title: Helicene quinones: redox-triggered chiroptical switching and chiral recognition of the semiquinone radical anion lithium salt by electron nuclear double resonance spectroscopy.
abstract: We present the synthesis and characterization of enantiomerically pure [6]helicene o-quinones (P)-(+)-1 and (M)-(-)-1 and their application to chiroptical switching and chiral recognition. (P)-(+)-1 and (M)-(-)-1 each show a reversible one-electron reduction process in their cyclic voltammogram, which leads to the formation of the semiquinone radical anions (P)-(+)-1(•-) and (M)-(-)-1(•-), respectively. Spectroelectrochemical ECD measurements give evidence of the reversible switching between the two redox states, which is associated with large differences of the Cotton effects [Δ(Δε)] in the UV and visible regions. The reduction of (±)-1 by lithium metal provides [Li(+){(±)-1(•-)}], which was studied by EPR and ENDOR spectroscopy to reveal substantial delocalization of the spin density over the helicene backbone. DFT calculations demonstrate that the lithium hyperfine coupling A((7)Li) in [Li(+){(±)-1(•-)}] is very sensitive to the position of the lithium cation. On the basis of this observation, chiral recognition by ENDOR spectroscopy was achieved by complexation of [Li(+){(P)-(+)-1(•-)}] and [Li(+){(M)-(-)-1(•-)}] with an enantiomerically pure phosphine oxide ligand.
authors.affiliation: Laboratory of Organic Chemistry, ETH Zurich , Vladimir-Prelog-Weg 3, CH-8093 Zurich, Switzerland.
authors.forename: David
authors.initials: D
authors.lastname: Schweinfurth
authors.forename: Michal
authors.initials: M
authors.lastname: Zalibera
authors.forename: Michael
authors.initials: M
authors.lastname: Kathan
authors.forename: Chengshuo
authors.initials: C
authors.lastname: Shen
authors.forename: Marcella
authors.initials: M
authors.lastname: Mazzolini
authors.forename: Nils
authors.initials: N
authors.lastname: Trapp
authors.forename: Jeanne
authors.initials: J
authors.lastname: Crassous
authors.forename: Georg
authors.initials: G
authors.lastname: Gescheidt
authors.forename: François
authors.initials: F
authors.lastname: Diederich