer reacts with either bromine or N-bromosuccinimide at the pyran rings only, giving (21).The part played by cation radicals in these reactions is discussed, related to the 1H nmr line broadening phenomena observed earlier, and explained in terms of a slight preferential concentration of unpaired spin at position 5 of a cation-radical derived from 6-hydroxychroman.