Support Effect of Boron Nitride on the First N-H Bond Activation of NH3 on Ru Clusters.

Support effect is an important issue in heterogeneous catalysis, while the explicit role of a catalytic support is often unclear for catalytic reactions. A systematic density functional theory computational study is reported in this paper to elucidate the effect of a model boron nitride (BN) support on the first N-H bond activation step of NH3 on Run (n = 1, 2, 3) metal clusters. Geometry optimizations and energy calculations were carried out using density functional theory (DFT) calculation for intermediates and transition states from the starting materials undergoing the N-H activation process. The primary findings are summarized as follows. The involvement of the model BN support does not significantly alter the equilibrium structure of intermediates and transition states in the most favorable pathway (MFP). Moreover, the involvement of BN support decreases the free energy of activation, ΔG≠, thus improving the reaction rate constant. This improvement is more obvious at high temperatures like 673 K than low temperatures like 298 K. The BN support effect leading to the ΔG≠ decrease is most significant for the single Ru atom case among all three cases studied. Finally, the involvement of the model BN may change the spin transition behavior of the reaction system during the N-H bond activation process. All these findings provide a deeper insight into the support effect on the N-H bond activation of NH3 for the supported Ru catalyst in particular and for supported transition metal catalysts in general.

Enantioselective Self-Assembly of a Homochiral Tetrahedral Cage Comprising Only Achiral Precursors.

How Nature synthesizes enantiomerically pure substances from achiral or racemic resources remains a mystery. In this study, we aimed to emulate this natural phenomenon by constructing chiral tetrahedral cages through self-assembly, achieved by condensing two achiral compounds-a trisamine and a trisaldehyde. The occurrence of intercomponent CH⋅⋅⋅π interactions among the phenyl building blocks within the cage frameworks results in twisted conformations, imparting planar chirality to the tetrahedrons. In instances where the trisaldehyde precursor features electron-withdrawing ester side chains, we observed that the intermolecular CH⋅⋅⋅π forces are strong enough to prevent racemization. To attain enantioselective self-assembly, a chiral amine was introduced during the imine formation process. The addition of three equivalents of chiral amino mediator to one equivalent of the achiral trisaldehyde precursor formed a trisimino intermediate. This chiral compound was subsequently combined with the achiral trisamino precursor, leading to an imine exchange reaction that releasing the chiral amino mediator and formation of the tetrahedral cage with an enantiomeric excess (ee) of up to 75 %, exclusively composed of achiral building blocks. This experimental observation aligns with theoretical calculations based on the free energies of related cage structures. Moreover, since the chiral amine was not consumed during the imine exchange cycle, it enabled the enantioselective self-assembly of the tetrahedral cage for multiple cycles when new batches of the achiral trisaldehyde and trisamino precursors were successively added.

Palladium-Catalyzed Directed Atroposelective C-H Iodination to Synthesize Axial Chiral Biaryl N-Oxides via Enantioselective Desymmetrization Strategy.

The discovery of enantioselective desymmetrization reactions to provide practical synthesis of enantio-enriched atropisomeric biaryls is a challenging topic in the field of asymmetric catalysis. Herein, we report a highly enantioselective desymmetrization reaction for the synthesis of axially chiral biaryl N-oxides by atroposelective C-H iodination by using Pd(II) coordinated by N-benzoyl-l-phenylalanine as a chiral catalyst at room temperature. A broad range of products were obtained in high yields (up to 99 %) with excellent enantioselectivities (up to 98 % ee). The products could be synthesized in gram scale, one of which was proved to be a powerful organocatalyst in asymmetric allylation reaction. Mechanistic evidence as well as DFT calculations point towards the factors that lead to high reactivity and excellent enantiocontrol in this reaction.

Effects of the Separation Distance between Two Triplet States Produced from Intramolecular Singlet Fission on the Two-Electron-Transfer Process.

