One-Pot Assembly of 2-Trifluoromethyl Benzothiazole and Benzoselenazole via Copper-Mediated Three-Component Cascade Reaction.

A domino one-pot synthesis of 2-(trifluoromethyl) benzothiazole via copper-mediated three-component cascade reaction starting from the easily accessible starting materials such as o-iodoanilines, methyl trifluoropyruvate, and elemental sulfur is reported. The present strategy displayed a comprehensive substrate scope and good functional group tolerance and enabled access to a variety of substituted 2-(trifluoromethyl) benzothiazoles. A 2-(trifluoromethyl) benzoselenazole has also been synthesized utilizing this reaction methodology.

Organocatalytic Enantioselective Nucleophilic Addition of Indole Imine 5-Methides.

Despite the enormous developments in the asymmetric transformations of indole imine methides (IIMs), the remote asymmetric induction involving IIMs remains challenging due to the spatial interaction requirement between the substrate and catalyst. Herein we report the first catalytic asymmetric nucleophilic addition to indole imine 5-methide (5-IIM), the only topological isomer of IIMs whose asymmetric addition remains unknown. Despite the challenging remote stereocontrol, high efficiency and respectable enantioselectivity were achieved to provide access to a range of enantioenriched indole-containing triaryl alkanes.

Conjugation-Induced Spin Delocalization in Helical Chiral Carbon Radicals via Through-Bond and Through-Space Effects.

A class of highly stable hydrocarbon radicals with helical chirality are synthesized, which can be isolated and purified by routine column chromatography on silica gel. These carbon-centered radicals are stabilized by through-bond delocalization and intramolecular through-space conjugation, which is evidenced by Density Functional Theory (DFT) calculation. The high stability enables to directly modify the carbon radical via palladium-catalyzed cross-coupling with the radical being untapped. The structures and optoelectronic properties are investigated with a variety of experimental methods, including Electron Paramagnetic Resonance (EPR), Ultraviolet Visisble Near Infrared (UV-vis-NIR) measurements, Cyclic Voltammetry (CV), Thermogravimetry Analysis (TGA), Circular Dichroism (CD) spectra, High-Performance Liquid Chromatography (HPLC), and X-ray crystallographic analysis. DFT calculations indicated that the 9-anthryl helical radical is more stable than its tail-to-tail σ-dimer over 13.2 kJ mol-1 , which is consistent with experimental observations.

A Chiral Relay Race: Stereoselective Synthesis of Axially Chiral Biaryl Diketones through Ring-Opening of Optical Dihydrophenan-threne-9,10-diols.

We report herein a point-to-axial chirality transfer reaction of optical dihydrophenanthrene-9,10-diols for the synthesis of axially chiral diketones. Two sets of conditions, namely a basic tBuOK/air atmosphere and an acidic NaClO/n-Bu4NHSO4, were developed to oxidatively cleave the C-C bond, resulting in the formation of axially chiral biaryl diketones. Finally, brief synthetic applications of the obtained chiral aryl diketones were demonstrated.

Dirhodium(II)/Phosphine Catalyst with Chiral Environment at Bridging Site and Its Application in Enantioselective Atropisomer Synthesis.

A dirhodium(II)/phosphine catalyst with a chiral environment at the bridging site was developed for the asymmetric arylation of phenanthrene-9,10-diones with arylboronic acids. In contrast to the classic chiral bridging carboxylic acid (or derivatives) ligand strategy of bimetallic dirhodium(II) catalysis, in this reaction, tuning both axial and bridging ligands realized the first Rh2(OAc)4/phosphine-catalyzed highly enantioselective carbonyl addition reaction. The kinetic analysis reveals that dirhodium(II) and arylboronic acid follow the first-order kinetics, while phenanthrene-9,10-dione is zeroth-order. These data supported the proposed catalytic cycle, where the key intermediate in the rate-determining step involved the dirhodium(II) complex and arylboronic acid. Finally, axially chiral biaryls were prepared based on a newly developed oxidative ring-opening reaction of α-hydroxyl ketones with a base and molecular oxygen, which featured a central-to-axial chirality transfer radical ß-scission step.

Divergent Synthesis of Helical Ketone Enabled by Rearrangement of Spiro Carbocation.

An acid-mediated electrophilic cyclization of 2-alkynyl-1,1'-biphenyls for the divergent synthesis of angular, bent, and zigzag fused nonplanar conjugated organic molecules was realized. The key feature of this reaction is a Wagner-Meerwein-type rearrangement via a spiro carbocation intermediate, which was formed by electrophilic cyclization of the 9H-fluoren-9-one derivative at the meta position. The products can be advanced to helical fluorenes, which exhibit high fluorescence quantum yields.

