The competition between Si-Si and Si-C cleavage in functionalised oligosilanes: their reactivity with elemental lithium.

The reaction of aryl-substituted disilanes with elemental lithium is a common method for the preparation of lithiosilanes and the subsequent synthesis of functionalised oligosilanes, especially of enantiomerically pure compounds. A series of alkyl- and arylsubstituted di- and trisilanes has been investigated with respect to their reactivity against elemental lithium. Thereby, depending on the substituents, silicon-silicon bond cleavage of the central Si-Si unit and/or silicon-carbon bond cleavage to arenes are observed. Quantum chemical studies provide a deeper insight into the ongoing processes. The reactive centre can be estimated by both, bond elongation after electron transfer and frontier orbital interactions (pi-bonding and sigma-antibonding part). Aromatic substituents at the silicon atoms proved to be necessary for the processing of any cleavage reaction in the studied systems by stabilising the radical anion after electron transfer at the corresponding di- or trisilane. Yet, selective cleavage reactions do not depend on the number of arenes.

Preparation of "Si-centered" chiral silanes by direct alpha-lithiation of methylsilanes.

The direct alpha-lithiation of methyl-substituted silanes as an efficient method for the preparation and elaboration of Si-chiral compounds is reported. Deprotonation of chiral oligosilanes occurs selectively and with high yields at the methyl group of the stereogenic silicon center, even in the presence of multiple methylsilyl or methylgermyl substituents. Computational studies have confirmed this preference as a consequence of pre-coordination of the lithiating agent by the amino side-arm and repulsion effects in the corresponding transition state. This complexation is also obvious from X-ray structure analyses of the alpha-lithiated silanes, which exhibit intriguing structure formation patterns differing in the type of aggregation and the amount of alkyllithium used. An alternative route to Si-chiral compounds is also presented, which involves desymmetrization of dimethylsilanes mediated by a chiral side-arm. Structure analyses and computational studies have shown that the diastereoselectivity of this alpha-lithiation is influenced by the selectivity of the formation of the stereogenic nitrogen upon complexation of the alkyllithium.

Structure formation principles and reactivity of organolithium compounds.

The structure-reactivity relationship is an important feature of organolithium compounds. The knowledge of the structure of reactive species is crucial for the elucidation of reaction mechanisms and the understanding of observed selectivities. This concept article gives an overview over the structural principles of lithium organics and their Lewis base coordinated complexes in the solid state. The transition from the oligomeric parent structures to smaller adducts, such as dimers and monomers, as well as special degrees of aggregation is presented. Besides the commonly used alkyllithium compounds, a short overview over the structural principles of the higher homologous silyllithium compounds is given. Moreover, the structure-reactivity relationship is depicted by means of the reactivity of the Lewis bases towards intramolecular decomposition reactions with the organolithium compound. Selected examples confirm the importance of structure elucidation for the understanding of mechanistic pathways and selectivities.

Rigidity effect of the dithioether spacer on the size of the luminescent cluster (Cu(2)I(2))(n) (n = 2, 3) in their coordination polymers.

Treatment of CuI with the flexible PhS(CH(2))(4)SPh dithioether ligand in MeCN solution affords the strongly luminescent metal-organic 2D coordination polymer [Cu(4)I(4){mu-PhS(CH(2))(4)SPh}(2)](n) (). The interpenetrated 2D network of is built upon by Cu(4)(mu(3)-I)(4) cubane-like clusters as secondary building units (SBUs), which are interconnected via bridging 1,4-bis(phenylthio)butane ligands. In contrast, the auto-assembly reaction of the unsaturated semi-flexible PhSCH(2)C[triple bond, length as m-dash]CCH(2)SPh ligand with CuI results in formation of the 3D metallopolymer [(Cu(6)I(6)){mu-PhSCH(2)C[triple bond, length as m-dash]CCH(2)SPh}(3)](n) (). The SBUs of luminescent consist of discrete Cu(6)(mu(3)-I)(6) hexagon prisms, which are coordinated with bridging 1,4-bis(phenythio)butyne ligands via Cu-S bonds. Contrary to the other rare literature-known examples of metallopolymers incorporating Cu(6)X(6) SBUs as connecting nodes, the CuCu interactions of [2.8484(6) A] are markedly shorter, being close to the sum of the Van der Waals radii of two Cu atoms ( approximately 2.8 A). The photophysics of these compounds, which exhibit reversible luminescence thermochromism, has been investigated in detail. The solid-state luminescence spectra of and feature at room temperature intense emissions around 560 and 555 nm, respectively. The luminescence properties of the unusual Cu(6)(mu(3)-I)(6) hexagon prism motif are rationalized by means of DFT and TDDFT computations.

