Synthesis of Highly Borylated 2‑Boraindanes through Multiple Borylation of Diynes

Boraindanes, composed of aromatic two-dimensional (2D) benzene and saturated three-dimensional (3D) boracycle moieties, are considered promising candidates for bioactive compounds/probes, agents for boron neutron capture therapy, and functional materials, as they combine the photophysical properties of the 2D aromatic and the high molecular recognition ability of the 3D boracycle. However, 2-boraindanes are difficult to access due to the difficulty of constructing two unstable C-(sp³)-B bonds. Herein, we present a straightforward synthesis of highly borylated, phenanthrene-fused 2-boraindane derivatives from biphenyl-linked terminal diacetylides through a B-B bond activation strategy. This methodology provides convenient access to a range of 2-boraindanes simply by changing the starting diyne. Spectroscopic measurements and computational analyses disclosed that the obtained highly borylated 2-boraindanes possess altered photophysical properties compared to the original 2D phenanthrene. The 2D/3D cyclic organoboron framework was utilized to construct a highly selective fluorescent probe for glucose.