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
2021
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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