Catalytic Enantioselective Synthesis of Alkenylhydrosilanes

Enantioselective Si-H Insertion Reactions of Diarylcarbenes for the Synthesis of Silicon-Stereogenic Silanes

We report the first example of enantioselective, intermolecular diarylcarbene insertion into Si-H bonds for the synthesis of silicon-stereogenic silanes. Dirhodium(II) carboxylates catalyze an Si-H insertion using carbenes derived from diazo compounds where selective formation of an enantioenriched silicon center is achieved using prochiral silanes. Fourteen prochiral silanes were evaluated with symmetrical and prochiral diazo reactants to produce a total of 25 novel silanes. Adding an ortho substituent on one phenyl ring of a prochiral diazo enhances enantioselectivity up to 95:5 er with yields up to 98%. Using in situ IR spectroscopy, the impact of the off-cycle azine formation is supported based on the structural dependence for relative rates of diazo decomposition. A catalytic cycle is proposed with Si-H insertion as the rate-determining step, supported by kinetic isotope experiments. Transformations of an enantioenriched silane derived from this method, including selective synthesis of a novel sila-indane, are demonstrated.

Characterization of hydrogen-substituted silylium ions in the condensed phase

Hydrogen-substituted silylium ions are long-sought reactive species. We report a protolysis strategy that chemoselectively cleaves either an Si-C(sp²) or an Si-H bond using a carborane acid to access the full series of [CHB₁₁H₅Br₆]^--stabilized R₂SiH⁺, RSiH₂ ⁺, and SiH₃ ⁺ cations, where bulky tert-butyl groups at the silicon atom (R = tBu) were crucial to avoid substituent redistribution. The crystallographically characterized molecular structures of [CHB₁₁H₅Br₆]^--stabilized tBu₂HSi⁺ and tBuH₂Si⁺ feature pyramidalization at the silicon atom, in accordance with that of tBu₃Si⁺[CHB₁₁H₅Br₆]^- Conversely, the silicon atom in the H₃Si⁺ cation adopts a trigonal-planar structure and is stabilized by two counteranions. This solid-state structure resembles that of the corresponding Brønsted acid.

Ozone Oxidation of Silylalkene: Mechanistic Study and Application for the Synthesis of Silacarboxylic Acid Derivatives

The addition-type ozone oxidation of silylalkenes is a highly efficient reaction to provide synthetically versatile α-silylperoxy carbonyl compounds. To gain insight into the reaction mechanism, we performed a computational study, which revealed that the reaction proceeds via [1,2]-Brook rearrangement-type silyl migration of primary ozonide. In sharp contrast to the addition-type reactions, the ozone oxidation of α-alkoxysilylalkenes proceeds in a cleavage-type manner to afford excellent yields of silacarboxylic acid esters via the 1,3-cycloelimination of primary ozonide prior to 1,2-silyl migration.

Asymmetric retro-[1,4]-Brook rearrangement of 3-silyl allyloxysilanes via chirality transfer from silicon to carbon

An asymmetric retro-[1,4]-Brook rearrangement of 3-silyl allyloxysilanes has been developed via Si-to-C chirality transfer. Mechanistic studies reveal that the silyl group migrates with retention of configuration. The stereochemical outcome of the newly formed stereogenic carbon center, which has remained a longstanding question, is also clarified, suggesting a diastereoselective Si to C chirality transfer without loss of enantiomeric excess.

Rhodium-Catalyzed Reaction of Silacyclobutanes with Unactivated Alkynes to Afford Silacyclohexenes

A Rh-catalyzed reaction of silacyclobutanes (SCBs) with unactivated alkynes has been developed to form silacyclohexenes with high chemoselectivity. Good enantioselectivity at the stereogenic silicon center was achieved using a chiral phosphoramidite ligand. The resulting silacyclohexenes are useful scaffolds for synthesizing structurally attractive silacyclic compounds.

Functionalized vinylsilanes via highly efficient and recyclable Pt-nanoparticle catalysed hydrosilylation of alkynes

A mild, selective and facile synthesis of vinylsilanes via a recyclable platinum nanoparticle catalysed hydrosilylation of alkynes is reported. Various functionalized alkynes are selectively hydrosilylated to furnish functional β-E vinylsilanes in high yields. The catalytic effectiveness, ease of catalyst recovery and recyclability of the polysiloxane stabilized Pt-nanoparticle catalyst are the major achievements of this work. Detailed in situ characterization using Electron Microscopy and controlled poisoning experiments supports the participation of Pt-nanoparticles as active catalysts.

