Fluorescent quenching of the 2-naphthoxide anion by aliphatic and aromatic halides. Mechanism and consequences of electron transfer reactions

The fluorescent excited state of the 2-naphthoxide ion (1) is quenched by aliphatic and aromatic halides according to an electron-transfer mechanism, with generation of the corresponding alkyl and aryl radicals by a concerted or consecutive C-X bond fragmentation reaction. Whereas bromo- and iodobenzene follow a concerted ET mechanism (C-X, BDE control), 1-bromonaphthalene exhibits a stepwise process (pi LUMO control). The photoinduced reaction of anion 1 with 1-iodoadamantane (2) in DMSO affords substitution products on C3, C6, and C8, 1-adamantanol, 1-adamantyl 2-naphthyl ether, and adamantane (3.2, 13.2, 12.2, 2.8, 2.5, and 14.1% yields, respectively). A complex mixture is also observed in the photochemical reaction of neopentyl iodide (3) with anion 1, which renders substitution on C1, C3, C6, C8, and 2-naphthyl neopentyl ether (8.1, 1.3, 19.1, 31.1, and 2.8% yields, respectively). The absence of reaction in the dark and the inhibition of the photoinduced reaction by the presence of the radical traps di-tert-butylnitroxide (DTBN) and 1,4-cyclohexadiene are evidence of a radical chain mechanism for these substitutions. On the other hand, only coupling at C1 is achieved by the photostimulated reaction of anion 1 with iodobenzene (5), to afford 41.9% of 1-phenyl-2-naphthol and 5.4% of disubstitution product. The regiochemistry of these reactions can be ascribed to steric hindrance and activation parameters.