Enantioselective synthesis of tricyclic amino acid derivatives based on a rigid 4-azatricyclo[5.2.1.0]decane skeleton.

An enantioselective route to four tricyclic amino acids and N-tosylamides, composed of a central norbornane framework with a 2-endo,3-endo-annelated pyrrolidine ring and a 5-endo-C1 or -C2 side chain, has been developed. A key intermediate was the chiral, N-Boc-protected ketone (1R,2S,6S,7R)-4-azatricyclo[5.2.1.0²,6]decan-8-one, available from inexpensive endo-carbic anhydride in five steps and 47% yield. The rigid scaffold makes these amino acid derivatives promising candidates for beta-turn-inducing building blocks in peptidomimetics and for chiral auxiliaries in asymmetric organocatalysis.

2-[(2-Hydr-oxy-2,2-diphenyl-ethyl)(meth-yl)amino]-N,N-dimethyl-ethanaminium bromide.

The title compound, C(19)H(27)N(2)O(+)·Br(-), is the hydro-bromide of the trapping product of lithia-ted N,N,N',N'-tetramethylethylenediamine (TMEDA) with benzophenone. Thereby, the N atom of the NMe(2) group is selectively protonated and the respective trapping product represents a potential tridentate ligand with one O and two N donor atoms. The H atoms at N (H2N) and O (H1O) are involved in hydrogen bonds with the Br(-). The mol-ecular structure shows all donor atoms to be arranged on one side of the mol-ecule, thus indicating a potential threefold coordination of a Lewis acid.

4,12-Bis(2,2-dibromo-vinyl)[2.2]paracyclo-phane.

In the title compound, C(20)H(16)Br(4), both vinylic substituents were introduced by a Corey-Fuchs reaction using 4,12-diform-yl[2.2]paracyclo-phane as starting material. The title compound may be used as a valuable precursor for the synthesis of diethyn-yl[2.2]paracyclo-phane. The title mol-ecule is centrosymmetric with a crystallographic center of inversion between the centers of the two phenyl rings. A strong tilting is observed with an inter-planar angle between the best aromatic plane and the vinyl plane of 49.4 (5)°. No significant inter-molecular inter-actions are found in the crystal.

Synthesis and reactivity of silylated tetrathiafulvalenes.

Novel organosilylated tetrathiafulvalenes (TTFs) possessing Si-H or Si-Si bonds have been synthesised. The crystal structures of several derivatives have been determined by X-ray diffraction, including that of dimeric (Si2Me4)(TTF)2 (11) incorporating a diatomic SiMe2-SiMe2 linker. Cyclic voltammetry measurements in all cases show two oxidation waves. DFT calculations were performed to rationalize the absence of an electronic communication between the two TTF moieties of 11 through the disilanyl spacer. The reactivity of the Si-H bond has been exploited to prepare the dinuclear complex [{Ru(CO)4}2{mu-(Me2Si)4TTF}] (14), starting from Ru3(CO)12 and TTF(SiMe2H)4 (1). Treatment of 14 with 2 equiv. of PPh3 or dppm results in selective substitution of a CO ligand trans to a SiMe2 group to afford mer-[{Ru(PPh3)(CO)3}2{mu-(Me2Si)4TTF}] (13) and mer-[{Ru(CO)3}2(eta1-dppm){mu-(Me2Si)4TTF}] (16). Attempts to transform the Si-H bonds of some TTF(SiMe2H)n (n = 1, 2) into Si-O functions using stoichiometric amounts of water in the presence of tris(dibenzylideneacetone)dipalladium(0) were unsuccessful. Quantitative cleavage of the C(TTF)-Si bond was observed instead of formation of TTF-based-siloxanes. Essays of catalytic bis-silylation of phenylacetylene with 11 and TTF(SiMe2-SiMe3) (9) in the presence of Pd(OAc)2/1,1,3,3-tetramethylbutylisocyanide failed. Again, cleavage of the C(TTF)-Si bond was noticed.

