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 (pi center dot center dot center dot pi) or edge-to-face (C-H center dot center dot center dot pi)inter-actions, 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 S-1 -> S-0 transition, resulting in this rare case of reversible CPL sign inversion upon application of an external stimulus.

Automated summary

A regioselective access to heterohelicenes through 1,3-dipolar cycloaddition of sydnones with arynes has been described. The origins of the unconventional regioselectivity were systematically investigated using density functional theory calculations, unveiling the key features that control this reactivity and leading to the discovery of a derivative with reversible CPL sign inversion properties.