Iridium-catalyzed (Z)-trialkylsilylation of terminal olefins

Triphasic Hydroxysilylation of Alkenes by Mechanically Piezoelectric Catalysis

The 1,2-hydroxysilylation of alkenes is crucial for synthesizing organosilicon compounds which are key intermediates in material science, pharmaceuticals, and organic synthesis. The development of strategies employing hydrogen atom transfer pathways is currently hindered by the existence of various competing reactions. Herein, we reported a novel mechanochemical strategy for the triphasic 1,2-hydroxysilylation of alkenes through a single-electron-transfer pathway. Our approach not only circumvents competitive reactions to enable the first-ever 1,2-hydroxysilylation of unactivated alkenes but also pioneers the research in mechanic force-induced triphasic reactions under ambient conditions. This gentle method offers excellent compatibility with various functional groups, operates under simple and solvent-free conditions, ensures rapid reaction time. Preliminary mechanistic investigations suggest that silylboronate can be transformed to a silicon radical by highly polarized Li2TiO3 particles and oxygen under ball-milling condition.

Regio- and Stereoselective Thianthrenation of Olefins To Access Versatile Alkenyl Electrophiles

Herein, we report a regioselective alkenyl electrophile synthesis from unactivated olefins that is based on a direct and regioselective C-H thianthrenation reaction. The selectivity is proposed to arise from an unusual inverse-electron-demand hetero-Diels-Alder reaction. The alkenyl sulfonium salts can serve as electrophiles in palladium- and ruthenium-catalyzed cross-coupling reactions to make alkenyl C-C, C-Cl, C-Br, and C-SCF3 bonds with stereoretention.

Three-component reaction to synthesize E-vinyl silyl anti-1,2-diols via sequential [1,4]-O-to-O/[1,4]-C-to-O silyl migrations

A three-component reaction of geminal bis(silyl) allyl silyl ether, aldehyde and electrophile has been developed to synthesize trisubstituted E-vinyl silyl anti-1,2-diols. The approach features a sequential [1,4]-O-to-O/[1,4]-C-to-O silyl migrations process.

Stereoselective Synthesis of Z-Vinylsilanes via Palladium-Catalyzed Direct Intermolecular Silylation of C(sp2)-H Bonds

An efficient and convenient palladium-catalyzed direct intermolecular silylation of C(sp²)-H bonds by using disilanes as the silicon source with the assistance of a readily removable bidentate directing group is reported. This strategy provided a regio- and stereoselective protocol for exclusive synthesis of Z-vinylsilanes with reasonable to excellent yields and good functional group compatibility. Silylation of the isolated palladacycle intermediate revealed the Z-stereoselective pathway. Moreover, the practicality and effectiveness of this method were illustrated by a gram-scale experiment and further functionalization of the silylation product.

Copper-Catalyzed Oxidative ipso-Annulation of Activated Alkynes with Silanes: An Approach to 3-Silyl Azaspiro[4,5]trienones

A novel strategy of silylation and dearomatization of activated alkynes with silanes to synthesize azaspiro[4,5]trienones is developed, which could be facilely achieved through a tandem difunctionalization of alkyne, dearomatization, and oxidation and provided a facile approach to produce useful 3-silyl azaspiro[4,5]trienones in an efficient manner.

Regio- and Stereoselective Dehydrogenative Silylation and Hydrosilylation of Vinylarenes Catalyzed by Ruthenium Alkylidenes

Development of regio- and stereoselective dehydrogenative silylation and hydrosilylation of vinylarenes with alkoxysilanes, catalyzed by ruthenium alkylidenes, is described. Varying L- and X-type ligands on ruthenium alkylidenes permits selective access to either (E)-vinylsilanes or β-alkylsilanes with high regio- and stereocontrol. cis,cis-1,5-Cyclooctadiene was identified as the most effective sacrificial hydrogen acceptor for the dehydrogenative silylation of vinylarenes, which allows use of a nearly equimolar ratio of alkenes and silanes.

