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Keywords:

  • boranes;
  • borates;
  • condensation;
  • Grignard reaction;
  • quantum chemistry

Abstract

1,2-Bis(pinacolboryl)benzene (1,2-C6H4(Bpin)2, 2) was synthesized in preparatively useful yields from 1,2-C6H4Br2, iPrO[BOND]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(Ar1)(Ar2)) through sequential Suzuki–Miyaura coupling reactions. Alternatively, it can be transformed into bis-borate Li2[1,2-C6H4(BH3)2] (3) through reduction with Li[AlH4]. In the crystal lattice, the diethyl ether solvate 3⋅OEt2 establishes a columnar structure that is reinforced by an intricate network of B[BOND](μ-H)[BOND]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(BH2SMe2)2 is not long-term stable but rather undergoes a condensation reaction to give 9,10-dihydro-9,10-diboraanthracene, HB(μ-C6H4)2BH, and BH3. 9,10-Dihydro-9,10-diboraanthracene was isolated from the reaction mixture as its SMe2 monoadduct (7), which dimerizes in the solid state through two B[BOND]H[BOND]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(μ-H)2B(μ-C6H4)2B(μ-H)2BH2 (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 1,2-C6H4(BH2)2 to the observed adducts of HB(μ-C6H4)2BH, has been elucidated in detail and the important role of B[BOND]C[BOND]B-bridged intermediates has been firmly established.