Addition and substitution reactions of hexahelicene: Bromination, nitration and acetylation

Bromine adds to hexahelicene at the C(5)‐C(6) and C(11)‐C(12) bond. The addition products eliminate hydrogen bromide on heating above 100°C, yielding mainly 5‐bromo‐ and 5,12‐dibromohexahelicene, respectively. The same elimination reactions occur on chromatography of the addition products over silica gel or alumina. In pyridine or triethylamine, however, bromine is eliminated. Bromination in the presence of iodine yields also the substitution products 5‐bromo‐ and 5,12‐dibromohexahelicene. Nitration and acetylation are less selective, but under sufficiently mild conditions 5‐ and 5,12‐substitution products can very well be obtained; in both cases an 8‐substituted hexahelicene can be isolated as a second product. Although several reactivity parameters ( N r , F r and μ r values) predict high reactivity towards electrophiles at C(5) the high selectivity of the aromatic substitutions could not be foreseen from F r (and N r ) values, whereas μ r values predict higher reactivity at C(3) than at C(8).