Ruthenium-Catalyzed Intermolecular Direct Silylation of Unreactive C(sp(3))-H Bonds

A Ru-catalyzed intermolecular silylation of unreactive, aliphatic C(sp(3))-H bonds has been described for the first time. This protocol features low catalyst loading, a relatively broad substrate spectrum, good functional group tolerance, and no sensitivity to air, which provides a convenient and practical pathway for the construction of C-Si bonds.

Modular Approach to Reductive C(sp2)-H and C(sp3)-H Silylation of Carboxylic Acid Derivatives through Single-Pot, Sequential Transition Metal Catalysis

We report a modular approach to catalytic reductive Csp2-H and Csp3-H silylation of carboxylic acid derivatives encompassing esters, ketones, and aldehydes. Choice of either an Ir(I)/Rh(I) or Rh(I)/Rh(I) sequence leads to either exhaustive reductive ester or reductive ketone/aldehyde silylation, respectively. Notably, a catalyst-controlled direct formation of doubly reduced silyl ethers is presented, specifically via Ir-catalyzed exhaustive hydrosilylation. The resulting silyl ethers undergo Csp2-H and benzylic Csp3-H silylation in a single vessel.

Transition metal-catalyzed C–H bond functionalizations by the use of diverse directing groups

In this review, a summary of transition metal-catalyzed C–H activation by utilizing the functionalities as directing groups is presented.

Highly efficient triphenyl(3-sulfopropyl)phosphonium functionalized phosphotungstic acid on silica as a solid acid catalyst for selective mono-allylation of acetals

A silica-supported zwitterion-functionalized phosphotungstic acid catalyst developed for the selective Hosomi–Sakurai reaction giving 93–99% yield of homoallyl ethers.

Enantioselective synthesis of α-aminosilanes by copper-catalyzed hydroamination of vinylsilanes

The synthesis of α-aminosilanes by a highly enantio- and regioselective copper-catalyzed hydroamination of vinylsilanes is reported. The system employs Cu-DTBM-SEGPHOS as the catalyst, diethoxymethylsilane as the stoichiometric reductant, and O-benzoylhydroxylamines as the electrophilic nitrogen source. This hydroamination reaction is compatible with differentially substituted vinylsilanes, thus providing access to amino acid mimics and other valuable chiral organosilicon compounds.

Rational design of a second generation catalyst for preparation of allylsilanes using the silyl-Heck reaction

Using rational ligand design, we have developed of a second-generation ligand, bis(3,5-di-tert-butylphenyl)(tert-butyl)phosphine, for the preparation of allylsilanes using the palladium-catalyzed silyl-Heck reaction. This new ligand provides nearly complete suppression of starting material alkene isomerization that was observed with our first-generation catalyst, providing vastly improved yields of allylsilanes from simple alkene starting materials. The studies quantifying the electronic and steric properties of the new ligand are described. Finally, we report an X-ray crystal structure of a palladium complex resulting from the oxidative addition of Me₃SiI using an analogous ligand that provides significant insight into the nature of the catalytic system.

Silyl-Heck Reactions for the Preparation of Unsaturated Organosilanes

Few methods exist to directly install silyl functionality onto olefins. This Synpacts highlights the state of the art of the silyl-Heck reaction and our recently developed conditions for preparing allyl and vinyl silanes from terminal alkenes using this method.

Hemilabile N-xylyl-N'-methylperimidine carbene iridium complexes as catalysts for C-H activation and dehydrogenative silylation: dual role of N-xylyl moiety for ortho-C-H bond activation and reductive bond cleavage

