Short-Lived 1,5-Biradicals Formed from Triplet 1-Alkoxy- and 1-(Benzyloxy)-9,10-anthraquinones

The cyclopropylmethyl and (trans-2-phenylcyclopropyl)methyl radical clocks were used to estimate the lifetimes of triplet state biradicals formed from substituted 1-alkoxy-9,10-anthraquinones by photoexcitation and subsequent 1,5-hydrogen atom transfer. Irradiation (350 nm) of 1-(cyclopropylmethoxy)-2-methyl-9,10-anthraquinone (1cp) in argon-purged methanol generated the primary anthrahydroquinone product (2). Upon exposure to air, 2 was rapidly converted to cyclopropanecarboxaldehyde and 1-hydroxy-2-X-9,10-anthraquinone (3). In contrast, irradiation of 1-{(trans-2-phenylcyclopropyl)methoxy}-2-benzyl-9,10-anthraquinone (1pcp) under similar conditions produced only small amounts of 3 and the corresponding aldehyde, trans-(2-phenylcyclopropyl)carboxaldehyde. In addition, products resulting from rearrangement of the 1,5-biradical to a homoallylic 1,8-biradical were also obtained. Using the known rate constant for the rearrangement of the phenylcyclopropylmethyl radical to the homoallylic radical and the observed product ratio, lifetimes of approximately 1−2 ns were estimated for 1,5-biradicals from these anthraquinones which are about an order of magnitude shorter than those reported for triplet state biradicals derived from structurally related benzophenones and acetophenones. The short lifetimes of these biradicals are attributed to the facile formation of a zwitterion which results from an intramolecular electron transfer from one radical site, which serves as electron donor, to the other radical site, which is a semianthraquinone and therefore serves as a good electron acceptor. If either the electron-donating or electron-accepting site is absent in the biradical, zwitterion formation is not observed and coupling of the biradical occurs resulting in a longer lifetime.