Palladium-Catalyzed Transient Chirality Transfer and Atroposelective C-H Functionalization to Access Quaternary Stereocenters.

Although palladium-catalyzed asymmetric C-H functionalization and Heck reactions represents one of the most important synthetic strategies for the construction of quaternary stereocenters, developing the enantioselective version of Pd -catalyzed carbopalladation-initiated cascade reactions still remains a formidable challenge. Herein, an unprecedent enantioselective [3+2] annulation of oxime ethers and alkynes has been developed, providing both spiro and nonspiro indenes bearing all-carbon quaternary stereocenters in good yields (up to 98 %) with excellent enantioselectivities (up to >99 % ee). This annulation is accomplished by merging the Pd -catalyzed atroposelective C-H activation/double carbopalladation and the transient axial-to-central chirality transfer process, constituting the first successful example of catalytic chirality transfer strategy involving axially chiral styrene intermediate.

3-Alkyl-2-pyridyl Directing Group-Enabled C2 Selective C-H Silylation of Indoles and Pyrroles via an Iridium Catalyst.

An iridium-catalyzed, directing group-enabled site selective intra- and intermolecular silylation of indoles and pyrroles with hydrosilanes has been developed under ligand-free conditions. Fine-tuning of the removable 3-alkyl-2-pyridyl directing group was found to be crucial for achieving high yields for C2-silylated indole and pyrrole products. Moreover, the scalability was demonstrated, and further transformations of the silylation products were achieved.

Atropisomers with Axial and Point Chirality: Synthesis and Applications.

Enantiopure atropisomers have become increasingly important in asymmetric synthesis and catalysis, pharmaceutical science, and material science since the discovery of inherent features of axial chirality originating from rotational restriction. Despite the advances made in this field to date, it remains highly desirable to construct structurally diverse atropisomers with potentially useful functions. We propose superposition to match axial and point chirality as a potentially useful strategy to access structurally complex and diverse building blocks for organic synthesis and pharmaceutical science because merging atropisomeric backbones with one or more extra chiral elements can topologically broaden three-dimensional environments to create complex scaffolds with multiple tunable parameters.

Combined Dynamic Kinetic Resolution and C-H Functionalization for Facile Synthesis of Non-Biaryl-Atropisomer-Type Axially Chiral Organosilanes.

Although asymmetric C-H functionalization has been available for the synthesis of structurally diverse molecules, catalytic dynamic kinetic resolution (DKR) approaches to change racemic stereogenic axes remain synthetic challenges in this field. Here, a concise palladium-catalyzed DKR was combined with C-H functionalization involving olefination and alkynylation for the highly efficient synthesis of non-biaryl-atropisomer-type (NBA) axially chiral oragnosilanes. The chemistry proceeded through two different and distinct DKR: first, an atroposelective C-H olefination or alkynylation produced axially chiral vinylsilanes or alkynylsilanes as a new family of non-biaryl atropisomers (NBA), and second, the extension of this DKR strategy to twofold o,o'-C-H functionalization led to the multifunctional axially chiral organosilicon compounds with up to >99 % ee.

Catalytic asymmetric cycloaddition of unsymmetrical EWG-activated alkenes to fully substituted pyrrolidines bearing three different carbonyl groups.

A unique 1,3-dipolar [3+2] cycloaddition of alkyl 4-oxo-4-arylbut-2-enoates bearing two different electron-withdrawing groups was completed by using the silver/(R)-DTBM-Segphos catalyst system, which gives the corresponding fully substituted pyrrolidines with four stereogenic centers in good yields and with excellent enantioselectivities (up to 98% ee).

PdCl(CHCN)-catalyzed regioselective C-H olefinations of 2-amino biaryls with vinylsilanes as unactivated alkenes.

The first example of palladium-catalyzed chelation-assisted C-H olefination of 2-amino biaryls using readily available vinylsilanes as unactivated olefinating reagents has been developed, affording valuable arylated vinylsilane compounds in moderate to excellent yields and with exclusive (E)-selectivities.

Desymmetrization-Oriented Enantioselective Synthesis of Silicon-Stereogenic Silanes by Palladium-Catalyzed C-H Olefinations.

A palladium-catalyzed chelation-assisted enantioselective C-H olefination of symmetrically diaryl-substituted tetraorganosilicon derivatives was developed, enabling the generation of nitrogen-containing silicon-stereogenic tetraorganosilicon compounds with modest to good yields and good to excellent enantioselectivities (up to 95.5:4.5 e.