Direct dehydrogenative silylation of pyridyl and iminyl substrates with triethylsilane was achieved using (L)Ir(cod)(X) (1) (L = a perimidine-based carbene ligand, X = OAc and OCOPh) complexes as catalysts under toluene refluxing conditions in the presence of norbornene as a hydrogen scavenger, and the silylated products were obtained in good yields. The isolated bis(cyclometalated)iridium complexes, (C(∧)C:)(C(∧)N)IrOAc (2) (C(∧)C: = a cyclometalated perimidine-carbene ligand and C(∧)N = a cyclometalated pyridyl- and iminyl-ligated aromatic substrate), were key intermediates, where cyclometalated five-membered metallacycles of substrates such as phenylpyridine were selectively formed before yielding mono-ortho-silylation products. The bis(cyclometalated)iridium complex ((Xy)C(∧)C:)(C(∧)N)IrOAc (2d) ((Xy)C(∧)C: = a cyclometalated N-xylyl-N'-methylperimidine-carbene ligand and C(∧)N = a 2-pyridylphenyl ligand), reacted with 2 equiv of Et3SiH to give an iridium hydride complex, (L(4))(C(∧)N)Ir(H)(SiEt3) (8d) (L(4) = N-CH3, N-3,5-(CH3)2C6H3 perimidine), via demetalation of a N-3,5-xylyl ring of the carbene ligand of 2d. The formation of 8d was confirmed by isolating the corresponding chloro complex (L(4))(C(∧)N)Ir(Cl)(SiEt3) (8d-Cl) by treatment with CCl4. The N-methyl moiety of the carbene ligand coordinated to 8d was cyclometalated in the presence of norbornene at room temperature to afford ((Me)C(∧)C:)(C(∧)N)Ir(SiEt3) (10d) ((Me)C(∧)C: = a cyclometalated N-xylyl-N'-methylperimidine-carbene), while at high temperature 8d reacted with norbornene and Et3SiH to afford the silylated product, 2-(2-triethylsilyl)phenylpyridine (3a) and norbornane. A deuterium labeling experiment using 2d and Et3SiD (excess) revealed the incorporation of deuterium atoms at two ortho-positions of the N-xylyl group (>90%) and at the 3-position of 2-pyridylphenyl ligand (ca. 40%) within 3 h at room temperature, indicating that the cyclometalation/demetalation of the N-xylylperimidine carbene and 2-phenylpyridine ligands were reversible processes. Isolation of these cyclometalated iridium complexes under controlled conditions and D-labeling experiments thus revealed a dual function of the N-aryl group bound to the perimidine-carbene ligand, which acted as both a neutral carbene ligand and a monoanionic ortho-metalated aryl-carbene ligand through reversible C-H bond activation and Ir-C bond cleavage of the N-aryl group during the catalytic cycle.

Activation of Si-Si Bonds for Copper(I)-Catalyzed Conjugate Silylation

Several alkyl- and vinylsilanes were prepared through the copper(I)-catalyzed conjugate silylation of α,β-unsaturated compounds. Optimal reaction conditions were first investigated to realize the conjugate addition of a nucleophilic silicon species to poorly electrophilic acceptors such as phenylvinyl sulfone by cleavage of the Si-Si bond of a disilane reagent. The scope of this reaction was extended to various electrophiles bearing different electron-withdrawing groups and afforded the desired substituted alkyl- and vinylsilanes. Among the wide range of commercially available disilanes, the reactivities of alkyl-, aryl-, and ethoxydisilane were also examined.

Palladium-catalyzed oxidative allylic C-H silylation

Palladium-catalyzed allylic C-H silylation was performed with use of hexamethyldisilane as the silyl source. These C-H functionalization reactions occur only in the presence of hypervalent iodine reagents or other strong oxidants and proceed with excellent regioselectivity, providing the linear allylic isomer of the allylsilane products. In demonstrating the first oxidative allylic C-H silylation of alkenes, this study marks an important advance for the catalytic C-H functionalization method.

Iridium-catalyzed arene ortho-silylation by formal hydroxyl-directed C-H activation

A strategy for the ortho-silylation of aryl ketone, benzaldehyde, and benzyl alcohol derivatives has been developed in which a hydroxyl group formally serves as the directing element for Ir-catalyzed arene C-H bond activation. One-pot generation of a (hydrido)silyl ether from the carbonyl compound or alcohol is followed by dehydrogenative cyclization at 80-100 °C in the presence of norbornene as a hydrogen acceptor and the combination of 1 mol % [Ir(cod)OMe]2 and 1,10-phenanthroline as a catalyst to form benzoxasiloles. The synthetic utility of the benzoxasilole products is demonstrated by conversion to phenol or biaryl derivatives by Tamao-Fleming oxidation or Hiyama cross-coupling. Both of these transformations of the C-H silylation products exploit the Si-O bond in the system and proceed by activation of the silyl moiety with hydroxide, rather than fluoride.