Double Boron-Deleting Annulation: Modular Access to Chiral Nonalternant B,N-Helicenes with Enhanced Chiroptical Properties

Abstract Direct ring manipulation of planar polycyclic conjugated hydrocarbons into three-dimensional chiral frameworks remains largely underexplored. Herein, we report an unprecedented modular synthesis of nonalternant B,N-embedded helical molecular carbons (HMCs) (BN-HMCs, 12 examples) featuring diaza-pentagon-heptagon pairs via double boron-deleting annulation (dBDA). The quadruple-borylated precursors containing two B2N3-moieties were modularly constructed by C–N cross-coupling and subsequent borylation. The final dBDA process realized structural evolution of boron-centered cove edges into inner helical rims and elicited a dramatic elevation in racemization barriers as corroborated by theoretical calculations and chiral resolution. Double helicenes 2a–2c and 4b derived from twisted precursors with S-shaped orientation of B2N3-units, whereas monohelicenes 4a, 4c–4g, and 5 were generated from C-shaped planar precursors. The reorganized nonalternant skeletons exhibited pronounced red-shifted emission profiles as well as distinct circularly polarized luminescence (CPL) activity. Specifically, the deletion of boron atoms resulted in narrowed energy gaps for red-to-near-infrared emission (e.g., 730 nm for double [7]helicene 2c). Notably, dibenzofuran-based HMC 4d showed a high luminescence dissymmetry factor (|glum| = 0.011) and spirofluorene-based HMC 5 exhibited CPL brightness (BCPL) of 198 M–1 cm–1. These are among the highest values for 1,4-azaborine-embedded (1,4 B,N) HMCs. By escaping from flatland, this work establishes a facile late-stage modification strategy toward nonalternant HMCs with tunable helical architectures, enhanced π-conjugation, and distinct chiroptical properties.