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Modular Synthesis of pH‐Sensitive Fluorescent Diaza[4]helicenes
Author(s) -
Wallabregue Antoine,
Sherin Petr,
Guin Joyram,
Besnard Céline,
Vauthey Eric,
Lacour Jérôme
Publication year - 2014
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.201402863
Subject(s) - chemistry , racemization , protonation , fluorescence , enantiomer , conjugated system , bond cleavage , stereochemistry , organic chemistry , catalysis , polymer , ion , quantum mechanics , physics
Configurationally stable diaza[4]helicenes have been prepared in two steps by using a particularly facile N–N bond‐cleavage reaction. The synthetic procedure uses hydrazine (NH 2 NH 2 ) for the introduction of a single nitrogen atom. The strategy is general, modular and highly tolerant to functional groups. A mechanistic rationale is proposed for the spontaneous N–N bond‐cleavage reaction. The resulting helical quinacridines are dyes that present absorption and emission properties that can be modulated as a function of pH; the pink quinacridine and green protonated forms (p K a ≈ 9.0) display distinct optical features in the near‐IR region. Single enantiomers were obtained by chiral stationary phase HPLC resolution. The absolute configurations were assigned by comparison of the ECD spectra of the conjugated acids with those of known dialkylquinacridinium derivatives. A rather high racemization barrier was measured by means of variable‐temperature ECD experiments (Δ G ‡ = 30.7 ± 4.0 kcal mol –1 at 140 °C).