Wagner-Meerwein-Type Rearrangements of Germapolysilanes - A Stable Ion Study

Installing Quaternary Germanium Centers in Sila-Diamondoid Cores via Skeletal Isomerization

This manuscript describes skeletal isomerization strategies to install one to four quaternary germanium atoms in the sila-adamantane core, in a cluster analogy to precision germanium doping in silicon-germanium alloys. The first strategy embodies an inorganic variant of single-atom skeletal editing, where we use a sila-Wagner-Meerwein bond shift cascade to exchange a peripheral Ge atom with a core Si atom. We can install up to four Ge atoms at the quaternary diamondoid centers based on controlling the SixGey stoichiometry of our precursor. We find that bridgehead Ge centers can be selectively functionalized over bridgehead Si centers in SiGe adamantanes; we use this chemistry in conjunction with scanning tunneling microscopy break-junction (STM-BJ) measurements to show that Si8Ge2 adamantane wires give a 60% increase in single-molecule conductance compared with Si10 adamantanes. These studies describe the first quantum transport measurements in sila-diamondoid structures, and demonstrate how main-chain Ge doping can be used to increase electronic transmission in sila-diamondoid-based molecular wires.

Selective synthesis of germasila-adamantanes through germanium-silicon shift processes

The regioselective synthesis of germasila-adamantanes with the germanium atoms in the bridgehead positions is described starting from cyclic precursors by a cationic sila-Wagner-Meerwein (SWM) rearrangement reaction. The SWM rearrangement allows also a deliberate shift of germanium atoms from the periphery and within the cage structures into the bridgehead positions. This opens the possibility for a synthesis of germasila-adamantanes of defined germanium content and controlled regiochemistry. In the same way that sila-adamantane can be regarded as a molecular building block of elemental silicon, the germasila-adamantane molecules represent cutouts of silicon/germanium alloys.

Dope it with germanium: selective access to functionalized Si5Ge heterocycles

The Cl^- diadduct [nBu₄N]₂[A·2Cl] of the mixed cyclohexatetrelane (SiCl₂)₅(GeMe₂), A, is accessible from Me₂GeCl₂, 6 eq. Si₂Cl₆, and 2 eq. [nBu₄N]Cl in one step (96%). Free, tenfold functionalized A can be released from the primary product by decomplexation with AlCl₃ (78%). Insight into the assembly mechanism of [nBu₄N]₂[A·2Cl] and the reactivity of A is provided.

Subvalent mixed SixGey oligomers: (Cl3Si)4Ge and Cl2(Me2EtN)SiGe(SiCl3)2

(Cl₃Si)₄Ge (1; 91%) is accessible from GeCl₄, the Si₂Cl₆/[nBu₄N]Cl silylation system, and excess SiCl₄. A key intermediate step involves Cl^- sequestration with AlCl₃ in the course of the reaction between the first-formed germanide [(Cl₃Si)₃Ge]^- and SiCl₄. The related adduct Cl₂(Me₂EtN)SiGe(SiCl₃)₂ (2; quantitative conversion) was prepared either by amine-induced cleavage of 1 or by a bottom-up synthesis starting from GeCl₄ and Si₂Cl₆.

Silylium Ions: From Elusive Reactive Intermediates to Potent Catalysts

The history of silyl cations has all the makings of a drama but with a happy ending. Being considered reactive intermediates impossible to isolate in the condensed phase for decades, their actual characterization in solution and later in solid state did only fuel the discussion about their existence and initially created a lot of controversy. This perception has completely changed today, and silyl cations and their donor-stabilized congeners are now widely accepted compounds with promising use in synthetic chemistry. This review provides a comprehensive summary of the fundamental facts and principles of the chemistry of silyl cations, including reliable ways of their preparation as well as their physical and chemical properties. The striking features of silyl cations are their enormous electrophilicity and as such reactivity as super Lewis acids as well as fluorophilicity. Known applications rely on silyl cations as reactants, stoichiometric reagents, and promoters where the reaction success is based on their steady regeneration over the course of the reaction. Silyl cations can even be discrete catalysts, thereby opening the next chapter of their way into the toolbox of synthetic methodology.

Single-Electron Transfer Reactions in Frustrated and Conventional Silylium Ion/Phosphane Lewis Pairs

Silylium ions undergo a single-electron reduction with phosphanes, leading to transient silyl radicals and the corresponding stable phosphoniumyl radical cations. As supported by DFT calculations, phosphanes with electron-rich 2,6-disubstituted aryl groups are sufficiently strong reductants to facilitate this single-electron transfer (SET). Frustration as found in kinetically stabilized triarylsilylium ion/phosphane Lewis pairs is not essential, and silylphosphonium ions, which are generated by conventional Lewis adduct formation of solvent-stabilized trialkylsilylium ions and phosphanes, engage in the same radical mechanism. The trityl cation, a Lewis acid with a higher electron affinity, even oxidizes trialkylphosphanes, such as tBu₃ P, which does not react with either B(C₆ F₅ )₃ or silylium ions.

Dispersion-Energy-Driven Wagner-Meerwein Rearrangements in Oligosilanes

The installation of structural complex oligosilanes from linear starting materials by Lewis acid induced skeletal rearrangement reactions was studied under stable ion conditions. The produced cations were fully characterized by multinuclear NMR spectroscopy at low temperature, and the reaction course was studied by substitution experiments. The results of density functional theory calculations indicate the decisive role of attractive dispersion forces between neighboring trimethylsilyl groups for product formation in these rearrangement reactions. These attractive dispersion interactions control the course of Wagner-Meerwein rearrangements in oligosilanes, in contrast to the classical rearrangement in hydrocarbon systems, which are dominated by electronic substituent effects such as resonance and hyperconjugation.

Cationic Si-H-Si Bridges in Polysilanes: Their Detection and Targeted Formation in Stable Ion Studies

The ionization of 1,1-dihydridocyclopentasilane 7 has been found to yield the cyclic polysilanylsilyl cation 8 instead of the expected hydrogen-substituted silylium ion 6. The silyl cation 8 is stabilized by the formation of an intramolecular Si-H-Si bridge, which also provides the thermodynamic driving force for its formation. In general, the preference for the formation of Si-H-Si bridges can be used to scavenge and identify transient intermediates in the Lewis acid induced rearrangement of polysilanes. The validity of this concept has been demonstrated for one central step in this chemistry, the ring-contraction reaction of cyclohexasilanes to form silylcyclopentasilanes.