Hydrogen- and Fluorine-Bridged Disilyl Cations and Their Use in Catalytic C−F Activation

The hydrogen-bridged disilyl cation 6 with an 1,8-naphthalenediyl backbone was synthesized and was characterized by NMR spectroscopy and X-ray crystallography, supported by quantum mechanical computations. The SiHSi linkage is symmetrical, corresponding to a single minimum potential, and the structural parameters are in agreement with the presence of a two electron−three center bond in 6. Treatment of disilyl cation 6 with alkyl fluorides yields the disilylfluoronium ion 10. The SiFSi group in the disilyl fluoronium ion 10 is symmetrical with an average SiF bond length of 175.9(8) and a bent angle β = 130°. Both cations catalyze the hydrodefluorination reaction of alkyl and benzyl fluorides to give alkanes

Hydrogen- and Fluorine-Bridged Disilyl Cations and Their Use in Catalytic C−F Activation

The hydrogen-bridged disilyl cation 6 with an 1,8-naphthalenediyl backbone was synthesized and was characterized by NMR spectroscopy and X-ray crystallography, supported by quantum mechanical computations. The SiHSi linkage is symmetrical, corresponding to a single minimum potential, and the structural parameters are in agreement with the presence of a two electron−three center bond in 6. Treatment of disilyl cation 6 with alkyl fluorides yields the disilylfluoronium ion 10. The SiFSi group in the disilyl fluoronium ion 10 is symmetrical with an average SiF bond length of 175.9(8) and a bent angle β = 130°. Both cations catalyze the hydrodefluorination reaction of alkyl and benzyl fluorides to give alkanes

Hydrogen- and Fluorine-Bridged Disilyl Cations and Their Use in Catalytic C−F Activation

The hydrogen-bridged disilyl cation 6 with an 1,8-naphthalenediyl backbone was synthesized and was characterized by NMR spectroscopy and X-ray crystallography, supported by quantum mechanical computations. The SiHSi linkage is symmetrical, corresponding to a single minimum potential, and the structural parameters are in agreement with the presence of a two electron−three center bond in 6. Treatment of disilyl cation 6 with alkyl fluorides yields the disilylfluoronium ion 10. The SiFSi group in the disilyl fluoronium ion 10 is symmetrical with an average SiF bond length of 175.9(8) and a bent angle β = 130°. Both cations catalyze the hydrodefluorination reaction of alkyl and benzyl fluorides to give alkanes

Bis-alkene Complexes of Stannylium and Germylium Ions

Bis-alkene complexes of triorgano-substituted cations of stannylium and germylium ions were synthesized by alkyl fragmentation reactions and were characterized by NMR spectroscopy and for the tin compound also by X-ray structure analysis of the corresponding perfluorinated tetraarylborate. According to experimental results and quantum mechanical computations, the central group 14 element adopts in these pentacoordinated cations a distorted trigonal-bipyramidal coordination geometry with the two CC double bonds occupying the apical positions. This geometrical arrangement places seven carbon atoms at a distance less than the sum of the van der Waals radii around the central atom. Quantum mechanical computations at the MP2/6-311G(d,p) (C,H) SDD (Sn, Ge, Si) level reveal substantial interaction energies between the group 14 element and the CC double bond and electron density distributions that are typical for noncovalent π-donor interactions

Bis-alkene Complexes of Stannylium and Germylium Ions

Bis-alkene complexes of triorgano-substituted cations of stannylium and germylium ions were synthesized by alkyl fragmentation reactions and were characterized by NMR spectroscopy and for the tin compound also by X-ray structure analysis of the corresponding perfluorinated tetraarylborate. According to experimental results and quantum mechanical computations, the central group 14 element adopts in these pentacoordinated cations a distorted trigonal-bipyramidal coordination geometry with the two CC double bonds occupying the apical positions. This geometrical arrangement places seven carbon atoms at a distance less than the sum of the van der Waals radii around the central atom. Quantum mechanical computations at the MP2/6-311G(d,p) (C,H) SDD (Sn, Ge, Si) level reveal substantial interaction energies between the group 14 element and the CC double bond and electron density distributions that are typical for noncovalent π-donor interactions

Hydrogen-Bridged Digermyl and Germylsilyl Cations

The synthesis of the digermyl and germylsilyl hydronium borates <b>7</b>[B­(C₆F₅)₄] and <b>8</b>[B­(C₆F₅)₄] is reported. Spectroscopic (IR, NMR) and structural data supported by the results of density functional calculations indicate in both cases a symmetric or almost symmetric E–H–E′ three-center–two-electron linkage (<b>7</b>, E = E′ = Ge; <b>8</b>, E = Si, E′ = Ge). The [B­(C₆F₅)₄]⁻ and the [HCB₁₁H₅Br₆]⁻ salts of both cations are active in catalytic hydrodefluorination reactions of alkyl and benzyl fluorides. No significant effect of the element atom E on the determined turnover numbers was found

