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 beta = 130 degrees . Both cations catalyze the hydrodefluorination reaction of alkyl and benzyl fluorides to give alkanes.

C-C coupling reactions between arylsilanes and alkylfluorides are efficiently catalyzed by disilyl cation 1. Primary as well as secondary alkylfluorides were quantitatively coupled with arylsilanes; however, in the case of tertiary fluorides, the hydrodefluorination reaction predominated. Primary alkylfluorides were found to give arenes with mostly rearranged alkyl substituents. In all cases subsequent Friedel-Crafts-type chemistry occurred.

Novel naphthoquinones were designed, synthesized, and tested as substrate-based inhibitors against the membrane-embedded protein quinol/fumarate reductase (QFR) from Wolinella succinogenes, a target closely related to QFRs from the human pathogens Helicobacter pylori and Campylobacter jejuni. 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 3a) were synthesized that vary in length and size of the alkyl side chains (3b-k). 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 (4a-c) and 2-methoxy-1,4-naphthoquinone (5a-b) series were also discovered as novel and potent inhibitors. Compounds 4a and 4b showed IC50 values in low micromolar range in the primary assay and no activity in the counter DT-diaphorase assay.

Stereoregular all-cis 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 iodophenylsubstituted starting material 16 ([p-I-C 6 H 4 -Si(O)-OSiMe 2 Vin] 4 ) and for the coupling product 21 ([biphenyl-CC-C 6 H 4 -Si(O)-OSiMe 2 Vin] 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.

A series of glutarimide derivatives which has two carbonyl coordination sites for intramolecular pentacoordination at silicon with a X(1+n)SiC(3-n)O moiety have been synthesised and characterized. The substituent (leaving group) effects on the Si-O bond exchange between the two coordination sites (resembling a pendulum) have been studied by comparison of the differently substituted (X = F, Cl, OTf, Br and I) structures. The activation parameters for the Si-O bond exchange process were measured by NMR and separately computed and are consistent with the strength of Si-O bond coordination and the nature of the leaving group, X. The temperature-dependent (29)Si NMR spectroscopy is supported by X-ray crystallography and shows that the tetrahedral reactant is converted into pentacoordinate intermediates by intramolecular O-Si association followed by reversion to a tetrahedral geometry by Si-X dissociation. The two association/dissociation patterns offer a model for nucleophilic substitution at a silicon atom. A continuum of structures on the S(N)2 reaction profile from the glutarimide derivatives correlates reasonably well with the structural data obtained from derivatives of lactams, diketopiperazines and quinolones.

The aryl-bridged disilyl cations 8 with an 1,8-naphthalenediyl backbone were synthesized and were identified by NMR spectroscopy. In addition the [B(C 6 F 5 ) 4 ] -salt of the tolyl-bridged cation 8b was characterized by X-ray crystallography. The NMR spectroscopic and the structural data indicate the arenium ion nature of disilyl cations 8. Results of quantum mechanical computations corroborate the experimental findings, and a Bader-type topological analysis of the electron density in model cation 6 gives a refined picture of the bonding in aryl-bridged disilyl cations 6 and 8.

Five- and six-membered cyclic silylated onium ions of group 15 elements I were synthesized by intramolecular cyclization of transient silylium ions II. Silylium ions II were prepared by the hydride transfer reaction from silanes III 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 IV were isolated instead. Cations I and IV 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 I 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 I[B(C6F5)4] versus molecular hydrogen.

The synthesis of the digermyl and germylsilyl hydronium borates 7[B(C 6 F 5 ) 4 ] and 8[B(C 6 F 5 ) 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 (7, E = E′ = Ge; 8, E = Si, E′ = Ge). The [B(C 6 F 5 ) 4 ] − and the [HCB 11 H 5 Br 6 ] − 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.