Copper‐Catalyzed Enantio‐ and Diastereoselective Addition of Silicon Nucleophiles to 3,3‐Disubstituted Cyclopropenes

A highly stereocontrolled syn‐addition of silicon nucleophiles across cyclopropenes with two different geminal substituents at C3 is reported. Diastereomeric ratios are excellent throughout (d.r.≥98:2) and enantiomeric excesses usually higher than 90 %, even reaching 99 %. This copper‐catalyzed C−Si bond formation closes the gap of the direct synthesis of α‐chiral cyclopropylsilanes.TU Berlin, Open-Access-Mittel - 201

Chiral Modification of the Tetrakis(pentafluorophenyl)borate Anion with Myrtanyl Groups

The synthesis and characterization of chiral [B(C6F5)4]– derivatives bearing a myrtanyl group instead of a fluoro substituent in the para position are described. These new chiral borates were isolated as their bench‐stable lithium, sodium, and cesium salts. The corresponding trityl salts were prepared and tested as catalysts in representative counteranion‐directed Diels–Alder reactions and Mukaiyama aldol additions but no enantioselectivity was obtained. Preformation of a chalcone‐derived silylcarboxonium ion with the chiral borate as counteranion did not lead to any asymmetric induction in a reaction with cyclohexa‐1,3‐diene.TU Berlin, Open-Access-Mittel - 201

Lewis acid catalyzed transfer hydromethallylation for the construction of quaternary carbon centers

The design and gram‐scale synthesis of a cyclohexa‐1,4‐diene‐based surrogate of isobutene gas is reported. Using the highly electron‐deficient Lewis acid B(C6F5)3, application of this surrogate in the hydromethallylation of electron‐rich styrene derivatives provided sterically congested quaternary carbon centers. The reaction proceeds by C(sp3)−C(sp3) bond formation at a tertiary carbenium ion that is generated by alkene protonation. The possibility of two concurrent mechanisms is proposed on the basis of mechanistic experiments using a deuterated surrogate.TU Berlin, Open-Access-Mittel - 201

Transferhydrosilylierung

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

Transfer hydrosilylation

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 Si-C 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

Stereospecific and chemoselective copper-catalyzed deaminative silylation of benzylic ammonium triflates

A method for the synthesis of benzylsilanes starting from the corresponding ammonium triflates is reported. Silyl boronic esters are employed as silicon pronucleophiles, and the reaction is catalyzed by copper(I) salts. Enantioenriched benzylic ammonium salts react stereospecifically through an SN2‐type displacement of the ammonium group to afford α‐chiral silanes with inversion of the configuration. A cyclopropyl‐substituted substrate does not undergo ring opening, thus suggesting an ionic reaction mechanism with no benzyl radical intermediate.DFG, 388910461, Ionische und radikalische Kreuzkupplungen zur Kohlenstoff‒Silicium-BindungsknüpfungTU Berlin, Open-Access-Mittel - 201

A unified survey of Si-H and H-H bond activation catalysed by electron-deficient boranes

The bond activation chemistry of B(C6F5)(3) and related electron-deficient boranes is currently experiencing a renaissance due to the fascinating development of frustrated Lewis pairs (FLPs). B(C6F5)(3)'s ability to catalytically activate Si-H bonds through eta(1) coordination opened the door to several unique reduction processes. The ground-breaking finding that the same family of fully or partially fluorinated boron Lewis acids allows for the related H-H bond activation, either alone or as a component of an FLP, brought considerable momentum into the area of transition-metal-free hydrogenation and, likewise, hydrosilylation. This review comprehensively summarises synthetic methods involving borane-catalysed Si-H and H-H bond activation. Systems corresponding to an FLP-type situation are not covered. Aside from the broad manifold of C=X bond reductions and C=X/C-X defunctionalisations, dehydrogenative (oxidative) Si-H couplings are also included.DFG, EXC 314, Unifying Concepts in Catalysi

BArF3-catalyzed imine hydroboration with pinacolborane not requiring the assistance of an additional Lewis base

The rarely used boron Lewis acid tris[3,5- bis(trifluoromethyl)phenyl]borane (BArF 3) is found to be an excellent catalyst for metal-free hydroboration of imines. In the presence of 1.0 mol % of BArF 3, several ketimines and aldimines undergo hydroboration with pinacolborane (HBpin) at room temperature without the aid of an external Lewis base. BArF 3 is more reactive than other Lewis-acidic boranes, including often-used tris(pentafluorophenyl)borane [B(C6F5)3]. The steric hindrance imparted by the six fluorine atoms ortho to the boron center in B(C6F5)3 accounts for this. Mechanistic control experiments indicate conventional Lewis-acid catalysis involving imine activation and hydride transfer from HBpin

Catalytic difunctionalization of unactivated alkenes with unreactive hexamethyldisilane through regeneration of silylium ions

A metal‐free, intermolecular syn‐addition of hexamethyldisilane across simple alkenes is reported. The catalytic cycle is initiated and propagated by the transfer of a methyl group from the disilane to a silylium‐ion‐like intermediate, corresponding to the (re)generation of the silylium‐ion catalyst. The key feature of the reaction sequence is the cleavage of the Si−Si bond in a 1,3‐silyl shift from silicon to carbon. A central intermediate of the catalysis was structurally characterized by X‐ray diffraction, and the computed reaction mechanism is fully consistent with the experimental findings.TU Berlin, Open-Access-Mittel - 201

Synthesis of Silylated Cyclobutanone and Cyclobutene Derivatives Involving 1,4‐Addition of Zinc‐Based Silicon Nucleophiles

A copper‐catalyzed conjugate silylation of various cyclobutenone derivatives with Me2PhSiZnCl ⋅ 2LiCl or (Me2PhSi)2Zn ⋅ xLiCl (x≤4) to generate β‐silylated cyclobutanones is reported. Trapping the intermediate enolate with ClP(O)(OPh)2 affords silylated enol phosphates that can be further engaged in Kumada cross‐coupling reactions to yield silylated cyclobutene derivatives