An Unusual Fluorine Atom Rearrangement in the Photocyclization of 1-Fluorohelicenes

Article abstract of DOI:10.1021/jo00152a022

Oxidative photocyclization of 1-fluoro<5>helicene and two of its chlorine-substituted derivatives leads to the formation of 8-fluorobenzoperylenes through an unprecedented inter-ring fluorine atom migration; photoelimination of HF to give benzoperylenes also occurs as a competing process.The rearrangement pathway predominates when the irradiation is carried out at 0 deg C in air-saturated benzene solution containing 10 ^-3 M iodine; the elimination pathway predominates at higher temperatures or in the absence of either oxygen or iodine.The rearrangement isrationalized by postulating a triplet-state photocyclization to give a dihydrobenzoperylene intermediate, followed by abstraction of the tertiary hydrogen atom from that intermediate to give a radical that subsequently undergoes a <1,3> fluorine atom shift by an S_Ni'mechanism.