To harvest two triplet excitons of singlet fission (SF) via a two-electron transfer efficiently, the revelation of the key factors that influence the two-electron-transfer process is necessary. Here, by using steady-state and transient absorption/fluorescence spectroscopy, we investigated the two-electron-transfer process from the two triplet excitons of intramolecular SF (iSF) in a series of tetracene oligomers (dimer, trimer, and tetramer) with 7,7,8,8-tetracyanoquinodimethane (TCNQ) as an electron acceptor in solution. Quantitative two-electron transfer could be conducted for the trimer and tetramer, and the rate for the tetramer is faster than that for the trimer. However, the maximum efficiency of the two-electron transfer in the dimer is relatively low (∼47%). The calculation result of the free energy change (ΔG) of the second-electron transfer for these three compounds (-0.024, -0.061, and -0.074 eV for the dimer, trimer, and tetramer, respectively) is consistent with the experimental observation. The much closer ΔG value to zero for the dimer should be responsible for its low efficiency of the two-electron transfer. Different ΔG values for these three oligomers are attributed to the different Coulomb repulsive energies between the two positive charges generated after the two-electron transfer that is caused by their various intertriplet distances. This result reveals for the first time the important effect of the Coulomb repulsive energy, which depends on the intertriplet distance, on the two-electron transfer process from the two triplet excitons of iSF.

Mechanistic Insights into Visible-Light-Driven Dearomative Fluoroalkylation Mediated by an Electron Donor-Acceptor Complex.

Electron donor-acceptor (EDA) complex photochemistry has become a burgeoning topic in the synthetic radical chemistry mediated by visible light; however, the theoretical insights into the reaction mechanisms are limited. Herein, accurate electronic structure calculations at the CASPT2//CASSCF/PCM level of theory were performed to investigate the paradigm example of EDA complex-enabled photoreaction for visible-light-driven dearomative perfluoroalkylation of ß-naphthol. The excitation energy levels of the EDA complex are controlled by noncovalent interactions because the photoinduced intermolecular charge is enhanced when the noncovalent interaction becomes weaker, leading to the broad spectra ranging from UVA (<380 nm) to visible light (>500 nm). The competitiveness of the radical-radical coupling over the radical chain pathway is also regulated due to the tunable radical concentrations varying the excitation wavelength.

Enantioselective synthesis of indole-based unnatural ß-Alkynyl α-amino acid derivatives via chiral phosphoric acid catalysis.

The synthesis of unnatural α-amino acid derivatives has attracted considerable interest in recent years, as they are ubiquitous in protein synthesis and peptide-based drug discovery. Herein, we reported the chiral phosphoric acid catalyzed asymmetric reaction of indoles with ß,γ-alkynyl-α-imino esters for the enantioselective synthesis of unnatural indole-based α-amino acid derivatives. This asymmetric organocatalysis protocol enables efficient synthesis of unnatural α-amino acid derivatives with a tetrasubstituted stereogenic center and an alkyne moiety with up to 99% yield and 98% ee, resulting in operationally simple conditions, short reaction time, and broad substrate scope.

Beyond PRNU: Learning Robust Device-Specific Fingerprint for Source Camera Identification.

Source-camera identification tools assist image forensics investigators to associate an image with a camera. The Photo Response Non-Uniformity (PRNU) noise pattern caused by sensor imperfections has been proven to be an effective way to identify the source camera. However, the PRNU is susceptible to camera settings, scene details, image processing operations (e.g., simple low-pass filtering or JPEG compression), and counter-forensic attacks. A forensic investigator unaware of malicious counter-forensic attacks or incidental image manipulation is at risk of being misled. The spatial synchronization requirement during the matching of two PRNUs also represents a major limitation of the PRNU. To address the PRNU's fragility issue, in recent years, deep learning-based data-driven approaches have been developed to identify source-camera models. However, the source information learned by existing deep learning models is not able to distinguish individual cameras of the same model. In light of the vulnerabilities of the PRNU fingerprint and data-driven techniques, in this paper, we bring to light the existence of a new robust data-driven device-specific fingerprint in digital images that is capable of identifying individual cameras of the same model in practical forensic scenarios. We discover that the new device fingerprint is location-independent, stochastic, and globally available, which resolves the spatial synchronization issue. Unlike the PRNU, which resides in the high-frequency band, the new device fingerprint is extracted from the low- and mid-frequency bands, which resolves the fragility issue that the PRNU is unable to contend with. Our experiments on various datasets also demonstrate that the new fingerprint is highly resilient to image manipulations such as rotation, gamma correction, and aggressive JPEG compression.

Recent advances in the asymmetric phosphoric acid-catalyzed synthesis of axially chiral compounds.

