Synthesis, bridgehead functionalization, and photoisomerization of 9,10-diboratatriptycene dianions

9,10-Diboratatriptycene salts M₂[RB(μ-C₆H₄)₃BR] (R = H, Me; M⁺ = Li⁺, K⁺, [n-Bu₄N]⁺) have been synthesized via [4 + 2] cycloaddition between doubly reduced 9,10-dihydro-9,10-diboraanthracenes M₂[DBA] and benzyne, generated in situ from C₆H₅F and C₆H₅Li or LiN(i-Pr)₂. [HB(μ-C₆H₄)₃BH]^2- reacts with CH₂Cl₂ to form quantitatively the bridgehead-derivatized [ClB(μ-C₆H₄)₃BCl]^2-, while twofold H^- abstraction with B(C₆F₅)₃ in the presence of SMe₂ leads cleanly to the diadduct (Me₂S)B(μ-C₆H₄)₃B(SMe₂). Photoisomerization of K₂[HB(μ-C₆H₄)₃BH] (THF, medium-pressure Hg lamp) provides facile access to diborabenzo[a]fluoranthenes, a little explored form of boron-doped polycyclic aromatic hydrocarbons. According to DFT calculations, the underlying reaction mechanism consists of three main steps: (i) photoinduced di-π-borate rearrangement, (ii) "walk reaction" of a BH unit, and (iii) boryl anion-like C-H activation.