Abstract
The new configurationally stable, unsymmetrical 2,12-diaza[6]helicene was synthesized as a racemate and the enantiomers were separated in an enantiopure state by semi-preparative HPLC on chiral stationary phase. Under selected alkylation conditions it was possible to obtain both the enantiopure 2-N-mono- and di-N-ethyl quaternary iodides. Metathesis with bis(trifluoromethanesulfonyl)imide anion gave low-melting salts which were tested as inherently chiral additives to achiral ionic liquids for the electrochemical enantiodiscrimination of chiral organic probes in voltammetric experiments. Remarkable differences in the oxidation potentials of the enantiomers of two probes, a chiral ferrocenyl amine and an aminoacid, were achieved; the differences increase with increasing additive concentration and number of alkylated nitrogen atoms.
Highlights
We have recently developed a research in the field of chiral ionic liquids (CILs), and more broadly organic ammonium salts, with the aim of designing new sensing materials endowed with powerful discrimination ability for enantiomeric pairs of different nature, functionalization and use
We investigated the effects of the structural relationships between the group constituting the stereogenic element responsible for chirality and the ammonium function characterizing the properties: we demonstrated that the coincidence of these moieties, which occurs in Inherently Chiral (IC) materials, strongly enhances the enantiodiscrimination ability of a CIL [1,2] while the structural independence of these units, which corresponds to the most popular design of CILs, is followed by more modest enantioselection results [3]
The starting materials and solvents for azahelicene synthesis were purchased from Carlo Erba (Milan, Italy) and used without further purification; photolysis reactions were performed on a Multirays instrument (Helios Italquartz, Cambiago, Italy) equipped with sets of 10 lamps of different wavelength; NMR spectra were recorded on Bruker AV400 and Bruker AV300 spectrometers
Summary
We have recently developed a research in the field of chiral ionic liquids (CILs), and more broadly organic ammonium salts, with the aim of designing new sensing materials endowed with powerful discrimination ability for enantiomeric pairs of different nature, functionalization and use. We investigated the effects of the structural relationships between the group constituting the stereogenic element responsible for chirality and the ammonium function characterizing the properties: we demonstrated that the coincidence of these moieties, which occurs in Inherently Chiral (IC) materials, strongly enhances the enantiodiscrimination ability of a CIL [1,2] while the structural independence of these units, which corresponds to the most popular design of CILs, is followed by more modest enantioselection results [3]. Bi-benzimidazolium (1), bicollidinium (2) and (P)-aza[6]helicenium bistriflimidate (3) salts. We synthesized the 5-octyl-5-aza[6]helicenium bistriflimidate (3), prompted by the consideration that aza[n]helicenes are a class of chiral molecules possessing peculiar electronic and chiroptical properties [4] and display high configurational stability when constituted by at least six fused 6-membered rings [5]
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