Dispersion-Energy-Driven Wagner-Meerwein Rearrangements in Oligosilanes

Aluminagerma[5]pyramidanes-Formation and Skeletal Rearrangement

The salt metathesis reaction of dipotassium germacyclopentadienediide with aluminum(III) dichlorides provides either half-sandwich alumole complexes of germanium(II) or aluminylene germole complexes. Their molecular structure and the delocalized bonding situation, revealed by density functional theory (DFT) calculations, are equally described as isomeric aluminagerma[5]pyramidanes with either the germanium or the aluminum atom in the apical position of the pentagonal pyramid. The product formation and the selectivity of the reaction depends on the third substituent of the aluminum dichloride. Aryl-substituents favor the formation of alumole complexes and Cp*-substituents that of the isomeric germole complexes. With amino-substituents at the aluminum atom mixtures of both isomers are formed and the positional exchange of the two heteroatoms is shown by NMR spectroscopy. The alumole complexes of germanium(II) undergo facile reductive elimination of germanium and form the corresponding alumoles.

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.

Ga+-catalyzed hydrosilylation? About the surprising system Ga+/HSiR3/olefin, proof of oxidation with subvalent Ga+ and silylium catalysis with perfluoroalkoxyaluminate anions

Already 1 mol% of subvalent [Ga(PhF)2]+[pf]- ([pf]- = [Al(ORF)4]-, RF = C(CF3)3) initiates the hydrosilylation of olefinic double bonds under mild conditions. Reactions with HSiMe3 and HSiEt3 as substrates efficiently yield anti-Markovnikov and anti-addition products, while bulkier substrates such as HSiiPr3 are less reactive. Investigating the underlying mechanism by gas chromatography and STEM analysis, we unexpectedly found that H2 and metallic Ga0 formed. Without the addition of olefins, the formation of R3Si-F-Al(ORF)3 (R = alkyl), a typical degradation product of the [pf]- anion in the presence of a small silylium ion, was observed. Electrochemical analysis revealed a surprisingly high oxidation potential of univalent [Ga(PhF)2]+[pf]- in weakly coordinating, but polar ortho-difluorobenzene of E 1/2(Ga+/Ga0; oDFB) = +0.26-0.37 V vs. Fc+/Fc (depending on the scan rate). Apparently, subvalent Ga+, mainly known as a reductant, initially oxidizes the silane and generates a highly electrophilic, silane-supported, silylium ion representing the actual catalyst. Consequently, the [Ga(PhF)2]+[pf]-/HSiEt3 system also hydrodefluorinates C(sp3)-F bonds in 1-fluoroadamantane, 1-fluorobutane and PhCF3 at room temperature. In addition, both catalytic reactions may be initiated using only 0.2 mol% of [Ph3C]+[pf]- as a silylium ion-generating initiator. These results indicate that silylium ion catalysis is possible with the straightforward accessible weakly coordinating [pf]- anion. Apparently, the kinetics of hydrosilylation and hydrodefluorination are faster than that of anion degradation under ambient conditions. These findings open up new windows for main group catalysis.

Hydrogen-Bridged Oligosilanylsilyl Mono- and Oligosilanylsilyl Dications

Hydrogen‐bridged oligosilanylsilyl borates 8 [B(C₆F₅)₄], 9[B(C₆F₅)₄] and diborates 10 [B(C₆F₅)₄]₂ have been prepared by hydride transfer between α‐ω‐dihydrido‐ (11) and branched tetrahydrido‐oligosilanes (13) and trityl cation. The obtained cyclic intramolecularly stabilized silylium ions 8, 9 and bissilylium ion 10 were characterized by low temperature NMR spectroscopy supported by the results of density functional calculations. The branched Si−H−Si monocation 9 undergoes at low temperatures a fast degenerate rearrangement, which exchanges the Si−H groups with a barrier of 31 kJ mol⁻¹ via an antarafacial transition state. Reaction of the branched monocation 9 with a second equivalent of trityl cation or of the branched oligosilane 13 with two equivalents of trityl cation, gives at −80 °C the corresponding bissilylium ion 10, an example for a new class of highly reactive poly‐Lewis acids.