In recent years, the synthesis of axially chiral compounds has received considerable attention due to their extensive application as biologically active compounds in medicinal chemistry and as chiral ligands in asymmetric catalysis. Chiral phosphoric acids are recognized as efficient organocatalysts for a variety of enantioselective transformations. In this review, we summarize the recent development of chiral phosphoric acid-catalyzed synthesis of a wide range of axially chiral biaryls, heterobiaryls, vinylarenes, N-arylamines, spiranes, and allenes with high efficiency and excellent stereoselectivity.

Synthesis of Axially Chiral Biaryl-2-amines by PdII -Catalyzed Free-Amine-Directed Atroposelective C-H Olefination.

A simple and ubiquitously present group, free amine, is used as a directing group to synthesize axially chiral biaryl compounds by PdII -catalyzed atroposelective C-H olefination. A broad range of axially chiral biaryl-2-amines can be obtained in good yields with high enantioselectivities (up to 97 % ee). Chiral spiro phosphoric acid (SPA) proved to be an efficient ligand and the loading could be reduced to 1 mol % without erosion of enantiocontrol in gram-scale synthesis. The resulting axially chiral biaryl-2-amines also provide a platform for the synthesis of a set of chiral ligands.

Iron-catalyzed asymmetric intramolecular cyclopropanation reactions using chiral tetramethyl-1,1'-spirobiindane-based bisoxazoline (TMSI-BOX) ligands.

The versatile application of chiral bisoxazoline (BOX) ligands in diverse metal-catalyzed asymmetric reactions results in growing demand for novel BOX ligands containing different motifs. Herein, the successful development of a chiral spiro bisoxazoline ligand (TMSI-BOX) on the basis of the tetramethyl-1,1'-spirobiindane motif and bisoxazoline chelating units is described. The corresponding Fe complexes of TMSI-BOX proved to be excellent catalysts in the asymmetric intramolecular cyclopropanation reaction of diazo derivatives, providing synthetically versatile [3.1.0]bicycloalkane derivatives bearing two contiguous quaternary chiral centers with high enantiomeric purity.

Atroposelective Phosphoric Acid Catalyzed Three-Component Cascade Reaction: Enantioselective Synthesis of Axially Chiral N-Arylindoles.

An efficient organocatalytic atroposelective three-component cascade reaction of 2,3-diketoesters, aromatic amines, and 1,3-cyclohexanediones has been developed for the highly enantioselective synthesis of axially chiral N-arylindoles. The success of this method derives from the use of a newly developed second-generation chiral spirocyclic phosphoric acid as the catalyst. In addition, this protocol was extended to the synthesis of an axially chiral monophosphorus ligand.

Total Syntheses of (+)-Sarcophytin, (+)-Chatancin, (-)-3-Oxochatancin, and (-)-Pavidolide†B: A Divergent Approach.

A concise and divergent approach for the total syntheses of four cembrane diterpenoids, namely (+)-sarcophytin, (+)-chatancin, (-)-3-oxochatancin, and (-)-pavidolide B, has been developed, and it also led to the structural revision of (-)-isosarcophytin. The key steps of the strategy feature a double Mukaiyama Michael addition/elimination, a Helquist annulation, two substrate-controlled facial-selective hydrations, and a pinacol rearrangement. The described syntheses not only achieved these natural products in an efficient manner, but also provided insight into the biosynthetic relationship between the two different skeletons.

Enantioselective Synthesis of Biaryl Atropisomers by Pd-Catalyzed C-H Olefination using Chiral Spiro Phosphoric Acid Ligands.

The discovery of proper ligands to simultaneously modulate the reactivity and effectively control the stereoselectivity is a central topic in the field of enantioselective C-H activation. Herein, we reported the synthesis of axially chiral biaryls by Pd-catalyzed atroposelective C-H olefination. A novel chiral spiro phosphoric acid, STRIP, was identified as a superior ligand for this transformation. A broad range of axially chiral quinoline derivatives were synthesized in good yields with excellent enantioselectivities (up to 98 % ee). Density functional theory was used to gain a theoretical understanding of the enantioselectivities in this reaction.

Synthesis and Application of Hexamethyl-1,1'-spirobiindane-Based Phosphine-Oxazoline Ligands in Ni-Catalyzed Asymmetric Arylation of Cyclic Aldimines.

With the vastly increasing applications of chiral phosphine-oxazoline (PHOX) hybrid ligands in various transition-metal-catalyzed reactions, novel PHOX ligands bearing innovative backbones are highly valuable and in great demand. This study describes the development of a new type of chiral PHOX ligands based on a hexamethyl-1,1'-spirobiindane scaffold and incorporating both a phosphine and an oxazoline moiety. The optimal ligand provided high yields and excellent enantioselectivities for the Ni-catalyzed asymmetric arylation of cyclic N-sulfonyl imines with arylboronic acids leading to chiral amines.

