Heterohelicenes through 1,3-Dipolar Cycloaddition of Sydnones with Arynes: Synthesis, Origins of Selectivity, and Application to pH-Triggered Chiroptical Switch with CPL Sign Reversal

Regioselective access
to heterohelicenes through the 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 that 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.