Access to Fully Alkylated Germanes by B(C₆F₅)₃‑Catalyzed Transfer Hydrogermylation of Alkenes

Various cyclohexa-2,5-dien-1-yl-substituted germanes are shown to serve as easy-to-handle surrogates of hydrogermanes, including gaseous MeGeH₃ and Me₂GeH₂. The Ge–H functional group is liberated by treatment with catalytic amounts of B­(C₆F₅)₃ and participates in situ in the B­(C₆F₅)₃-catalyzed hydrogermylation of alkenes. The range of suitable alkenes is broad, and the overall procedure provides a convenient access to tetraalkyl-substituted germanes at room temperature. Transfer hydrogermylation of internal alkynes works equally well and selectively forms the <i>trans</i> or <i>cis</i> diastereomer depending on the electronic bias of the CC bond

Two-Directional Desymmetrization by Double 1,4-Addition of Silicon and Boron Nucleophiles

The two-directional desymmetrization of prochiral precursors with α,β-unsaturated branches by catalyst-controlled 1,4-addition of silicon and likewise boron nucleophiles allows for a general enantioselective access to <i>syn</i>,<i>anti</i>-triols with 1,<i>n</i> + 1,2<i>n</i> + 1 (<i>n</i> = 2 and 3) substitution patterns. The utility is demonstrated in the synthesis of the C17–C25 fragment of dermostatin A

Hidden Enantioselective Hydrogenation of N‑Silyl Enamines and Silyl Enol Ethers in Net CN and CO Hydrosilylations Catalyzed by Ru–S Complexes with One Monodentate Chiral Phosphine Ligand

Ruthenium thiolate complexes with one chiral monodentate phosphine ligand are applied to enantioselective hydrosilylation of enolizable imines and ketones. The structural features of the catalyst exclude the presence of more than one phosphine ligand at the ruthenium center in the enantioselectivity-determining step. The enantiomeric excesses obtained in these reduction reactions are moderate (up to 66% ee), but the stereochemical outcome enables an experimental analysis of the reaction pathways operative in this catalysis. A two-step sequence consisting of successive N–Si/O–Si dehydrogenative coupling and enamine/enol ether hydrogenation is the prevailing mechanism of action. Both steps involve cooperative bond activation at the Ru–S bond of the coordinatively unsaturated ruthenium complex: Si–H bond activation in the dehydrogenative coupling and heterolytic H–H splitting in the hydrogenation. Previously documented side reactions such as deprotonation/protonation equilibria as well as competing direct CN or CO hydrogenation have been excluded

Catalytic Access to Indole-Fused Benzosiloles by 2‑Fold Electrophilic C–H Silylation with Dihydrosilanes

A protocol for the catalytic synthesis of indole-fused benzosiloles starting from 2-aryl-substituted indoles and dihydrosilanes is reported. Compared to known procedures, this method does not require prefunctionalized starting materials and, hence, allows for a rapid access to those siloles. The net reaction is a 2-fold electrophilic C–H silylation catalyzed by cationic Ru–S complexes. Both reaction steps were separately investigated, and these results eventually led to the development of a two-step procedure. By preparing new Ru–S complexes with different weakly coordinating anions (WCAs), it is also shown that the latter can have a dramatic influence on the outcome of these reactions. Furthermore, the substrate scope of the new method is discussed

Brønsted Acid-Catalyzed Transfer Hydrogenation of Imines and Alkenes Using Cyclohexa-1,4-dienes as Dihydrogen Surrogates

Cyclohexa-1,4-dienes are introduced to Brønsted acid-catalyzed transfer hydrogenation as an alternative to the widely used Hantzsch dihydropyridines. While these hydrocarbon-based dihydrogen surrogates do offer little advantage over established protocols in imine reduction as well as reductive amination, their use enables the previously unprecedented transfer hydrogenation of structurally and electronically unbiased 1,1-di- and trisubstituted alkenes. The mild procedure requires 5.0 mol % of Tf₂NH, but the less acidic sulfonic acids TfOH and TsOH work equally well

Merging Platinum-Catalyzed Alkene Hydrosilylation with SiH₄ Surrogates: Salt-Free Preparation of Trihydrosilanes

The monosilane (SiH₄) surrogate di­(cyclohexa-2,5-dien-1-yl)­silane is shown to be compatible with platinum-catalyzed hydrosilylation of α-olefins. The cyclohexa-2,5-dien-1-yl substituents in the monohydrosilylation adducts serve as protecting groups, and treatment with catalytic amounts of B­(C₆F₅)₃ liberates the Si–H bonds along with benzene. By this, trihydrosilanes become accessible in two steps without the formation of salt waste

Copper(I)-Catalyzed Regio- and Chemoselective Single and Double Addition of Nucleophilic Silicon to Propargylic Chlorides and Phosphates

Copper(I)-catalyzed propargylic substitution of linear precursors with (Me₂PhSi)₂Zn predominantly yields the γ isomer independent of the propargylic leaving group. The thus formed allenylic silane reacts regioselectively with another equivalent of (Me₂PhSi)₂Zn, yielding a bifunctional building block with allylic and vinylic silicon groups. The reaction rates of both steps are well-balanced for chloride (γ:α ≥ 99:1) where the propargylic displacement occurs quantitatively prior to the addition step. Substitutions of α-branched propargylic phosphates are also reported

Catalytic Access to Indole-Fused Benzosiloles by 2‑Fold Electrophilic C–H Silylation with Dihydrosilanes

A protocol for the catalytic synthesis of indole-fused benzosiloles starting from 2-aryl-substituted indoles and dihydrosilanes is reported. Compared to known procedures, this method does not require prefunctionalized starting materials and, hence, allows for a rapid access to those siloles. The net reaction is a 2-fold electrophilic C–H silylation catalyzed by cationic Ru–S complexes. Both reaction steps were separately investigated, and these results eventually led to the development of a two-step procedure. By preparing new Ru–S complexes with different weakly coordinating anions (WCAs), it is also shown that the latter can have a dramatic influence on the outcome of these reactions. Furthermore, the substrate scope of the new method is discussed

Enantioselective Diels–Alder Reactions of Cyclohexa-1,3-diene and Chalcones Catalyzed by Intramolecular Silicon–Sulfur Lewis Pairs as Chiral Lewis Acids

The stereoselective preparation of diastereomeric dihydrosilepine-derived silicon cations decorated with another binaphthyl unit at the silicon atom is described. A sulfide donor attached to that additional binaphthyl substituent forms an intramolecular Lewis pair with the electron-deficient silicon atom, as verified by ²⁹Si NMR spectroscopy. Both chiral sulfur-stabilized silicon cations act as catalysts in the difficult Diels–Alder reaction of cyclohexa-1,3-diene and chalcone derivatives. Both Lewis acids induce enantioselectivity, but the <i>S</i>,<i>S</i> relative configuration is superior to the <i>S</i>,<i>R</i> configuration. With the former diastereomer, enantiomeric excesses of close to 60% are obtained. These values are the highest achieved to date in this seemingly trivial cycloaddition

Aerobic Palladium(II)-Catalyzed Dehydrogenation of Cyclohexene-1-carbonyl Indole Amides: An Indole-Directed Aromatization

A palladium­(II)-catalyzed oxidative dehydrogenation of cyclohexene-1-carbonyl indole amides yielding the corresponding benzoylindoles is reported. The new aromatization is also applied to functionalized indoles such as tryptamine and tryptophan. The tethered indole is likely acting as a directing group for allylic C–H bond activation, and there is evidence for a mechanism proceeding through 1,3-diene formation followed by aromatization