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

9,10-Diboratatriptycene salts M-2[RB(mu-C6H4)(3)BR] (R = H, Me; M+ = Li+, K+, [n-Bu4N](+)) 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 C6H5F and C6H5Li or LiN(i-Pr)(2). [HB(mu-C6H4)(3)BH](2-) reacts with CH2Cl2 to form quantitatively the bridgehead-derivatized [ClB(mu-C6H4)(3)BCl](2-), while twofold H- abstraction with B(C6F5)(3) in the presence of SMe2 leads cleanly to the diadduct (Me2S)B(mu-C6H4)(3)B(SMe2). Photoisomerization of K-2[HB(mu-C6H4)(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-pi-borate rearrangement, (ii) "walk reaction" of a BH unit, and (iii) boryl anion-like C-H activation.