Synthesis of 2-(4,5-Diphenyl-2-imidazolyl)-5,6,11,12-tetraphenylnaphthacene via 5,12-Dihydro-5,12-dioxo-6,11-diphenyl-2-naphthacenecarbaldehyde

Zhi Zhi HU and Masaru KIMURA*†

Department of Chemistry, Faculty of Science, Okayama University; Tsushimanaka 3-1-1, Okayama-shi 700-8530 Japan
Institute for Fundamental Research of Organic Chemistry(IFOC), Kyushu University; Hakozaki 6-10-1, Fukuoka-shi 812-8581 Japan

In order to prepare a high efficient chemiluminescent imidazole system, we tried to introduce a 2-imidazolyl group into the second position of 5,6,11,12-tetraphenylnaphthacene (rubrene) which may help it to gain a very efficient chemiluminescent compound because of efficient fluorescence and low emission energy in an energy acceptor-emitter of the rubrene moiety. Synthesis of 5,6,11,12-tetraphenyl naphthacene-2-carbaldehyde was planned first for preparation of imidazole ring, but it was too labile to deal with. Therefore, we decided to prepare the imidazole ring using stable 5,12-dihydro-5,12-dioxo-6,11-diphenyl-2-naphthacenecarbaldehyde (8) prior to preparation of air-sensitive naphthacene part. We prepared 6-methyl-4a,5,8,8a-tetrahydro-1,4-naphthoquinone (1) followed by aromatization with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) to give 6-methyl-1,4-naphthoquinone (2). Diels-Alder (DA) reaction of 2 with 1,3-diphenylisobenzofuran gave 6,11-epoxy-5a,6,11,11a-tetrahydro-2-methyl-6,11-diphenyl-5,12-naphthacenedione (3). Dehydration of 3 was carried out with BBr3 to give 2-methyl-6,11-diphenyl-5,12-naphthacenequinone (4). 2-Dibromomethyl-6,11-diphenyl-5,12-naphthacenequinone (6) was prepared by bromination of 4 with N-bromosuccinimide (NBS) in the presence of benzoyl peroxide. 5,12-Dihydro-5,12-dioxo-6,11-diphenyl-2-naphthacenecarbaldehyde (8) as a key intermediate for formation of the imidazole ring was prepared by hydrolysis of 6. 6,11-Diphenyl-2-(4,5-diphenyl-2-imidazolyl)-5,12- naphthacenequinone (9) was prepared by reaction of 8 with benzil and AcONH4. Strongly fluorescent 2-(4,5-diphenyl-2-imidazolyl)-5,6,11,12-tetraphenylnaphthacene (11) was prepared by reaction of imidazolyl derivative 9 with phenyllithium, followed by treatment with Fe/CH3COOH.

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