Bent phenyl groups in lithiosilanes--crystal structures and interpretation of this unanticipated feature.

Based on the crystal structures of PMDTA coordinated lithiosilanes, the bend of aromatic substituents in anionic group fourteen and neutral group fifteen compounds (third row and higher) is explained by means of quantum chemical calculations, which indicate the deformation to be a result of Pauli-repulsion of the electrons in the occupied frontier orbitals.

Structure/reactivity studies on an alpha-lithiated benzylsilane: chemical interpretation of experimental charge density.

Modern organic synthesis (e.g., of natural products) is virtually impossible without employment of enantiomerically enriched compounds. In many cases, alkyllithium compounds are key intermediates for the generation of these stereogenic substances. In recent years, the lithiated carbon atom in silicon-substituted benzyllithium compounds has become a focus of interest because it is possible to maintain its stereogenic information. Starting from a highly enantiomerically enriched benzylsilane, (R,S)-2 x quinuclidine could be obtained, and the absolute configuration at the metalated carbon atom was determined by X-ray diffraction analysis. In solution, a quartet was found in the (13)C NMR spectrum for the metalated carbon atom because of coupling between carbon and lithium, indicating a fixed lithium carbon contact at room temperature. After reaction of (R,S)-2 x quinuclidine with trimethylchlorostannane, the trapped product (S,S)-4 was obtained with a dr > or = 98:2 with inversion of the configuration at the metalated carbon. Multipole refinement against high-resolution diffraction data and subsequent topological analysis of the benchmark system (R,S)-2 x quinuclidine provide insight in the electronic situation and thus the observed stereochemical course of the transformations. Surprisingly, the negative charge generated at the carbanion hardly couples into the phenyl ring. The neighboring silicon atom counterbalances this charge by a pronounced positive charge. Therefore, the alpha-effect of the silicon atom is caused not just by a polarization of the electron density but also by an electrostatic bond reinforcement. Furthermore, the experimentally determined electrostatic potential unequivocally explains the observed back side attack of an electrophile under inversion of the stereogenic center with high diastereomeric ratios.

(S)-1,2-Dimethyl-1,1,2-triphenyl-2-(4-piperidiniometh-yl)disilane chloride.

The title compound, C(26)H(34)NSi(2) (+)·Cl(-), shows chirality at silicon. Because of its highly selective synthesis with an e.r. of >99:1 by means of a racemic resolution with mandelic acid, the free disilane is of great importance to the chemistry of highly enanti-omerically enriched lithio-silanes and their trapping products. N-H⋯Cl hydrogen bonding is present between the protonated nitro-gen atom of the piperidino group and the chloride counter-anion. The silicon-silicon distance as well as silicon-carbon and carbon-nitro-gen bond lengths are in the same ranges as in other quaternary, functionalized di- and tetra-silanes.

(1R,2R)-N,N'-Dimethyl-cyclo-hexane-1,2-diamine.

The molecule of the title compound, C(8)H(18)N(2), possesses C(2) symmetry. Owing to its stereochemistry, it is used in the synthesis of chiral ligands and metal complexes for asymmetric synthesis. The cyclo-hexane ring shows a chair conformation with the amino groups in equatorial positions. Contrary to the literature, the title compound is not a liquid, but a crystalline solid at room temperature (293 K). The absolute configuration is assigned from the synthesis.

Crystal structures of the chiral lithiosilanes [(Lis)-PhMe2SiLi x THF x (-)-sparteine] and [Ph2(NEt2)SiLi x (-)-sparteine].

The (-)-sparteine-coordinated lithiosilanes (Lis)-4 and 5 crystallize as monomers with coordination numbers of 4 and 3 on lithium and represent the first structurally characterized enantiopure lithiosilanes. Both systems are also monomeric in solution with Si-Li contacts detected by 29Si-7Li couplings in the 29Si NMR spectrum (for 5 even at room temperature).