Es steht außer Frage, dass Transferhydrierung ein gängiges Verfahren in der Industrie und der akademischen Welt ist. Trotz ihrer Vielfältigkeit war die konzeptionell verwandte Transferhydrosilylierung bis zur jüngsten Entwicklung einer radikalischen und einer ionischen Variante im Grunde unbekannt gewesen. Die neuen Methoden basieren beide auf dem Motiv eines siliciumsubstituierten Cyclohexa-1,4-diens und hängen von der Aromatisierung der entsprechenden radikalischen bzw. kationischen Cyclohexadienylzwischenstufen begleitet von homo- oder heterolytischer Spaltung der Si-C-Bindung ab. Die radikalische und ionische Transferhydrosilylierung werden in diesem Kurzaufsatz miteinander in Bezug gesetzt, und frühe Hinweise auf die Möglichkeit einer Transferhydrosilylierung werden auch berücksichtigt. Der derzeitige Stand der Forschung befindet sich zweifellos noch in den Anfängen, aber die jüngsten Fortschritte lassen das vielversprechende Potenzial von Transferhydrosilylierungen bereits erkennen.

The preparation of a series of planar chiral, ferrocenyl-substituted hydrosilanes as precursors of ferrocene-stabilized silicon cations is described. These molecules also feature stereogenicity at the silicon atom. The generation and ²⁹Si NMR spectroscopic characterization of the corresponding silicon cations is reported, and problems arising from interactions of the electron-deficient silicon atom and adjacent C(sp³)−H bonds or aromatic π donors are discussed. These issues are overcome by tethering another substituent at the silicon atom to the ferrocene backbone. The resulting annulation also imparts conformational rigidity and steric hindrance in such a way that the central chirality at the silicon atom is set with complete diastereocontrol. These chiral Lewis acid catalysts were then tested in difficult Diels–Alder reactions, but no enantioinduction was seen.

A phosphinic amide is introduced as a directing group for the ortho C−H alkenylation of anilines. The new donor group distinguishes itself from existing ones by assisting the C−H bond activation of anilides without (NH group) and with alkylation (NMe group) at the amide nitrogen atom. The reactivity is even reversed with the methyl-substituted anilide being more reactive than its unsubstituted counterpart. Electron-donating substituents at the arene ring enhance their reactivity while halogenation is not tolerated. The phosphinic amide also enables the C-7-selective C−H alkenylation of indoline.

An electrophilic aromatic substitution (S_EAr) with a catalytically generated silicon electrophile is reported. Essentially any commercially available base-metal salt acts as an initiator/catalyst when activated with NaBAr^F₄ . The thus-generated Lewis acid then promotes the S_EAr of electron-rich arenes with hydrosilanes but not halosilanes. This new C−H silylation was optimized for FeCl₂ /NaBAr^F₄ , affording good yields at catalyst loadings as low as 0.5 mol %. The procedure is exceedingly straightforward and comes close to typical Friedel–Crafts methods, where no added base is needed to absorb the released protons.

Transfer hydrogenation is without question a common technology in industry and academia. Unlike its countless varieties, conceptually related transfer hydrosilylations had essentially been unreported until the recent development of a radical and an ionic variant. The new methods are both based on a silicon-substituted cyclohexa-1,4-diene and hinge on the aromatization of the corresponding cyclohexadienyl radical and cation intermediates, respectively, concomitant with homo- or heterolytic fission of the SiC bond. Both the radical and ionic transfer hydrosilylation are brought into context with one other in this Minireview, and early insight into the possibility of transfer hydrosilylation is included. Although the current state-of-the-art is certainly still limited, the recent advances have already revealed the promising potential of transfer hydrosilylation.

