Publications (2)15.83 Total impact
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Article: Synthesis, coupling, and condensation reactions of 1,2-diborylated benzenes: an experimental and quantum-chemical study.
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ABSTRACT: 1,2-Bis(pinacolboryl)benzene (1,2-C(6)H(4) (Bpin)(2), 2) was synthesized in preparatively useful yields from 1,2-C(6)H(4)Br(2), iPrO-Bpin, and Mg turnings in the presence of 1,2-C(2)H(4)Br(2) as an entrainer. Compound 2 is a versatile starting material for the synthesis of (un)symmetrically substituted benzenes (i.e., 1,2-C(6)H(4)(Ar(1))(Ar(2))) through sequential Suzuki-Miyaura coupling reactions. Alternatively, it can be transformed into bis-borate Li(2)[1,2-C(6)H(4)(BH(3))(2)] (3) through reduction with Li[AlH(4)]. In the crystal lattice, the diethyl ether solvate 3·OEt(2) establishes a columnar structure that is reinforced by an intricate network of B-(μ-H)-Li interactions. Hydride-abstraction from compound 3 with Me(3)SiCl leads to the transient ditopic borane 1,2-C(6)H(4)(BH(2))(2), which can either be used in situ for subsequent hydroboration reactions or trapped as its stable NMe(2)Et diadduct (6). In SMe(2) solution, the putative diadduct 1,2-C(6)H(4)(BH(2)·SMe(2))(2) is not long-term stable but rather undergoes a condensation reaction to give 9,10-dihydro-9,10-diboraanthracene, HB(μ-C(6)H(4))(2)BH, and BH(3). 9,10-Dihydro-9,10-diboraanthracene was isolated from the reaction mixture as its SMe(2) 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 CH(2)Cl(2) provides the unique exo-adduct H(2)B(μ-H)(2)B(μ-C(6)H(4))(2)B(μ-H)(2)BH(2) (8, elucidated by X-ray crystallography). Quantum-chemical calculations on various conceivable isomers of [1,2-C(6)H(4)(BH(2))(2)](2) revealed that compound 8 was the most stable of these species. Moreover, the calculations confirmed the experimental findings that the NMe(2)Et diadduct of 1,2-C(6)H(4)(BH(2))(2) is significantly more stable than the corresponding SMe(2) complex and that the latter complex is not able to compete successfully with borane-dimerization and -condensation. The reaction cascade in SMe(2), which proceeds from 1,2-C(6)H(4)(BH(2))(2) to the observed adducts of HB(μ-C(6)H(4))(2)BH, has been elucidated in detail and the important role of B-C-B-bridged intermediates has been firmly established.Chemistry 07/2012; 18(36):11284-95. · 5.93 Impact Factor -
Article: Main-chain boron-containing oligophenylenes via ring-opening polymerization of 9-H-9-borafluorene.
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ABSTRACT: 9-H-9-Borafluorene (H(8)C(12)BH; 5) can be generated in situ from 9-Br-9-borafluorene and Et(3)SiH in benzene or hexane. Monitoring of the reaction by NMR spectroscopy at rt in C(6)D(6) reveals that 5 forms C(1)-symmetric dimers (5)(2) under these conditions. DFT calculations on conceivable isomers of (5)(2) and a comparison of calculated and experimentally determined (1)H, (13)C, and (11)B NMR shift values lead to the conclusion that (5)(2) is not a classical dimer H(8)C(12)B(μ-H)(2)BC(12)H(8), but contains one B-H-B three-center, two-electron bond together with a boron-bridging phenyl ring. Addition of 1 equiv of pyridine to (5)(2) leads to the clean formation of the pyridine adduct H(8)C(12)BH(py) (5·py). Likewise, (5)(2) can be employed in hydroboration reactions, as evidenced by its transformation with 0.5 equiv of tert-butylacetylene, which gives the hydroboration products tBuC(H)(2)C(H)(BC(12)H(8))(2) (9) and tBuC(H)C(H)BC(12)H(8) in almost quantitative yield. (5)(2) is not long-term stable in benzene solution. Addition of pyridine to aged reaction mixtures allowed the isolation of the adduct (py)H(2)B-C(6)H(4)-C(6)H(4)-(py)BC(12)H(8) (10·py(2)) of a ring-opened dimer of 5. Storage of a hexane solution of 9-Br-9-borafluorene and Et(3)SiH for 1-2 weeks at rt leads to the formation of crystals of a ring-opened pentamer H[-(H)B-(C(6)H(4))(2)-](4)BC(12)H(8) (11) of 5 (preparative yields are obtained after 1-4 months). The polymer main chain of 11 is reinforced by four intrastrand B-H-B three-center, two-electron bonds. Apart from the main product 11, we have also isolated minor amounts of closely related oligomers carrying different chain ends, i.e., H(8)C(12)B-(C(6)H(4))(2)[-(H)B-(C(6)H(4))(2)-](2)BC(12)H(8) (12) and H[-(H)B-(C(6)H(4))(2)-](5)BH(2) (13). When the reaction between 9-Br-9-borafluorene and Et(3)SiH is carried out in refluxing toluene, the cyclic dimer [-(μ-H)B-(C(6)H(4))(2)-](2) (14) can be obtained in a crystalline yield of 25%. The compounds 9, 10·py(2), 11, 12, 13, and 14 have been structurally characterized by X-ray crystallography. The entire reaction pathway leading from 5 to 10, 11, 12, 13, and 14 has been thoroughly elucidated by DFT calculations and we propose a general mechanistic scenario applicable for ring-opening polymerization reactions of 9-borafluorenes.Journal of the American Chemical Society 03/2011; 133(12):4596-609. · 9.91 Impact Factor