Pincer cobalt complex-catalyzed Z-selective hydrosilylation of terminal alkynes

Co-Catalyzed Z-selective hydrosilylation of terminal alkynes produces (Z)-β-vinylsilanes, which are further converted to (Z)-disubstituted alkenes by Pd-catalyzed cross couplings.

Renovation of Optically Active Phenanthrolines as Powerful Chiral Ligands for Versatile Asymmetric Metal Catalysis

In the field of asymmetric synthesis, the development of new chiral ligands has been regarded as an attractive challenge for decades. Novel chiral ligands can often have a great impact on synthetic protocols. In this context, we are currently interested in the application of 1,10-phenanthroline (phen) as an entirely new class of chiral ligand. To handle this issue, we designed a chiral phen ligand that provides the N,N,O-tridentate coordination of the phen moiety and an additional phenolic hydroxyl group. As phen possesses greater coordination ability with various ions, our chiral phen ligand would be valuable as one of the "privileged" chiral ligands applied to a broad range of metal catalysts and new reactions. This account summarizes the results of the application of the chiral phen ligand to various kinds of metal catalysis.

Enantioselective Synthesis of Silacyclopentanes

A variety of functionalized silacyclopentanes were synthesized by highly enantioselective β-eliminations of silacyclopentene oxides followed by stereospecific transformations. The reaction mechanism of the β-elimination was elucidated by DFT calculations. An in vitro biological assay with an oxy-functionalized silacyclopentane showed substantial binding to a serotonin receptor protein.

Palladium-catalyzed Si–C bond-forming silylation of aryl iodides with hydrosilanes: an enhanced enantioselective synthesis of silicon-stereogenic silanes by desymmetrization

An enantioselective Pd-catalyzed silicon–carbon bond-forming silylation reaction of aryl iodides with hydrosilanes for the synthesis of silicon-stereogenic silanes has been developed with good enantioselectivity under mild conditions.

Rhodium-catalyzed asymmetric synthesis of silicon-stereogenic silicon-bridged arylpyridinones

A rhodium-catalyzed regio- and enantioselective synthesis of silicon-stereogenic silicon-bridged arylpyridinones was developed. A reasonable catalytic cycle was also experimentally established.

A rhodium-catalyzed regio- and enantioselective synthesis of silicon-stereogenic silicon-bridged arylpyridinones has been developed through [2 + 2 + 2] cycloaddition of silicon-containing prochiral triynes with isocyanates. High yields and enantioselectivities have been achieved by employing an axially chiral monophosphine ligand, and this process could be applied to catalytic asymmetric synthesis of silicon-stereogenic chiral polymers for the first time. The reaction mechanism of the present catalysis has also been experimentally investigated to establish a reasonable catalytic cycle, advancing the mechanistic understanding of the rhodium-catalyzed pyridinone synthesis by [2 + 2 + 2] cycloaddition reactions.

Rhodium-catalyzed intramolecular alkynylsilylation of alkynes

Rhodium-catalyzed intramolecular alkynylsilylation of alkynes is described, including its enantioselective variant for the creation of a silicon stereogenic center.

Novel phosphorus–nitrogen–silicon flame retardants and their application in cycloaliphatic epoxy systems

In this work, we prepared two novel reactive-type halogen-free and UV-curable phosphorus–nitrogen–silicon synergistic flame retardants.

From an α-functionalized silicon-stereogenic N,O-silane to a monomeric and tetracoordinate tBuLi adduct with lithium-centered chirality

Donor-functionalized silanes with stereogenic silicon centers are extremely rare. A convenient stereocontrolled route to a nitrogen-oxygen-functionalized silicon-chiral compound with an additional aminomethyl function is presented. This silane was directly achieved in stereochemically pure form by a simple nucleophilic substitution reaction. Owing to the unique asymmetry of this silane and the presence of three donor functions, the first monomeric butyllithium compound with lithium-centered chirality could be isolated; the configuration was assigned by X-ray crystallography. This [silane⋅ tBuLi] complex undergoes an unexpected deprotonation/stereospecific substitution sequence in toluene, leading to the development of a convenient one-pot synthesis of a functionalized silicon-chiral benzylsilane, which proceeds with inversion of configuration and complete preservation of the stereochemical integrity at silicon.