Synthese und Struktur von Lithium-tris(trimethylsilyl)silanid � 1,5 DME

Multifaceted behavior of a doubly reduced arylborane in B-H-bond activation and hydroboration catalysis

Alkali-metal salts of 9,10-dimethyl-9,10-dihydro-9,10-diboraanthrancene (M₂[DBA-Me₂]; M⁺ = Li⁺, Na⁺, K⁺) activate the H-B bond of pinacolborane (HBpin) in THF already at room temperature. For M⁺ = Na⁺, K⁺, the addition products M₂[4] are formed, which contain one new H-B and one new B-Bpin bond; for M⁺ = Li⁺, the H^- ion is instantaneously transferred from the DBA-Me₂ unit to another equivalent of HBpin to afford Li[5]. Although Li[5] might commonly be considered a [Bpin]^- adduct of neutral DBA-Me₂, it donates a [Bpin]⁺ cation to Li[SiPh₃], generating the silyl borane Ph₃Si-Bpin; Li₂[DBA-Me₂] with an aromatic central B₂C₄ ring acts as the leaving group. Furthermore, Li₂[DBA-Me₂] catalyzes the hydroboration of various unsaturated substrates with HBpin in THF. Quantum-chemical calculations complemented by in situ NMR spectroscopy revealed two different mechanistic scenarios that are governed by the steric demand of the substrate used: in the case of the bulky Ph(H)C[double bond, length as m-dash]NtBu, the reaction requires elevated temperatures of 100 °C, starts with H-Bpin activation which subsequently generates Li[BH₄], so that the mechanism eventually turns into "hidden borohydride catalysis". Ph(H)C[double bond, length as m-dash]NPh, Ph₂C[double bond, length as m-dash]O, Ph₂C[double bond, length as m-dash]CH₂, and iPrN[double bond, length as m-dash]C[double bond, length as m-dash]NiPr undergo hydroboration already at room temperature. Here, the active hydroboration catalyst is the [4 + 2] cycloadduct between the respective substrate and Li₂[DBA-Me₂]: in the key step, attack of HBpin on the bridging unit opens the bicyclo[2.2.2]octadiene scaffold and gives the activated HBpin adduct of the Lewis-basic moiety that was previously coordinated to the DBA-B atom.

Salts of Tris(pentafluoroethyl)silylchalcogenolates [Si(C2F5)3E]- with E = S, Se, and Te: Synthesis, Structure, and Reactivity

Unlike silanolates [SiR₃O]^- (R = alkyl, aryl), which have been intensely studied, the heavier derivatives [SiR₃E]^- (E = S, Se, Te) have been much less examined. Among such species, virtually nothing is known about perfluoroalkyl-substituted silylchalcogenolates. In this contribution, a convenient synthesis of tris(pentafluoroethyl)silylchalcogenolate salts [{(Et₂N)₃P═N}₃PN(H)^tBu][Si(C₂F₅)₃E] (E = S, Se, Te; ^tBu = tert-butyl) is presented. All representatives were isolated and fully characterized by multinuclear NMR spectroscopy, IR spectroscopy, mass spectrometry, elemental analysis, and X-ray diffraction studies. Furthermore, first reactivity studies of these novel species toward selected metal halide complexes were performed. In this course, metal complexes [HgPh{SSi(C₂F₅)₃}] (2) and [Au(PPh₃){SSi(C₂F₅)₃}] (3) were isolated and characterized.

