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

Abstract

The targeted hydrolysis of the 9,10-dihydro-9,10-diboraanthracene adduct (Me₂S)HB(C₆H₄)₂BH(SMe₂) (1) with 0.5 equiv of H₂O leads to formation of the borinic acid anhydride [(Me₂S)HB(C₆H₄)₂B]₂O (2) and thereby provides access to the field of unsymmetrically 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)₂B]₂O (7) is employed, which can be synthesized from BrB(C₆H₄)₂BBr (6) by treatment with 1 equiv of MesMgBr and subsequent hydrolysis. The reaction of 7 with LiAlH₄ in tetrahydrofuran (THF) furnishes Li[MesB(C₆H₄)₂BH₂] (8); hydride elimination with Me₃SiCl leads to formation of the THF adduct MesB(C₆H₄)₂BH(THF) (9·THF). Alternatively, 7 can be transformed into the bromoborane MesB(C₆H₄)₂BBr (10) by treatment with BBr₃. A Br/H-exchange reaction between 10 and Et₃SiH yields the donor-free borane MesB(C₆H₄)₂BH (9), which forms B-H-B bridged dimers (9)₂ in the solid state. The vinyl borane MesB(C₆H₄)₂BC(H)=C(H)Mes (14) is accessible from MesC≡CH and either 9·THF or 9. Compared with the related compound Mes₂BC(H)=C(H)Mes, the electronic absorption and emission spectra of 14 reveal bathochromic shifts of Δλ(abs)=17 nm and Δλ(em)=74 nm, which can be attributed to the rigid, fully delocalized π framework of the [MesB(C₆H₄)₂B] chromophore.