Es wird über eine elektrophile aromatische Substitution (S_EAr) mit einem katalytisch erzeugten Siliciumelektrophil berichtet. Bei Aktivierung mit NaBAr^F₄ wirkt im Grunde jedes käufliche Nichtedelmetallsalz als Initiator/Katalysator. Die auf diese Weise gebildete Lewis-Säure vermittelt dann die S_EAr von elektronenreichen Arenen mit Hydrosilanen, nicht aber mit Halogensilanen. Diese neue C-H-Silylierung ist für FeCl₂/NaBAr^F₄ optimiert und erbringt gute Ausbeuten bei Katalysatorbeladungen von lediglich 0.5 Mol-%. Die Vorgehensweise ist denkbar einfach und kommt typischen Friedel-Crafts-Reaktionsvorschriften nahe, bei denen keine zusätzliche Base zum Abfangen der freigesetzten Protonen vonnöten ist.

The B(C₆F₅)₃-catalyzed hydrogenation is applied to aldoxime triisopropylsilyl ethers and hydrazones bearing an easily removable phthaloyl protective group. The CN reduction of aldehyde-derived substrates (oxime ethers and hydrazones) is enabled by using 1,4-dioxane as the solvent known to participate as the Lewis-basic component in FLP-type heterolytic dihydrogen splitting. More basic ketone-derived hydrazones act as Lewis bases themselves in the FLP-type dihydrogen activation and are therefore successfully hydrogenated in nondonating toluene. The difference in reactivity between aldehyde- and ketone-derived substrates is also reflected in the required catalyst loading and dihydrogen pressure.

An enantio- and regioselective allylic silylation of linear allylic phosphates that makes use of catalytically generated cuprate-type silicon nucleophiles is reported. The method relies on soft bis(triorganosilyl) zincs as silicon pronucleophiles that are prepared in situ from the corresponding hard lithium reagents by transmetalation with ZnCl₂. With a preformed chiral N-heterocyclic carbene–copper(I) complex as catalyst, exceedingly high enantiomeric excesses are achieved. The new method is superior to existing ones using a silicon–boron reagent as the source of the silicon nucleophile.

A CH silylation of pyridines that seemingly proceeds through electrophilic aromatic substitution (S_EAr) is reported. Reactions of 2- and 3-substituted pyridines with hydrosilanes in the presence of a catalyst that splits the SiH bond into a hydride and a silicon electrophile yield the corresponding 5-silylated pyridines. This formal silylation of an aromatic CH bond is the result of a three-step sequence, consisting of a pyridine hydrosilylation, a dehydrogenative CH silylation of the intermediate enamine, and a 1,4-dihydropyridine retro-hydrosilylation. The key intermediates were detected by ¹H NMR spectroscopy and prepared through the individual steps. This complex interplay of electrophilic silylation, hydride transfer, and proton abstraction is promoted by a single catalyst.

Die Freisetzung von Wasserstoff aus entsprechend donorsubstituierten Cyclohexa-1,4-dienen nach Hydridabstraktion durch die starke Bor-Lewis-Säure Tris(pentafluorphenyl)boran, B(C₆F₅)₃, wird vorgestellt. Dieses Verfahren ist dann an die FLP-artige Hydrierung (FLP=frustiertes Lewis-Paar) von Iminen und stickstoffenthaltenden Heteroaromaten, die durch dieselbe Lewis-Säure katalysiert wird, gekoppelt. Die Nettoreaktion ist eine B(C₆F₅)₃-katalysierte, d. h. übergangsmetallfreie, Transferhydrierung mit leicht zugänglichen Cyclohexa-1,4-dienen als Reduktionsmitteln. Konkurrierende Reaktionspfade mit oder ohne Einbeziehung von freiem Wasserstoff werden diskutiert.