Sila-Peterson Reaction of Cyclic Silanides

Sila-Peterson type reactions of the 1,4,4-tris(trimethylsilyl)-1-metallooctamethylcyclohexasilanes (Me₃Si)₂Si₆Me₈(SiMe₃)M (2a, M = Li; 2b, M = K) with various ketones were investigated. The obtained products strongly depend on the nature of the ketone component. With 2-adamantanone 2a,b afforded the moderately stable silene 3. 3 is the first example of an Apeloig–Ishikawa–Oehme-type silene with the tricoordinate silicon atom incorporated into a cyclopolysilane framework and could be characterized by NMR and UV spectroscopy as well as by trapping reactions with water, methanol, and MeLi. The reaction of 2b with aromatic ketones also follows a sila-Peterson type mechanism with formation of carbanionic species. With 1,2-diphenylcyclopropenone 2b reacted by conjugate 1,4-addition to give a spirocyclic carbanion. In most cases the underlying reaction mechanism could be elucidated by the isolation and characterization of unstable intermediates and final products after proper derivatization.

The Tris(pentafluoroethyl)silanide Anion

Tris(pentafluoroethyl)silane, which is conveniently accessible by the treatment of Si(C₂ F₅ )₃ X (X=Cl, Br) with Bu₃ SnH, was successfully employed for hydrosilylation reactions. In the presence of a palladium catalyst, hydrosilylation of phenylacetylene with Si(C₂ F₅ )₃ H affords the product of an α-addition whereas the reaction of trimethylsilylacetylene with the silane leads to the β-trans product. Tris(pentafluoroethyl)silane can be deprotonated by sterically demanding bases such as lithium diisopropylamide at low temperatures to give the corresponding silanide ion. The addition of crown ethers or cryptands to this highly reactive species enabled the isolation and characterization of salt-like tris(pentafluoroethyl)silanide at room temperature.

Syntheses and Structures of the First Heavy-Alkali-Metal Tris(trimethylsilyl)germanides

The first heavy-alkali-metal tris(trimethylsilyl)germanides were obtained in high yield and purity by a simple one-pot reaction involving the treatment of tetrakis(trimethylsilyl)germane, Ge(SiMe3)4, with various alkali metal tert-butoxides. The addition of different sizes of crown ethers or the bidentate TMEDA (TMEDA=N,N,N',N'-tetramethylethylenediamine) provided either contact or separated species in the solid state, whereas in aromatic solvents the germanides dissociate into separated ions, as shown by 29Si NMR spectroscopic studies. Here we report on two series of germanides, one displaying M-Ge bonds in the solid state with the general formula [M(donor)n Ge(SiMe3)3] (M=K, donor=[18]crown-6, n=1, 1; Rb, donor=[18]crown-6, n=1, 4; and M=K, donor=TMEDA, n=2, 6). The silicon analogue of 6, [K(tmeda)2Si(SiMe3)3] (7) is also included to provide a point of reference. The second group of compounds consists of separated ions with the general formula [M(donor)2][Ge(SiMe3)3] (M=K, donor=[15]crown-5, 2; M=K, donor=[12]crown-4, 3; and M=Cs, donor=[18]crown-6, 5). While all target compounds are highly sensitive towards hydrolysis, use of the tridentate nitrogen donor PMDTA (PMDTA=N,N,N',N'',N''-pentamethyldiethylenetriamine) afforded even more reactive species of the composition [K(pmdta)2Ge(SiMe3)3] (8). We also include the silanide analogue [K(pmdta)2Si(SiMe3)3] (9) for sake of comparison. The compounds were typically characterized by X-ray crystallography, and 1H, 13C, and 29Si NMR and IR spectroscopy, unless extremely high reactivity, as observed for the PMDTA adducts 8 and 9, prevented a more detailed characterization.

Branched Star-Type Polysilyllithium Compounds: The Effects of β-Silyl Substitution and of Complexation on Their Molecular Structure

It was fortunate that the first extended branched polysilyl anion (Me3 SiMe2 Si)3 Si(-) could be crystallographically characterized in two forms-as the THF complex 1 and as the dimer 2. Comparison with the analogous (Me3 Si)3 SiLi systems 3 and 4 shows that β-Me3 Si substitution and complexation by THF elongate the Si-Li distance by 0.09-0.14 Å and 0.04-0.07 Å, respectively.