Unsymmetrically Substituted 9,10-Dihydro-9,10-diboraanthracenes as Versatile Building Blocks for Boron-Doped π-Conjugated Systems

The targeted hydrolysis of the 9,10-dihydro-9,10-diboraanthracene adduct (Me2S)HB(C6H4)(2)BH(SMe2) (1) with 0.5 equiv of H2O leads to formation of the borinic acid anhydride [(Me2S)HB(C6H4)(2)B](2)O (2) and thereby provides access to the field of un-symmetrically substituted 9,10-dihydro-9,10-diboraanthracenes. Compound 2 reacts with tBuC CH to give the corresponding vinyl derivative in an essentially quantitative conversion. Subsequent cleavage of the B-O-B bridge by LiAlH4 with formation of hydridoborate functionalities is possible but is accompanied by partial B-C(vinyl) bond degradation. This situation changes when the related mesityl derivative [MesB(C6H4)(2)B](2)O (7) is employed, which can be synthesized from BrB(C6H4)(2)BBr (6) by treatment with 1 equiv of MesMgBr and subsequent hydrolysis. The reaction of 7 with LiAlH4 in tetrahydrofuran (THF) furnishes Li[MesBA(C6H4)(2)BH2] (8); hydride elimination with Me3SiCl leads to formation of the THF adduct MesB(C6H4)(2)BH(THF) (9 center dot THF). Alternatively, 7 can be transformed into bromoborane MesB(C6H4)(2)BBr (10) by treatment with BBr3. A Br/H-exchange reaction between 10 and Et3SiH yields the donor-free borane MesB(C6H4)(2)BH (9), which forms B-H-B bridged dimers (9)(2) in the solid state. The vinyl borane MesB(C6H4)(2)BC(H)=C(H) Mes (14) is accessible from MesC CH and either 9 center dot THF or 9. Compared with the related compound Mes(2)BC(H)=C(H)Mes, the electronic absorption and emission spectra of 14 reveal bathochromic shifts of Delta lambda(abs) = 17 nm and Delta lambda(em) = 74 nm, which can be attributed to the rigid, fully delocalized pi framework of the [MesBA(C6H4)(2)B] chromophore.