Selective preparation of polycyclic aromatic hydrocarbons. Part 5.1 Bromination of 2,7-di-tert-butylpyrene and conversion into pyrenoquinones and their pyrenoquinhydrones

The bromination of 2,7-di-tert-butylpyrene 1 with 1.1 mol equiv. and 2.2 mol equiv. of bromine in a carbon tetrachloride solution afforded 1-bromo-2,7-di-tert-butylpyrene 2a and a mixture of 1,6-dibromo- (3a) and 1,8-dibromo-2,7-di-tert-butylpyrene (3b) in 85 and 73% yield, respectively. On the other hand, when the same reaction was carried out with 6.0 mol equiv. of bromine in the presence of iron powder, an acid-catalysed rearrangement of bromine atoms was observed to give 4,5,9,10-tetrabromo-2,7-di-tert-butylpyrene 4 in 90% yield. The same orientation in the chlorination of substrate 1 was also observed to afford 2,7-di-tert-butyl-1-chloropyrene 2b. The conversion of tetrabromide 4 into 2,7-di-tert-butylpyrene-4,5,9,10-tetraone 15 was carried out by the reaction of compound 4 with sodium methoxide in the presence of copper(I) iodide, followed by demethylation of the corresponding 4,5,9,10-tetramethoxy derivative 12 with boron tribromide. The strong charge-transfer complex of tetraone 15 and its hydroquinone 13 was observed as being due to the pyrene skeleton. The mechanism of the above novel bromination is also discussed.