Eine C-H-Silylierung von Pyridinen wurde entwickelt, die scheinbar dem Mechanismus einer elektrophilen aromatischen Substitution (S_EAr) folgt. Reaktionen von 2- und 3-substituierten Pyridinen mit Hydrosilanen in Gegenwart eines Katalysators, welcher die Si-H-Bindung in ein Hydrid und ein Siliciumelektrophil spaltet, ergeben die entsprechenden 5-silylierten Pyridine. Diese formale Silylierung einer aromatischen C-H-Bindung ist das Ergebnis einer dreistufigen Reaktionssequenz bestehend aus einer Pyridinhydrosilylierung, einer dehydrierenden C-H-Silylierung der Enaminzwischenstufe und einer Retro-Hydrosilylierung des 1,4-Dihydropyridins. Die Schlüsselzwischenstufen wurden ¹H-NMR-spektroskopisch nachgewiesen und schrittweise einzeln hergestellt. Dieses komplexe Wechselspiel elektrophiler Silylierungen, Hydridübertragungen und Protonenabstraktionen wird durch einen einzigen Katalysator vermittelt.

The strong boron Lewis acid tris(pentafluorophenyl)borane, B(C₆F₅)₃, is shown to abstract a hydride from suitably donor-substituted cyclohexa-1,4-dienes, eventually releasing dihydrogen. This process is coupled with the FLP-type (FLP=frustrated Lewis pair) hydrogenation of imines and nitrogen-containing heteroarenes that are catalyzed by the same Lewis acid. The net reaction is a B(C₆F₅)₃-catalyzed, i.e., transition-metal-free, transfer hydrogenation using easy-to-access cyclohexa-1,4-dienes as reducing agents. Competing reaction pathways with or without the involvement of free dihydrogen are discussed.

Es wird über eine allgemeine Reaktionsvorschrift zur katalytischen Synthese von an einem oder beiden Benzolringen funktionalisierten Dibenzosilolen berichtet, die von leicht verfügbaren ortho-silylierten Biphenylen ausgeht. Diese Methode ermöglicht einen schnellen Zugang zu Silolbausteinen mit Chloratomen an beiden Phenylengruppen und beschreibt somit einen katalytischen Weg zu direkt polymerisierbaren Dibenzosilolen. Es wird außerdem gezeigt, dass trotz hochgradig elektrophiler Zwischenstufen eine beachtliche Breite an Lewis-basischen, beispielsweise sauerstoff- und stickstoffhaltigen, funktionellen Gruppen toleriert wird. Der Mechanismus dieser intramolekularen elektrophilen aromatischen Substitution (S_EAr) durchläuft ein schwefelstabilisiertes Siliciumkation, das katalytisch aus der Hydrosilanvorstufe generiert wird.

A general procedure for the catalytic preparation of dibenzosiloles functionalized at one or both benzene rings starting from readily available ortho-silylated biphenyls is reported. This method provides rapid access to silole building blocks substituted with chlorine atoms at both phenylene groups, thereby allowing catalytic access to directly polymerizable dibenzosiloles. Moreover, it is shown that, despite the involvement of highly electrophilic intermediates, a considerable range of Lewis-basic, for example, oxygen- and nitrogen-containing, functional groups is tolerated. The mechanism of this intramolecular electrophilic aromatic substitution (S_EAr) proceeds through a sulfur-stabilized silicon cation, generated catalytically from the hydrosilane precursor.

An unprecedented transfer silylation of alcohols catalyzed by the strong Lewis acid B(C₆F₅)₃ is described. Gaseous Me₃SiH is released in situ by B(C₆F₅)₃-catalyzed decomposition of 3-trimethylsilylcyclohexa-1,4-diene and subsequently reacts with an alcohol in a dehydrogenative Si–O coupling promoted by the same boron catalyst. Benzene and dihydrogen are formed during the reaction, but no salt waste is. This expedient protocol is applicable to several silicon groups, and the preparation of trimethylsilyl ethers presented here is potentially useful for alcohol derivatization prior to GLC analysis.

The copper(I) alkoxide-catalyzed release of a silicon-based cuprate reagent from a silicon–boron pronucleophile is applied to the addition across carbon–carbon triple bonds. Commercially available CuBr⋅Me₂S was found to be a general precatalyst that secures high regiocontrol for both aryl- and alkyl-substituted terminal as well as internal alkynes. The solvent greatly influences the regioisomeric ratio, favoring the linear regioisomer with terminal acceptors. This facile protocol even allows for the transformation of internal acceptors with remarkable levels of regio- and diastereocontrol.