Nitrenoid from Oxime: A Practical Synthesis of Planar Chiral Ferrocenyl Phenanthridines via Nitrene-Involved Ring-Expansion Reaction.

Nitrenes and nitrenoids are highly reactive species and the proposed key intermediates in nitrogen-containing heterocyclic compound synthesis. In this work, we developed a practical method for the synthesis of phenanthridines by the reaction of oximes and Grignard reagents (with or without diethylzinc) via ring-expansion of magnesium coordinated nitrenoid complex as the key step. The method has been used to synthesize optically active planar chiral ferrocenyl phenanthridines.

Synthesis of Atropisomers with Biaryl and Vinylaryl Chirality via Pd-Catalyzed Point-to-Axial Chirality Transfer Ring-Opening Reaction.

A palladium-catalyzed domino cyclization/ring-opening reaction between 8H-indeno[1,2-c]thiophen-8-ols and N-(2-bromophenyl)propiolamide derivatives is reported. The reaction features a highly stereospecific point chirality to axial chirality transfer process, which enables the synthesis of molecules containing two chiral axes from the compounds with only one stereogenic center.

Torsional strain inversed chemoselectivity in a Pd-catalyzed atroposelective carbonylation reaction of dibenzothiophenium.

A palladium-catalyzed enantioselective ring-opening/carbonylation of cyclic diarylsulfonium salts is reported. In comparison to thioethers, the sulfonium salts displayed high reactivity and enabled the reaction to be performed under mild conditions (room temperature). The steric repulsion of the two non-hydrogen substituents adjacent to the axis led cyclic diarylsulfonium salts to be distorted, which enabled the ring-opening reaction to proceed with significant preference for breaking the exocyclic C-S bond.

Synthesis of 2-Hydroxy-2'-aroyl-1,1'-biaryls via Oxidative Ring-Opening of 9H-Fluoren-9-ols.

An Oxone-mediated oxidative ring-opening reaction of 4,5-disubstituted 9H-fluoren-9-ols by cleavage of a carbon-carbon bond is reported. 2-Hydroxy-2'-aroyl-1,1'-biaryls can be efficiently prepared by simply heating the mixture of fluoren-9-ols, Oxone, and 1,1,1,3,3,3-hexafluoroisopropanol at 60 °C for 4 h. The persulfate-involved ring-expansion processes were proposed and supported by the DFT calculations.

Construction of 2-Amino-2'-ketonyl Biaryls via Acid-Mediated Ring Opening of 9H-Fluoren-9-ols with Organic Azides.

A direct cross-coupling between 9H-fluoren-9-ols and organic azides for the synthesis of steric hindered 2-amino-2'-ketonyl biaryls was reported. The reaction featured an acid-mediated azidation/ring-expansion/hydrolysis cascade, which formally realized the C-N bond coupling reaction via cleavage of a C-C single bond. This method was applicable to chiral helical structure to give bulky axially chiral biaryls with full stereospecificity.

Synthesis of Planar Chiral 2-Aryl Aroylferrocenes via Palladium-Catalyzed C-C Bond-Cleavage/Ring-Opening Reaction.

A palladium-catalyzed ring-opening reaction of optically active ferrocenyl tertiary alcohols for the construction of planar chiral ketones is reported. The stereochemistry of the hydroxyl group in ferrocenyl alcohols markedly affects reaction: ferrocenyl alcohols with a ß-hydroxyl group show better reactivity and chemoselectivity than the corresponding α-hydroxyl analogues. The treatment of α-hydroxyl substrates with trifluoroacetic acid successfully realizes the inversion of the orientation of hydroxyl group to the corresponding ß-analogues.

Ring-Expansion Strategy for α-Aryl Azahelicene Construction: Building Blocks for Optoelectronic Materials.

An acid-mediated rapid synthesis of α-aryl azahelicenes via C-C bond cleavage of helical 9H-fluoren-9-ols is reported. The newly introduced aryl ring and pyridine moieties provide an excellent opportunity to further tune the properties of azahelicences: i.e., photoluminescence. The novel α-aryl azahelicenes showcase high circularly polarized luminescence (CPL) efficiencies (4.5 × 10-3) as well as CPL brightness (BCPL), reaching 7.39 M-1 cm-1, which indicates a potential application as chiral emitters.