Photoinduced electron-transfer reactions of homonaphthoquinones with amine and arene donors

Abstract

Photoreactions of monoaryl- and diaryl-homonaphthoquinones bearing methyl, chloro and bromo substituents have been studied in the presence of amine and arene donors. The products of these photolyses are dependent on the substituents and the identity of the added donors. Irradiation of bromo-substituted diarylhomoquinones 1a, b with amines such as triethylamine and diethylamine in various aprotic solvents resulted in the formation of (±)- and meso-3,3′-bis(diphenylmethylene)-2,2′,3,3′-tetrahydro-2,2′-bi-1,4-naphthoquinones 2a, bvia the dimerization of intermediary allyl radicals arising from bromide release accompanied by ring-opening of the cyclopropane ring. Besides dimer 2, a 1:1 amine adduct 4 was also obtained with N,N-dimethylaniline as donor. However, methyl- and chloro-substituted homoquinones remained intact due to the poor leaving ability of these substituents. A similar reaction of bromo-substituted methylphenylhomoquinones 1e gave naphthofuran derivative 6via 2-(α-phenylvinyl)-1,4-naphthoquinone III. In contrast, photoreactions of 1b in the presence of arene donors, naphthalene and methoxy-substituted benzenes, gave no dimeric product but instead gave photoisomerized 2-bromo-3-diphenylmethylene-2,3-dihydronaphthoquinone 3 and its photocyclized xanthylium salt 7. This reaction does not occur in nonpolar solvents. In the case of xanthene as donor, the dimer 2b was again formed in addition to the 3 and 7. These reaction features were interpreted in terms of a difference between proton donating ability of the donor cation radicals.