The influence of ring strain on the formation of Si-H-Si stabilised oligosilanylsilyl cations

Hydride transfer between Si(SiMe₂H)₄ (2) and [Ph₃C][B(C₆F₅)₄] in 3-fluorotoluene yields the intermolecularly silane-stabilised silylium ion [((HMe₂Si)₃SiSiMe₂)₂H]⁺ ([5]+), independently of the amount of [Ph₃C][B(C₆F₅)₄] (0.5, 1.0 or 2.0 equiv.) used. The cyclic silane-stabilised silylium ion [4]+ is not detected by NMR spectroscopy. This result demonstrates the influence of ring strain effects on the formation of intra- ([4]+) or intermolecularly ([5]+) Si-H-Si bridged silyl cations.

Selective One-Pot Syntheses of Mixed Silicon-Germanium Heteroadamantane Clusters

Si x Ge y alloys are emerging materials for modern semiconductor technology. Well-defined model systems of the bulk structures aid in understanding their intrinsic characteristics. Three such model clusters have now been realized in the form of the Si x Ge y heteroadamantanes [0] , [1] , and [2] via selective one-pot syntheses starting from Me 2 GeCl 2 , Si 2 Cl 6 , and [nBu 4 N]Cl. Compound [0] contains 6 GeMe 2 and 4 SiSiCl 3 vertices, while one and two of the GeMe 2 groups are replaced by SiCl 2 moieties in compounds [1] and [2] , respectively. Chloride ion-mediated rearrangement quantitatively converts [2] into [1] at room temperature and finally into [0] at 60 °C, which is not only remarkable in view of the rigidity of these cage structures but also sheds light on the assembly mechanism.

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.

Optical properties of organosilicon compounds containing sigma-electron delocalization by quasiparticle self-consistent GW calculations

We investigate theoretically the electronic and optical absorption properties of two sub-classes of oligosilanes: (i) Si(CH₃)₄, Si₄(CH₃)₈, and Si₈(CH₃)₈ that contain Si dot, ring and cage, respectively, and exhibit typical SiC and SiSi bonds; and (ii) persilastaffanes Si₇H₆(CH₃)₆ and Si₁₂H₆(CH₃)₁₂, which contain extended delocalized σ-electrons in SiSi bonds over three-dimensional Si frameworks. Our modeling is performed within the GW approach up to the partially self-consistent GW₀ approximation, which is more adequate for reliably predicting the optical band gaps of materials. We examine how the optical properties of these organosilicon compounds depend on their size, geometric features, and Si/C composition. Our results indicate that the present methodology offers a viable way of describing the optical excitations of tailored functional Si-C-based clusters and molecular optical tags with potential use as efficient light absorbers/emitters in molecular optical devices.

Synthesis of a Counteranion-Stabilized Bis(silylium) Ion

The preparation of a molecule with two alkyl‐tethered silylium‐ion sites from the corresponding bis(hydrosilanes) by two‐fold hydride abstraction is reported. The length of the conformationally flexible alkyl bridge is crucial as otherwise the hydride abstraction stops at the stage of a cyclic bissilylated hydronium ion. With an ethylene tether, the open form of the hydronium‐ion intermediate is energetically accessible and engages in another hydride abstraction. The resulting bis(silylium) ion has been NMR spectroscopically and structurally characterized. Related systems based on rigid naphthalen‐n,m‐diyl platforms can only be converted into the dications when the positively charged silylium‐ion units are remote from each other (1,8 versus 1,5 and 2,6).

Delayed catalyst function enables direct enantioselective conversion of nitriles to NH2-amines

Accessing enantiomerically enriched amines often demands oxidation-state adjustments, protection and deprotection processes, and purification procedures that increase cost and waste, limiting applicability. When diastereomers can be formed, one isomer is attainable. Here, we show that nitriles, largely viewed as insufficiently reactive, can be transformed directly to multifunctional unprotected homoallylic amines by enantioselective addition of a carbon-based nucleophile and diastereodivergent reduction of the resulting ketimine. Successful implementation requires that competing copper-based catalysts be present simultaneously and that the slower-forming and less reactive one engages first. This challenge was addressed by incorporation of a nonproductive side cycle, fueled selectively by inexpensive additives, to delay the function of the more active catalyst. The utility of this approach is highlighted by its application to the efficient preparation of the anticancer agent (+)-tangutorine.