Rhodium-Catalyzed Asymmetric Addition of Organoboronic Acids to Aldimines Using Chiral Spiro Monophosphite-Olefin Ligands: Method Development and Mechanistic Studies.

The synthesis of a novel type of chiral spiro monophosphite-olefin (SMPO) ligands based on a hexamethyl-1,1'-spirobiindane scaffold was accomplished starting from Bisphenol C. The optimal ligand could serve as an elegant chiral bidentate ligand in the Rh-catalyzed asymmetric 1,2-addition of organoboronic acids to various acyclic/cyclic aldimines, leading to chiral amines with high yields and excellent enantioselectivities. Detailed stereochemical models for enantioselective induction were elucidated through DFT calculations and postulated the origins of the higher enantioselectivity of phosphite-olefin ligands.

Development and application of chiral spirocyclic phosphoric acids in asymmetric catalysis.

The development of suitable chiral catalysts, which represent powerful, economically feasible tools for the preparation of optically active organic molecules, is a fundamental endeavour in synthetic chemistry. Chiral phosphoric acids derived from axially chiral 1,1'-bi-2-naphthol (BINOL) are a widely used class of strong Brønsted acid catalysts and have been applied successfully in many asymmetric catalyzed reactions. Accordingly, the development and application of chiral spirocyclic phosphoric acids derived from axially chiral 1,1'-spirobiindane-7,7'-diol (SPINOL) have received increasing attention in the last eight years. This review covers the synthetic methods and the application of chiral spirocyclic phosphoric acids in asymmetric catalysis.

Synthesis and application of a new hexamethyl-1,1'-spirobiindane-based chiral bisphosphine (HMSI-PHOS) ligand in asymmetric allylic alkylation.

The emerging and versatile application of chiral bisphosphine ligands in diverse asymmetric reactions has resulted in great demand for novel bisphosphine ligands containing different innovative backbones. Herein, a new type of chiral spiro bisphosphine ligand (HMSI-PHOS) has been developed on the basis of a hexamethyl-1,1'-spirobiindane motif. A series of new HMSI-PHOS ligands were synthesized and used in Pd-catalyzed asymmetric allylic alkylation reactions, and moderate to good yields and enantioselectivities were obtained with phenyl substituted ligands.

Synthesis and application of a new chiral monodentate spiro phosphoramidite ligand based on hexamethyl-1,1'-spirobiindane backbone in asymmetric hydroamination/arylation of alkenes.

The design and synthesis of a new chiral monodentate spiro phosphoramidite ligand based on a hexamethyl-1,1'-spirobiindane scaffold has been accomplished. The ligand could serve as an elegant chiral monodentate ligand in the Pd-catalyzed asymmetric hydroamination/arylation of alkenes leading to chiral imidazolidin-2-ones with good enantioselectivities.

Asymmetric organocatalytic synthesis of chiral 3,3-disubstituted oxindoles via a 1,6-conjugate addition reaction.

A highly efficient synthesis of chiral 3,3-disubstituted oxindoles was developed using a chiral spirocyclic phosphoric acid catalyzed 1,6-conjugate addition reaction of para-quinone methides derived from N-unprotected isatins with indoles. The reaction proceeds under mild reaction conditions to provide indole-containing N-unprotected oxindoles bearing quaternary stereocenters in good yields and with moderate to excellent enantioselectivities (up to 97% ee).

Organocatalytic asymmetric synthesis of benzazepinoindole derivatives with trifluoromethylated quaternary stereocenters by chiral phosphoric acid catalysts.

An enantioselective aza-Friedel-Crafts reaction of trifluoromethyl dihydrobenzoazepinoindoles with pyrroles catalyzed by a chiral spirocyclic phosphoric acid was developed. This methodology provides a facile route to CF3- and pyrrole-containing benzazepinoindoles bearing quaternary stereocenters in good yields and with moderate to excellent enantioselectivities (up to 93% ee). Indoles were also investigated as electron-rich aromatic substrates to afford the corresponding chiral heterocycles with good yields and considerable enantioselectivities. The introduction of CF3 shows a remarkable fluorine effect and increases the activation and stereoinduction.