Helicene Quinones: Redox-Triggered Chiroptical Switching and Chiral Recognition of the Semiquinone Radical Anion Lithium Salt by Electron Nuclear Double Resonance Spectroscopy

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(7Li) 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.