Palladium-Catalyzed Asymmetric Silaboration of Allenes

An enantioselective silaboration of allenes was achieved using an achiral silylborane in the presence of a palladium catalyst bearing a chiral monodentate phosphine ligand. (R)-2-Bis(3,5-dimethylphenyl)phosphino-1,1‘-binaphthyl gave the highest enantioselectivities in the addition of (diphenylmethylsilyl)pinacolborane to the internal CC bond of terminal allenes at 0 °C, giving the corresponding β-borylallylsilanes in high yields with high enantiomeric excesses. The enantioselectivity depended on the bulkiness of substituents of allenes:  the enantiomeric excesses were found to be 91−93% ee (R = tert- and sec-alkyl), 88−90% ee (R = aryl), and 80−82% ee (R = prim-alkyl and Me) at 0 °C. Perfect chirality transfer was observed in the intramolecular cyclization reactions of the functionalized allylsilanes, affording highly enantioenriched cyclic alkenylboranes, which underwent Suzuki−Miyaura coupling with aryl halides

Site- and Regioselective Silaborative C–C Cleavage of 1‑Alkyl-2-Methylenecyclopropanes Using a Platinum Catalyst with a Sterically Demanding Silylboronic Ester

1-Alkyl-2-methylenecyclopropanes react with silylboronic esters under mild conditions in the presence of a phosphine-free platinum catalyst, giving 3-substituted 2-boryl-4-silyl-1-butenes through selective cleavage of the less hindered proximal C–C bond of the cyclopropane ring. The steric bulk of the silyl group of the silylboronic esters was critical for efficient formation of the silaboration products, and <i>i</i>-PrPh₂Si–B­(pin) was developed as a silylboronic ester of choice

Kinetic Resolution of Racemic 1-Alkyl-2-methylenecyclopropanes via Palladium-Catalyzed Silaborative C−C Cleavage

Kinetic resolution of racemic 1-alkyl-2-methylenecyclopropanes via silaborative C−C cleavage was efficiently catalyzed by a palladium complex bearing a chiral phosphoramidite, affording 2-boryl-3-silylmethyl-1-alkenes as major products with up to 92% ee. Enantioenrichment through parallel kinetic resolution, where the slower reacting enantiomer was converted to the constitutional isomer of the major product, may be involved as the crucial stereodiscriminating step

Nickel-Catalyzed Ring-Opening Hydroacylation of Methylenecyclopropanes: Synthesis of γ,δ-Unsaturated Ketones from Aldehydes

Nickel-Catalyzed Ring-Opening Hydroacylation of Methylenecyclopropanes: Synthesis of γ,δ-Unsaturated Ketones from Aldehyde

Switch of Regioselectivity in Palladium-Catalyzed Silaboration of Terminal Alkynes by Ligand-Dependent Control of Reductive Elimination

The regioselectivity in the addition of silylboronic esters to terminal alkynes can be switched by the choice of phosphorus ligands on the palladium catalysts. The silaboration proceeds with normal regisoselectivity in the presence of (η3-C3H5)Pd(PPh3)Cl (1.0 mol %) to give 1-boryl-2-silyl-1-alkenes in high yields. In sharp contrast, selective formation of the inverse regioisomers, 2-boryl-1-silyl-1-alkenes, takes place when the reaction is carried out with a palladium catalyst bearing P(t-Bu)2(biphenyl-2-yl). A reaction mechanism for the change of regioselectivity that involves reversible insertion/β-boryl elimination steps is proposed

Palladium-Catalyzed Asymmetric Silaborative C−C Cleavage of <i>meso</i>-Methylenecyclopropanes