Thermodynamic versus kinetic control in substituent redistribution reactions of silylium ions steered by the counteranion

An in-depth experimental and theoretical study of the substituent exchange reaction of silylium ions is presented. Apart from the substitution pattern at the silicon atom, the selectivity of this process is predominantly influenced by the counteranion, which is introduced with the trityl salt in the silylium ion generation. In contrast to Müller's protocol for the synthesis of triarylsilylium ions under kinetic control, the use of Reed's carborane anions leads to contact ion pairs, allowing selective formation of trialkylsilylium ions under thermodynamic control. DFT calculations finally revealed an unexpected mechanism for the rate-determining alkyl exchange step, which is initiated by an unusual 1,2-silyl migration in the intermediate ipso-disilylated arenium ion. The resulting ortho-disilylated arenium ion can then undergo an alkyl transfer via a low-barrier five-centered transition state.

Linear MgCp*2 vs Bent CaCp*2: London Dispersion, Ligand-Induced Charge Localizations, and Pseudo-Pregostic C-H···Ca Interactions

In the family of metallocenes, MgCp*₂ (Cp* = pentamethylcyclopentadienyl) exhibits a regular linear sandwich structure, whereas CaCp*₂ is bent in both the gas phase and solid state. Bending is typically observed for metal ions which possess a lone pair. Here, we investigate which electronic differences cause the bending in complexes lacking lone pairs at the metal atoms. The bent gas-phase geometry of CaCp*₂ suggests that the bending must have an intramolecular origin. Geometry optimizations with and without dispersion effects/d-type polarization functions on MCp₂ and MCp*₂ gas-phase complexes (M = Ca, Mg) establish that attractive methyl···methyl London dispersion interactions play a decisive role in the bending in CaCp*₂. A sufficient polarizability of the metal to produce a shallow bending potential energy curve is a prerequisite but is not the reason for the bending. Concomitant ligand-induced charge concentrations and localizations at the metal atoms are studied in further detail, for which real-space bonding and orbital-based descriptors are used. Low-temperature crystal structures of MgCp*₂ and CaCp*₂ were determined which facilitated the identification and characterization of intermolecular pseudo-pregostic interactions, C-H···Ca, in the CaCp*₂ crystal structure.

New reactivity at the silicon bridge in sila[1]ferrocenophanes

We describe two new types of reactivity involving silicon-bridged [1]ferrocenophanes. In an attempt to form a [1]ferrocenophane with a bridging silyl cation, the reaction of sila[1]ferrocenophane [Fe(η-C₅H₄)₂Si(H)TMP] (12) (TMP = 2,2,6,6-tetramethylpiperidyl) towards the hydride-abstraction reagent trityl tetrakis(pentafluorophenyl)borate ([CPh₃][B(C₆F₅)₄]) was explored. This yielded the unusual dinuclear species [Fe(η-C₅H₄)₂Si(TMP·H)(η-C₅H₃)Fe(η-C₅H₄)Si(H)TMP][B(C₆F₅)₄] [13][B(C6F5)4] in low yield. The formation of [13]+ is proposed to involve abstraction of hydride from the silicon bridge in 12 with subsequent C-H bond cleavage of a cyclopentadienyl group by the resulting electrophilic transient silyl cation intermediate. We also explored the reaction of dimethylsila[1]ferrocenophane [Fe(η-C₅H₄)₂SiMe₂] (1) with the Au(i) species AuCl(PMe₃). This was found to result in addition of the Au-Cl bond across the Cp_ipso-Si bond to yield the ring-opened species [1'-(chlorodimethylsilyl)-ferrocenyl](trimethylphosphine)gold(i), [Fe(C₅H₄SiMe₂Cl){C₅H₄Au(PMe₃)}] (14). This represents the first example of ring-opening addition of a metallocenophane with a reagent possessing a transition metal-halogen bond.

Catalytic diastereo- and enantioselective additions of versatile allyl groups to N-H ketimines

There are many biologically active organic molecules that contain one or more nitrogen-containing moieties, and broadly applicable and efficient catalytic transformations that deliver them diastereoselectively and/or enantioselectively are much sought after. Various methods for enantioselective synthesis of α-secondary amines are available (for example, from additions to protected/activated aldimines), but those involving ketimines are much less common. There are no reported additions of carbon-based nucleophiles to unprotected/unactivated (or N-H) ketimines. Here, we report a catalytic, diastereo- and enantioselective three-component strategy for merging an N-H ketimine, a monosubstituted allene and B₂(pin)₂, affording products in up to 95% yield, >98% diastereoselectivity and >99:1 enantiomeric ratio. The utility of the approach is highlighted by synthesis of the tricyclic core of a class of compounds that have been shown to possess anti-Alzheimer activity. Stereochemical models developed with the aid of density functional theory calculations, which account for the observed trends and levels of enantioselectivity, are presented.