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 (Me 2 S)HB(C 6 H 4 ) 2 BH(SMe 2 ) ( 1 ) with 0.5 equiv of H 2 O leads to formation of the borinic acid anhydride [(Me 2 S)HB(C 6 H 4 ) 2 B] 2 O ( 2 ) and thereby provides access to the field of unsymmetrically substituted 9,10‐dihydro‐9,10‐diboraanthracenes. Compound 2 reacts with t BuCCH to give the corresponding vinyl derivative in an essentially quantitative conversion. Subsequent cleavage of the B‐O‐B bridge by LiAlH 4 with formation of hydridoborate functionalities is possible but is accompanied by partial BC(vinyl) bond degradation. This situation changes when the related mesityl derivative [MesB(C 6 H 4 ) 2 B] 2 O ( 7 ) is employed, which can be synthesized from BrB(C 6 H 4 ) 2 BBr ( 6 ) by treatment with 1 equiv of MesMgBr and subsequent hydrolysis. The reaction of 7 with LiAlH 4 in tetrahydrofuran (THF) furnishes Li[MesB(C 6 H 4 ) 2 BH 2 ] ( 8 ); hydride elimination with Me 3 SiCl leads to formation of the THF adduct MesB(C 6 H 4 ) 2 BH(THF) ( 9 ⋅THF). Alternatively, 7 can be transformed into the bromoborane MesB(C 6 H 4 ) 2 BBr ( 10 ) by treatment with BBr 3 . A Br/H‐exchange reaction between 10 and Et 3 SiH yields the donor‐free borane MesB(C 6 H 4 ) 2 BH ( 9 ), which forms B‐H‐B bridged dimers ( 9 ) 2 in the solid state. The vinyl borane MesB(C 6 H 4 ) 2 BC(H)C(H)Mes ( 14 ) is accessible from MesCCH and either 9 ⋅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 Δ λ (abs)=17 nm and Δ λ (em)=74 nm, which can be attributed to the rigid, fully delocalized π framework of the [MesB(C 6 H 4 ) 2 B] chromophore.