A Modular Cascade Synthetic Strategy Toward Structurally Constrained Boron-Doped Polycyclic Aromatic Hydrocarbons.

A novel synthetic strategy was developed for the construction of difficult-to-access structurally constrained boron-doped polycyclic aromatic hydrocarbons (sc-B-PAHs) via a cascade reaction from the readily available ortho-aryl-substituted diarylalkynes. This domino process involves borylative cyclization, 1,4-boron migration and successive two-fold electrophilic borylation. Two types of sc-B-PAHs bearing B-doped [4]helicene (1 a-1 i) or BN-doped [4]helicene (1 n-1 t) and double [4]helicene (1 u-1 v) are constructed by this cascade reaction. Remarkably, this synthetic strategy is characterized by modest yields (20-50 %) and broad substrate scope (18 examples) with versatile functional group tolerance. The resultant sc-B-PAHs show good stability under ambient conditions and are thoroughly investigated by X-ray crystallography, UV/Vis absorption and fluorescence spectroscopy, and cyclic voltammetry. Interestingly enough, BN-doped [4]helicene 1 o forms a unique alternating π-stacked dimer of enantiomers within a helical columnar superstructure, while BN-doped double [4]helicene 1 u establishes an unprecedented π-stacked trimeric sandwich structure with a rare 2D lamellar π-stacking. The synthetic approach reported herein represents a powerful tool for the rapid generation of novel sc-B-PAHs, which are highly attractive for the elucidation of the structure-property relationship and for potential optoelectronic applications.