Enantioselective synthesis of axially chiral vinyl arenes through palladium-catalyzed C-H olefination.

A palladium-catalyzed ketoxime-chelation-assisted enantioselective C-H olefination of 2-arylcyclohex-2-enone oxime ether with a wide range of olefins as coupling partners was developed. Employing ketoxime ether as an efficient directing group, a variety of axially chiral vinyl arenes was synthesized by Pd(ii)-catalyzed C-H functionalization with excellent enantioselectivities (96 → 99% ee) under mild conditions.

Ligand-Controlled Inversion of Diastereo- and Enantioselectivity in Silver-Catalyzed Azomethine Ylide-Imine Cycloaddition of Glycine Aldimino Esters with Imines.

A highly diastereo- and enantioselective silver-catalyzed azomethine ylide-imine (AYI) cycloaddition reaction of glycine aldimino esters with imines was developed in which the Xing-Phos-controlled syn-selective or DTBM-Segphos-induced anti-selective AYI cycloaddition reaction could be applied to the synthesis of a variety of stereodivergent 1-alkyl-2,5-substituted imidazolidines with high yields and excellent enantioselectivities (up to 99% ee) as well as good diastereoselectivities (up to 99:1 dr) under mild reaction conditions.

Asymmetric Synthesis of Glutamic Acid Derivatives by Silver-Catalyzed Conjugate Addition-Elimination Reactions.

The enantioselective construction of a family of chiral glycine-derived aldimino esters is described. The asymmetric tandem conjugate addition-elimination procedure is characterized by its exceptional mild reaction conditions and features with an exquisite enantioselectivity profile using commercially available silver/DTBM-SegPhos catalyst, allowing for the facile preparation of a variety of substituted and chiral glutamic acid derivatives (up to 99% ee) bearing Schiff base in a straightforward manner.

Cs₂CO₃-Initiated Trifluoro-Methylation of Chalcones and Ketones for Practical Synthesis of Trifluoromethylated Tertiary Silyl Ethers.

It was found that 1,2-trifluoromethylation reactions of ketones, enones, and aldehydes were easily accomplished using the Prakash reagent in the presence of catalytic amounts of cesium carbonate, which represents an experimentally convenient, atom-economic process for this anionic trifluoromethylation of non-enolisable aldehydes and ketones.

Lewis-Base-Mediated Diastereoselective Silylations of Alcohols: Synthesis of Silicon-Stereogenic Dialkoxysilanes Controlled by Chiral Aryl BINMOLs.

In the past years, stereoselective functionalizations of hydroxyl groups of alcohol substrates with chlorosilanes leading to silyl ether formation have evolved from a functional-group protection to an enantioselective synthetic strategy. This work comprises a controlled desymmetrization of dichlorosilanes by using a family of structurally specific chiral diols, chiral 1,1'-binaphthalene-2-α-arylmethanol-2'-ol (Ar-BINMOL). This process led to the facile construction of silicon-stereogenic organosilicon compounds with high yields and good diastereoselectivities.

Mechanistic Insights into Palladium-Catalyzed Silylation of Aryl Iodides with Hydrosilanes through a DFT Study.

The catalytic cycles of palladium-catalyzed silylation of aryl iodides, which are initiated by oxidative addition of hydrosilane or aryl iodide through three different mechanisms characterized by intermediates R Si-Pd -H (Cycle A), Ar-Pd -I (Cycle B), and Pd (Cycle C), have been explored in detail by hybrid DFT. Calculations suggest that the chemical selectivity and reactivity of the reaction depend on the ligation state of the catalyst and specific reaction conditions, including feeding order of substrates and the presence of base. For less bulky biligated catalyst, Cycle C is energetically favored over Cycle A, through which the silylation process is slightly favored over the reduction process.

Transition-Metal-Free Oxidative C(sp )-H Hydroxylation of Terpyridines: A HOMO-Raising Strategy for the Construction of a New Super-Stable Terpyridine Chromophores.

Direct functionalization of terpyridines is an increasingly important topic in the field of dyes and catalysis as well as supramolecular chemistry, but its synthesis and transformation is usually challenging. Herein, a HOMO-raising strategy is reported for the construction of a super-stable novel terpyridine chromophores, in which the selective oxidation of terpyridines at its 3-position was determined successfully to the synthesis of phenol-functionalization of terpyridines (TPyOHs) bearing a hydrogen bonding group. The corresponding TPyOHs displayed strong aggregation-induced emission and exhibited highly selective and visual detection of Zn cation with a record green terpyridine-based luminophore with nanomolar sensitivity (125 nm).

