SHORT-LIVED 1,5-BIRADICALS FORMED FROM TRIPLET 1-ALKOXY-9,10-ANTHRAQUINONE AND 1-(BENZYLOXY)-9,10-ANTHRAQUINONE

The cyclopropylmethyl and (trans-2-phenylcyclopropyl)methyl radical cl ocks were used to estimate the lifetimes of triplet state biradicals f ormed from substituted 1-alkoxy-9,10-anthraquinones by photoexcitation and subsequent Ig-hydrogen atom transfer. Irradiation (350 nm) of 1-( cyclopropylmethoxy)-2-methyl-9, 10-anthraquinone (Icp) in argon-purged methanol generated the primary anthrahydroquinone product(2). Upon ex posure to air, 2 was rapidly converted to cyclopropanecarboxaldehyde a nd 1-hydroxy-2-X-9,10-anthraquinone (3). In contrast, irradiation of y lcyclopropyl)methoxy}-2-benzyl-9,10-anthraquinone (1pcp) under similar conditions produced only small amounts of 3 and the corresponding ald ehyde, trans-(2-phenylcyclopropyl)carboxaldehyde. In addition, product s 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 homoallyl ic radical and the observed product ratio, lifetimes of approximately 1-2 ns were estimated for 1,5-biradicals from these anthraquinones whi ch are about an order of magnitude shorter than those reported for tri plet state biradicals derived from structurally related benzophenones and acetophenones. The short lifetimes of these biradicals are attribu ted to the facile formation of a zwitterion which results from an intr amolecular electron transfer from one radical site, which serves as el ectron donor, to the other radical site, which is a semianthraquinone and therefore serves as a good electron acceptor. If either the electr on-donating or electron-accepting site is absent in the biradical, zwi tterion formation is not observed and coupling of the biradical occurs resulting in a longer lifetime.