Studies on the Photochemical Reactions of 1,4-Naphthoquinones

Abstract

The photochemical reactivity of various 2-alkylamino-1,4-naphthoquinones is the subject of this study. Typically, a 2-alkylamino-3-halogeno-1,4-naphthoquinone (~5.0 x 10-5 mol) was dissolved in benzene (10 mL). The mixture was kept cold in ice-water and irradiated by a medium pressure mercury arc lamp. After the reaction was completed, the reaction mixture was purified by column chromatography on silica gel. The reaction of some 2-bromo-3-alkyllamino-1,4-naphthoquinones, 2,3, and 4, 2-amino-3-bromo-1,4-naphthoquinone 13 was isolated as the product through dealkylation, and characterized by ¹H and ¹³C-NMR spectra. The reactions of 1 and 5 were found stable under irradiation. To gain further insight into the role of the halogeno substituent, here we have extended our study to various 2-alkylamino-3-chloro-1,4-naphthoquinones. In the reaction of 7,8, and 9, the dealkylated product 12 resulted. The reactions of 6 and 10, on the other hand were found stable under irradiation. These results suggest that the α-hydrogen in the alkylamino group of 2-alkylamino-3-halogeno-1,4-naphthoquinones is important for photochemical dealkylation. Additionally, the alkylamino-1,4-naphthoquinone compounds 11 and 12, which lack a bromo or chloro substituent, were also tested and found stable under irradtiation. This result suggests that the bromo or chloro substituent is also important for photochemical dealkylation. Here, we also present further details about rate of reactivities of the different 2-alkylmino-1,4-naphthoquinones. After approximately 23 hours of irradiation, typically the 2-halo-3-isopropylamino-naphthoquinones 3 and 8 completely de-alkylated to products 13 and 14. In the irradiation of 4 and 9, starting compound remained. The relative spatial relationship of the excited carbonyl group to the α-hydrogen atom on the alkylamino group also appears to be an important factor that affects the rate of dealkylation.