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.

Helicene-Based Chiral Auxiliaries and Chirogenesis

“…The synthesis was carried out by using the Nickel-catalyzed [2 + 2 + 2] cyclotrimerization reaction of triyne 150, followed by the deprotection of the methoxy group in 151 to result in helicene diol 152 in a racemic form. The enantiomers of 152 were separated on chiral HPLC, followed by the silver oxide-mediated oxidation, to give the quinone 149 in quantitative yield (Scheme 26) [83].…”

“…ENDOR spectra of diastereomeric species, 153 (black) and 154 (red) in THF at 210 K, only the low-frequency region is shown. Reprinted from [83]. © 2014, American Chemical Society.…”

“…Although this topic is not directly related to the category of helicene applications, it is an important background for the whole helicene chemistry; hence, a brief description should be included as a special section. The major methods of obtaining enantiopure helicenes from a racemic mixture are based on optical resolution by chiral HPLC [60,77,78,83,101], diastereomeric salt formation [51,60,76] (for representative resolving agents, see Figure 22), chromatographic separation of diastereomers [52,56,74], and enzymatic-based approaches [97]. …”

Redox Switching of Orthoquinone‐Containing Aromatic Compounds with Hydrogen and Oxygen Gas

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Unique redox switching of orthoquinone-containing pentacyclic aromatic compounds with molecular hydrogen and oxygen in the presence of ap alladium nanoparticle catalyst (SAPd) is disclosed. [7] Thus,two different states of a pconjugated system could be produced by aredox reaction. Clear switching between orthoquinone and aromatic hydroquinone compounds was observed on the basis of their photoluminescence properties.

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“…Va rious switching technologies of the p-conjugated molecules have also been reported, such as photochromism, [1] thermochromism, [1] electrochromism, [2] piezochromism, [3] solvatochromism, [4] and halochromism, [5] as well as redoxmediated chromism induced by electricity [2] or chemical reagents. [7] Thus,two different states of a pconjugated system could be produced by aredox reaction. We were convinced that aromatic compounds containing an orthoquinone moiety were interesting candidates for redox switching,b ecause the orthoquinone could be reduced to the corresponding hydroquinone,and the hydroquinone could also be quickly oxidized to the quinone (Scheme 1).…”