Spiro‐Fused vs 575‐Ringed Boron‐Doped Polycyclic π‐Systems: Selective Synthesis, Reactivity and Properties

Abstract Incorporation of heteroatoms and/or non‐hexagonal rings into polycyclic aromatic hydrocarbons (PAHs) can alter their intrinsic structures and physical properties. However, it is challenging to construct PAHs featuring boron/carbon composition and non‐hexagonal combination. Herein, we disclose the selective synthesis of spiro‐type and pentagon/heptagon‐containing boron‐doped polycyclic π‐systems by the Scholl reaction. Two spiro‐fused organoboranes 1 and 2 and one 575‐ringed boron‐doped nanographene 3 were synthesized. The key point is that the boron‐edged π‐system enabled unexpected spiro‐formation via dearomatization, whereas the boron‐centered π‐system enabled desired 575‐ringed π‐extension via cyclodehydrogenation. The thus‐obtained molecules exhibit the intriguing but fully distinct reactivity, electronic structures and properties, for instance, while 2 may react with Brønsted acid to display reversible chemochromism and further convert into 1 , 3 possesses very broad absorption, short excited‐state lifetime and no fluorescence nature. As disclosed, the boron atom together with the spirocycle region or 575‐ringed structural motif significantly contribute to their characteristic properties. Thus, this study sheds light on control over the Scholl reaction using organoborane π‐systems and will promote the exploration of more sophisticated polycyclic π‐systems as organic reactive and optical scaffolds.