Palladium-Catalyzed Desymmetrization of Silacyclobutanes with Alkynes to Silicon-Stereogenic Silanes: A Density Functional Theory Study.

The palladium-catalyzed desymmetrization of silacyclobutanes using electron-deficient alkynes proceeds with high enantioselectivity in the presence of a chiral P ligand; this provides a facile approach for the synthesis of novel silicon-stereogenic silanes. In this work, we used hybrid density functional theory (DFT) to elucidate the mechanism of the palladium-catalyzed desymmetrization of silacyclobutanes with dimethyl acetylenedicarboxylate. Full catalytic cycle including two different initiation modes that were proposed to be a possible initial step to the formation of the 1-pallada-2-silacyclopentane/alkyne intermediate-the oxidative addition of the palladium complex to the silacyclobutane Si-C bond (cycle MA) or coordination of the Pd complex with the alkyne C≡C bond (cycle MB)-have been studied.

An efficient approach toward formation of polycyclic coumarin derivatives via carbocation-initiated [4+2] cycloaddition and atom-economical photo-irradiated cyclization.

Facile carbocation-initiated [4+2] cycloaddition of o-anisole-substituted propargyl silyl ethers and ynamides led to the formation of 4-vinylcoumarins. Subsequent intramolecular cyclization of 3-aryl-4-vinylcoumarins afforded polycyclic coumarin derivatives 11,12-dihydronaphtho[1,2-c]chromen-5-ones in excellent yields under mild photo-irradiated conditions established by fluorescence analysis-oriented screening.

Catalytic asymmetric bromochlorination of aromatic allylic alcohols promoted by multifunctional Schiff base ligands.

It was found that the tridentate O,N,O-type Schiff base ligand bearing suitable substituents was a highly effective promoter in the catalytic asymmetric bromochlorination reaction, in which the corresponding aromatic bromochloroalcohols with vicinal halogen-bearing stereocenters were formed with perfect regioselectivity, with moderate to excellent enantioselectivities (up to 93% ee), and with good yields and chemoselectivities.

Lewis acid catalyzed [2+2] cycloaddition of ynamides and propargyl silyl ethers: synthesis of alkylidenecyclobutenones and their reactivity in ring-opening and ring expansion.

A family of four-membered enones, polysubstituted alkylidenecyclobutenones, were easily prepared by a Lewis acid catalyzed [2+2] cycloaddition of ynamides and propargyl silyl ethers. This challenging regioselective [2+2] cycloaddition enables the efficient construction and conversion of four-membered enones, which provides high-value and structurally diverse products through the unexpected ring-opening and ring expansion of alkylidenecyclobutenone with Grignard reagents, organolithium, primary amines, and water.

Asymmetric Michael Addition of Aldimino Esters with Chalcones Catalyzed by Silver/Xing-Phos: Mechanism-Oriented Divergent Synthesis of Chiral Pyrrolines.

The mechanism-oriented reaction design for the divergent synthesis of chiral molecules from simple starting materials is highly desirable. In this work, aromatic amide-derived nonbiarylatropisomer/silver (silver/Xing-Phos) complex was used to catalyze the Michael addition of glycine aldimino esters to chalcones and successfully applied to the subsequent cyclocondensation to afford substituted cis-Δ(1)-pyrroline derivatives with up to 98 % ee. Besides the inherent performance of the chiral Ag/Xing-Phos catalyst system, it was found that the workup of such reactions played an important role for the stereoselective construction of stereodivergent Δ(1)-pyrrolines, in which an epimerization of the cis-Δ(1)-pyrrolines to the trans-isomers during was revealed.

[Spectroscopic analysis of Er3+ in Er3+ /Yb3+ co-doped LiNbO3 crystal].

At room temperature, alpha-polarized absorption spectra of as-grown and annealed Er3+/Yb3+ co-doped, Z-cut LiNbO3 single crystals, grown by using Czochralski method, were measured in the wavelength range of 300-1 650 nm. The spectroscopic properties of Er3+ were analysed by using Judd-Ofelt theory. The experimental values of the electron transition strengths of several major transitions from the ground state to excited-state manifolds were evaluated from the measured integrated absorption coefficients of Er3+.