High-Yield Syntheses and Reactivity Studies of 1,2-Diborylated and 1,2,4,5-Tetraborylated Benzenes

Treatment of 1,2-dibromobenzene (1,2-C6H4Br2) or 1,2,4,5-tetrabromobenzene (1,2,4,5-C6H2Br4) with 2 equiv or 4 equiv of n-BuLi in the presence of excess iso-propoxy(pinacol)borane ((i-PrO)Bpin) furnishes 1,2-C6H4(Br)(Bpin) (1) or 1,4,2,5-C6H2(Br)2(Bpin)2 (3) with excellent selectivity. The subsequent reaction of 1 or 3 with Mg turnings and more (i-PrO)Bpin gives the di- and tetraborylated benzenes 1,2-C6H4(Bpin)2 (2) and 1,2,4,5-C6H2(Bpin)4 (4) in overall yields of about 65%. For the Grignard transformation step, it is essential to continuously add 1 equiv (1) or 2 equiv (3) of 1,2-dibromoethane as an entrainer over a period of 1 h. Compounds 1 and 2 have been transformed into the ortho-functionalized trihydroborates Li[1,2-C6H4(Br)(BH3)] (Li[7]) and Li[1,2-C6H4(Bpin)(BH3)] (Li[8]) by means of 1 equiv of Li[AlH4]. Using 3 equiv of Li[AlH4], 2 can also be converted into the ditopic lithium trihydroborate Li2[1,2-C6H4(BH3)2] (Li2[9]); even the tetratopic derivative Li4[1,2,4,5-C6H2(BH3)4] (Li4[10]) is accessible from 4 and 4 equiv of Li[AlH4]. The compounds Li[7], Li[8], Li2[9], and Li4[10] have been crystallographically characterized as ether solvates, but still show Ar–BH3-η2-Li interactions as the dominant mode of coordination. In the cases of Li2[9] and Li4[10] an intricate three-dimensional network and a zigzag polymer, respectively, are established by the contact ion pairs in the crystal lattice.