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

9,10-Diboratatriptycene salts M(2)[RB(μ-C(6)H(4))(3)BR] (R = H, Me; M(+) = Li(+), K(+), [n-Bu(4)N](+)) have been synthesized via [4 + 2] cycloaddition between doubly reduced 9,10-dihydro-9,10-diboraanthracenes M(2)[DBA] and benzyne, generated in situ from C(6)H(5)F and C(6)H(5)Li or LiN(i-Pr)(2). [HB(μ-C(6)H(4))(3)BH](2−) reacts with CH(2)Cl(2) to form quantitatively the bridgehead-derivatized [ClB(μ-C(6)H(4))(3)BCl](2−), while twofold H(−) abstraction with B(C(6)F(5))(3) in the presence of SMe(2) leads cleanly to the diadduct (Me(2)S)B(μ-C(6)H(4))(3)B(SMe(2)). Photoisomerization of K(2)[HB(μ-C(6)H(4))(3)BH] (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.