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.