A procedure for the synthesis of otherwise difficult-to-make N-silylated enamines, that is masked enamines derived from primary amines, is reported. The approach is based on formation of a silyliminium ion and subsequent abstraction of the acidified α-proton rather than α-deprotonation of the enolizable imine followed by reaction with an electrophilic silicon reagent. The silicon electrophile, stabilized by a sulfur atom, is generated by cooperative activation of an SiH bond at the RuS bond of a tethered ruthenium(II) thiolate complex. After transfer of the silicon cation onto the imine nitrogen atom, the remaining ruthenium(II) hydride fulfills the role of the base. Deprotonation and release of dihydrogen close the catalytic cycle. The net reaction is a dehydrogenative SiN coupling of enolizable imines and hydrosilanes.

The silylzincation of terminal ynamides is achieved through a radical-chain process involving (Me₃Si)₃SiH and R₂Zn. A potentially competing polar mechanism is excluded on the basis of diagnostic control experiments. The unique feature of this addition across the CC bond is its trans selectivity. One-pot electrophilic substitution of the CZn bond by Cu^I-mediated CC bond formation and subsequent manipulation of the CSi bond provides a modular access to Z-α,β-disubstituted enamides.

The hydrogenation of oximes and oxime ethers is usually hampered by NO bond cleavage, hence affording amines rather than hydroxylamines. The boron Lewis acid B(C₆F₅)₃ is found to catalyze the chemoselective hydrogenation of oxime ethers at elevated or even room temperature under 100 bar dihydrogen pressure. The use of the triisopropylsilyl group as a protecting group allows for facile liberation of the free hydroxylamines.

Die Hydrierung von Oximen und Oximethern geht normalerweise mit der Spaltung der N-O-Bindung einher und setzt daher Amine und eben nicht Hydroxylamine frei. Die Bor-Lewis-Säure B(C₆F₅)₃ katalysiert die chemoselektive Hydrierung von Oximethern bei erhöhter oder sogar Raumtemperatur unter 100 bar Wasserstoffdruck. Die Verwendung der Triisopropylsilylgruppe als Schutzgruppe erlaubt eine mühelose Freisetzung der Hydroxylamine.

The silylzincation of terminal ynamides is achieved through a radical-chain process involving (Me₃Si)₃SiH and R₂Zn. A potentially competing polar mechanism is excluded on the basis of diagnostic control experiments. The unique feature of this addition across the CC bond is its trans selectivity. One-pot electrophilic substitution of the CZn bond by Cu^I-mediated CC bond formation and subsequent manipulation of the CSi bond provides a modular access to Z-α,β-disubstituted enamides.

Diese Arbeit befasst sich mit einer noch verbliebenen, bedeutenden Herausforderung der asymmetrischen Addition von Siliciumnukleophilen an typische prochirale Akzeptoren, der enantioselektiven 1,2-Addition an Aldimine. Die Aktivierung der Si-B-Bindung des Siliciumpronukleophils durch ein Kupfer(I)-Alkoholat mit McQuades chiralem sechsgliedrigen N-heterocyclischen Carben als Ligand setzt das Siliciumnukleophil frei, das mit hohen Enantioselektivitäten an verschiedenartige Aldimine addiert. Die neue Methode bietet einen katalytischen asymmetrischen Zugang zu α-silylierten Aminen.

A remaining major challenge in the asymmetric addition of silicon nucleophiles to typical prochiral acceptors, the enantioselective 1,2-addition to aldimines, is addressed. Activation of the SiB bond in the silicon pronucleophile by a copper(I) alkoxide with McQuade’s chiral six-membered N-heterocyclic carbene as a supporting ligand releases the silicon nucleophile, which adds to various aldimines with high levels of enantioselectivity. The new method provides a catalytic asymmetric access to α-silylated amines.

