Synthesis, Coupling, and Condensation Reactions of 1,2-Diborylated Benzenes: An Experimental and Quantum-Chemical Study

1,2-Bis(pinacolboryl)benzene (1,2-C6H4(Bpin)(2), 2) was synthesized in preparatively useful yields from 1,2-C6H4Br2, iPrO-Bpin, and Mg turnings in the presence of 1,2-C2H4Br2 as an entrainer. Compound 2 is a versatile starting material for the synthesis of (un)symmetrically substituted benzenes (i.e., 1,2-C6H4(Ar-1)(Ar-2)) through sequential Suzuki-Miyaura coupling reactions. Alternatively, it can be transformed into bis-borate Li-2[1,2-C6H4-(BH3)(2)] (3) through reduction with Li[AlH4]. In the crystal lattice, the diethyl ether solvate 3 center dot OEt2 establishes a columnar structure that is reinforced by an intricate network of B-(mu-H)-Li interactions. Hydride-abstraction from compound 3 with Me3SiCl leads to the transient ditopic borane 1,2-C6H4-(BH2)(2), which can either be used in situ for subsequent hydroboration reactions or trapped as its stable NMe2Et diadduct (6). In SMe2 solution, the putative diadduct 1,2-C6H4(BH2 center dot SMe2)(2) is not long-term stable but rather undergoes a condensation reaction to give 9,10-dihydro-9,10-diboraanthracene, HB(mu-C6H4) 2BH, and BH3. 9,10-Dihydro9,10-diboraanthracene was isolated from the reaction mixture as its SMe2 monoadduct (7), which dimerizes in the solid state through two B-H-B bridges ((7)(2), elucidated by X-ray crystallography). In contrast, hydride-abstraction from compound 3 in THF or CH2Cl2 provides the unique exo-adduct H2B(mu-H)(2)B(mu-C6H4)(2)B(mu-H)(2)BH2 (8, elucidated by X-ray crystallography). Quantum-chemical calculations on various conceivable isomers of [1,2-C6H4-( BH2)(2)](2) revealed that compound 8 was the most stable of these species. Moreover, the calculations confirmed the experimental findings that the NMe2Et diadduct of 1,2-C6H4(BH2)(2) is significantly more stable than the corresponding SMe2 complex and that the latter complex is not able to compete successfully with borane-dimerization and -condensation. The reaction cascade in SMe2, which proceeds from 1,2C(6)H(4)-(BH2)(2) to the observed adducts of HB(mu-C6H4)(2)BH, has been elucidated in detail and the important role of B-C-B-bridged intermediates has been firmly established.