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

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.