A perfectly γ-selective copper(I)-catalyzed allylic substitution of protected δ-hydroxy allylic chlorides with a silicon nucleophile generated by Si–B bond activation provides diastereoselective access to β-hydroxy α-chiral allylic silanes with an anti relative configuration. The high levels of diastereocontrol of this rare allylic displacement are interpreted according to a model suggested by Nakamura where diastereofacial selectivity originates from steric control rather than an oxygen-directing effect in the Felkin–Anh transition state.

The purpose of this systematic experimental and theoretical study is to deeply understand the unique bonding situation in ferrocene-stabilized silylium ions as a function of the substituents at the silicon atom and to learn about the structure parameters that determine the ²⁹Si NMR chemical shift and electrophilicity of these strong Lewis acids. For this, ten new members of the family of ferrocene-stabilized silicon cations were prepared by a hydride abstraction reaction from silanes with the trityl cation and characterized by multinuclear ¹H and ²⁹Si NMR spectroscopy. A closer look at the NMR spectra revealed that additional minor sets of signals were not impurities but silylium ions with substitution patterns different from that of the initially formed cation. Careful assignment of these signals furnished experimental proof that sterically less hindered silylium ions are capable of exchanging substituents with unreacted silane precursors. Density functional theory calculations provided mechanistic insight into that substituent transfer in which the migrating group is exchanged between two silicon fragments in a concerted process involving a ferrocene-bridged intermediate. Moreover, the quantum-chemical analysis of the ²⁹Si NMR chemical shifts revealed a linear relationship between δ(²⁹Si) values and the Fe⋅⋅⋅Si distance for subsets of silicon cations. An electron localization function and electron localizability indicator analysis shows a three-center two-electron bonding attractor between the iron, silicon, and C′_ipso atoms, clearly distinguishing the silicon cations from the corresponding carbenium ions and boranes. Correlations between ²⁹Si NMR chemical shifts and Lewis acidity, evaluated in terms of fluoride ion affinities, are seen only for subsets of silylium ions, sometimes with non-intuitive trends, indicating a complicated interplay of steric and electronic effects on the degree of the Fe⋅⋅⋅Si interaction.

BINAP(O), rarely used as a chiral ligand, was found to induce significantly higher levels of enantioselection in representative desymmetrizing Mizoroki–Heck cyclizations where conventional BINAP produces essentially racemic material. BINAP(O) is a ligand with a stronger and weaker donor atom, and that hemilabile nature lends itself the ability to act as either a bi- or monodentate ligand. On that basis, we introduce mechanistic models where the weak donor, in one case, mediates the equilibration of diastereomeric alkene–palladium(II) complexes and, in another case, dissociates from the palladium(II) atom, thereby rendering BINAP(O) as a monodentate ligand. These new findings, along with the recently reported effects in intermolecular Mizoroki–Heck reactions, suggest that BINAP(O) ought to be included into ligand screenings.

The reduction of CO groups with silanes catalyzed by electron-deficient boranes follows a counterintuitive mechanism in which the SiH bond is activated by the boron Lewis acid prior to nucleophilic attack of the carbonyl oxygen atom at the silicon atom. The borohydride thus formed is the actual reductant. These steps were elucidated by using a silicon-stereogenic silane, but applying the same technique to the related reduction of CN groups was inconclusive due to racemization of the silicon atom. The present investigation now proves by the deliberate combination of our axially chiral borane catalyst and axially chiral silane reagents (in both enantiomeric forms) that the mechanisms of these hydrosilylations are essentially identical. Unmistakable stereochemical outcomes for the borane/silane pairs show that both participate in the enantioselectivity-determining hydride-transfer step. These experiments became possible after the discovery that our axially chiral C₆F₅-substituted borane induces appreciable levels of enantioinduction in the imine hydrosilylation.