TY - JOUR AU - Wallabregue, Antoine AU - Sherin, Petr AU - Guin, Joyram AU - Besnard, Céline AU - Vauthey, Eric AU - Lacour, Jérôme TI - Modular Synthesis of pH-Sensitive Fluorescent Diaza[4]helicenes PY - 2014 JF - European Journal of Organic Chemistry JA - Eur. J. Org. Chem. SN - 1434-193X IS - 29 SP - 6431 EP - 6438 L1 - http://onlinelibrary.wiley.com/doi/10.1002/ejoc.201402863/pdf L2 - http://onlinelibrary.wiley.com/enhanced/doi/10.1002/ejoc.201402863 L3 - http://doi.wiley.com/10.1002/ejoc.201402863 M3 - 10.1002/ejoc.201402863 UR - http://dx.doi.org/10.1002/ejoc.201402863 KW - fluorescence KW - pH sensing KW - hHelicenes KW - chiral resolution KW - reaction mechanisms N2 - 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 (NH2NH2) 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 (pKa ≈ 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 (ΔG‡ = 30.7 ± 4.0 kcal mol–1 at 140 °C). ID - 1434 ER -