An enantioselective silaborative C−C cleavage of meso-methylenecyclopropanes (meso-MCPs) was achieved by using a palladium catalyst bearing a chiral monodentate phosphine ligand. The (R)-2-bis(3,5-dimethylphenyl)phosphino-1,1‘-binaphthyl gave the highest enantioselectivity in the reactions with (methyldiphenylsilyl)pinacolborane at 50 °C, affording derivatives of 2-boryl-4-silyl-1-butene in high yields with high enantiomeric excesses. The reactions of bicyclic MCPs that have fused five- to eight-membered rings gave the corresponding products with 89−91% ee, whereas a little lower ee was observed in the reaction of a non-fused MCP (81% ee). Synthetic applications of the products were demonstrated through synthesis of β-silyl ketones by H2O2 oxidation and synthesis of stereodefined homoallylic alcohols via homologation−allylboration sequence, in which the chiral β-substituent of the allylborane has critical impact on the diastereochemical outcome

Switch of Regioselectivity in Palladium-Catalyzed Silaboration of Terminal Alkynes by Ligand-Dependent Control of Reductive Elimination

The regioselectivity in the addition of silylboronic esters to terminal alkynes can be switched by the choice of phosphorus ligands on the palladium catalysts. The silaboration proceeds with normal regisoselectivity in the presence of (η3-C3H5)Pd(PPh3)Cl (1.0 mol %) to give 1-boryl-2-silyl-1-alkenes in high yields. In sharp contrast, selective formation of the inverse regioisomers, 2-boryl-1-silyl-1-alkenes, takes place when the reaction is carried out with a palladium catalyst bearing P(t-Bu)2(biphenyl-2-yl). A reaction mechanism for the change of regioselectivity that involves reversible insertion/β-boryl elimination steps is proposed

Temperature-Stable Linear Dielectric Response of Low-Temperature Sintered La-Doped Bi₂SiO₅ Ceramics

Doping of a small amount of La3+ into ferroelectric Bi2SiO5 induces the disorder of SiO4 chains in its crystal structure, leading to a transition into a paraelectric phase with a superior temperature stability of the dielectric permittivity. In this study, we attempted to fabricate bulk La-doped Bi2SiO5 ceramics through sintering with the aim of applying Bi2SiO5 to ceramic capacitors. (Bi1–xLax)2SiO5 fine powders with a La content x of up to 0.05 were synthesized by a sol–gel method and then sintered at low temperatures below 740 °C. The undoped Bi2SiO5 decomposed into secondary phases after sintering at temperatures over 640 °C due to its metastable nature, whereas La doping retarded the thermal decomposition, enabling sintering at higher temperatures. As a result, high relative densities near 90% were achieved for (Bi1–xLax)2SiO5 ceramics with x = 0.03 and 0.05 without the formation of secondary phases. The dielectric peak due to the ferroelectric–paraelectric phase transition at around 400 °C disappeared with the increase in La content. The obtained (Bi0.97La0.03)2SiO5 ceramic consequently exhibited a temperature-stable dielectric permittivity over a wide temperature range between −160 and 500 °C. A highly linear large-field dielectric response of the (Bi0.97La0.03)2SiO5 ceramic was observed under varying electric fields up to 220 kV cm–1 (at 20 °C) and at varying temperatures between −60 and 60 °C (at 100 kV cm–1)

Weak Ferroelectricity in <i>n</i> = 2 Pseudo Ruddlesden–Popper-Type Niobate Li₂SrNb₂O₇

Li2SrNb2O7 (LSNO) crystallizes in a structure closely related to n = 2 Ruddlesden–Popper-type compounds, which are generally formed by intergrowth of two-dimensional perovskite-type blocks and rocksalt-type layers. The present study demonstrates the coexistence of spontaneous polarization and an anti-ferroelectric-like nonlinear response in LSNO at 80 K, suggesting weak ferroelectricity below the phase transition temperature of 217 K. A combination of first-principles calculations and single-crystal X-ray diffractions clarifies a polar P21cn structure for the ground state of LSNO, where an in-plane antiferroelectric displacement and an out-of-plane polar shift simultaneously take place. The present study offers a new perspective to design ferroelectric and antiferroelectric materials with Ruddlesden–Popper-type compounds