Cyclic Silylated Onium Ions of Group 15 Elements

Five- and six-membered cyclic silylated onium ions of group 15 elements <b>I</b> were synthesized by intramolecular cyclization of transient silylium ions <b>II</b>. Silylium ions <b>II</b> were prepared by the hydride transfer reaction from silanes <b>III</b> using trityl cation as hydride acceptor. It was found that smaller ring systems could not be obtained by this approach. In these cases tritylphosphonium ions <b>IV</b> were isolated instead. Cations <b>I</b> and <b>IV</b> were isolated in the form of their tetrakispentafluorphenyl borates and characterized by multinuclear NMR spectroscopy and, in two cases, by X-ray diffraction analysis. Cyclic onium ions <b>I</b> showed no reactivity similar to that of isoelectronic intramolecular borane/phosphane frustrated Lewis pairs (FLPs). The results of DFT computations at the M05-2X level suggest that the strength of the newly formed Si–E linkage is the major reason for inertness of <b>I</b>[B­(C₆F₅)₄] versus molecular hydrogen

Selective Derivatization and Characterization of Bifunctional “Janus-Type” Cyclotetrasiloxanes

Stereoregular all-<i>cis</i> cyclotetrasiloxanes [R-Si­(O)-R′]₄ with different functional groups attached to the opposite faces of the ring skeleton were derivatized without stereoisomerization or cleavage of the (SiO)₄ ring and with high selectivity using standard synthetic methods. The solid-state structures obtained for the iodophenyl-substituted starting material <b>16</b> ([<i>p</i>-I-C₆H₄-Si­(O)-OSiMe₂Vin]₄) and for the coupling product <b>21</b> ([biphenyl-CC-C₆H₄-Si­(O)-OSiMe₂Vin]₄) show a pronounced differentiation in the steric requirements of the different sides of the ring, resulting in characteristic crystal packing. In combination with the observed high thermal and chemical stability, these data demonstrate the high potential of cyclotetrasiloxanes for a wide range of applications

Selective Derivatization and Characterization of Bifunctional “Janus-Type” Cyclotetrasiloxanes

Stereoregular all-<i>cis</i> cyclotetrasiloxanes [R-Si­(O)-R′]₄ with different functional groups attached to the opposite faces of the ring skeleton were derivatized without stereoisomerization or cleavage of the (SiO)₄ ring and with high selectivity using standard synthetic methods. The solid-state structures obtained for the iodophenyl-substituted starting material <b>16</b> ([<i>p</i>-I-C₆H₄-Si­(O)-OSiMe₂Vin]₄) and for the coupling product <b>21</b> ([biphenyl-CC-C₆H₄-Si­(O)-OSiMe₂Vin]₄) show a pronounced differentiation in the steric requirements of the different sides of the ring, resulting in characteristic crystal packing. In combination with the observed high thermal and chemical stability, these data demonstrate the high potential of cyclotetrasiloxanes for a wide range of applications

Design, Synthesis, and Biological Testing of Novel Naphthoquinones as Substrate-Based Inhibitors of the Quinol/Fumarate Reductase from <i>Wolinella succinogenes</i>

Novel naphthoquinones were designed, synthesized, and tested as substrate-based inhibitors against the membrane-embedded protein quinol/fumarate reductase (QFR) from <i>Wolinella succinogenes</i>, a target closely related to QFRs from the human pathogens <i>Helicobacter pylori</i> and <i>Campylobacter jejuni</i>. For a better understanding of the hitherto structurally unexplored substrate binding pocket, a structure–activity relationship (SAR) study was carried out. Analogues of lawsone (2-hydroxy-1,4-naphthoquinone <b>3a</b>) were synthesized that vary in length and size of the alkyl side chains (<b>3b</b>–<b>k</b>). A combined study on the prototropic tautomerism of 2-hydroxy-1,4-naphthoquinones series indicated that the 1,4-tautomer is the more stable and biologically relevant isomer and that the presence of the hydroxyl group is crucial for inhibition. Furthermore, 2-bromine-1,4-naphthoquinone (<b>4a</b>–<b>c</b>) and 2-methoxy-1,4-naphthoquinone (<b>5a</b>–<b>b</b>) series were also discovered as novel and potent inhibitors. Compounds <b>4a</b> and <b>4b</b> showed IC₅₀ values in low micromolar range in the primary assay and no activity in the counter DT-diaphorase assay