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Hetero-helicenes Synthesis Through 1,3-Dipolar-Cycloaddition of Sydnones with Arynes: Synthesis, Origins of Selectivity and Application to pH Triggered Chiroptical Switch with CPL-sign Reversal

submitted on 12.01.2021, 10:04 and posted on 13.01.2021, 08:43 by Expédite Yen-Pon, Floris Buttard, Lucas Frédéric, Pierre Thuéry, Frédéric Taran, Grégory Pieters, Pier Alexandre Champagne, Davide Audisio
The regioselective access to hetero-helicenes through 1,3-dipolar cycloaddition of sydnones with arynes is described. Novel access to sydnones and poly-(hetero)aromatic aryne precursors allowed the introduction of chemical diversity over multiple positions of the helical scaffolds. The origins of the unconventional regioselectivity during the cycloaddition steps was systematically investigated using density functional theory (DFT) calculations, unveiling the key features which control this reactivity, namely face-to-face (π…π) or edge-to-face (C-H…π) interactions, primary orbital interactions and distortion from coplanarity in the transition structures (TSs) of the transformation. From the library of 24 derivatives synthesized, a pyridyl containing derivative displayed reversible, red-shifted, pH triggered chiroptical switching properties, with CPL-sign reversal. It is found that protonation of the helicene causes a change of the angle between the electric and magnetic dipole moments related to the S1 → S0 transition, resulting in this rare case of reversible CPL sign inversion upon application of an external stimulus.




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