Steric Effects on Single-Molecule Conductance in Flat-Lying Phenanthrene.

A previous combined experimental and theoretical study found that the position of anchoring groups on a phenanthrene (PHE) backbone played a large role in determining the single-molecule conductance of the PHE derivative. However, a consistent 0.1 G0 feature was found across all PHE derivatives. To understand this, the previously investigated PHE derivatives were placed flat on a simulated Au substrate with a scanning tunneling microscope (STM) tip over PHE and conductance was calculated using the non-equilibrium Green's function technique in conjunction with density functional theory (NEGF-DFT). The location of the tip was varied to find the most conductive and most energetically favorable arrangements, which did not coincide. Furthermore, the variation in conductance found in erect junctions was not present when PHE derivatives were lying flat, with all derivatives calculated to have conductance values around 0.1 G0.

A Three-Dimensional Non-Fullerene Acceptor with Contorted Hexabenzocoronene and Perylenediimide for Organic Solar Cells.

Although fullerene derivatives such as [6,6]-phenyl-C61/C71-butyric acid methyl ester (PC61BM/PC71BM) have dominated the the photoactive acceptor materials in bulk heterojunction organic solar cells (OSCs) for decades, they have several drawbacks such as weak absorption, limited structural tunability, prone to aggregation, and high costs of production. Constructing non-fullerene small molecules with three-dimensional (3D) molecular geometry is one of the strategies to replace fullerenes in OSCs. In this study, a 3D molecule, contorted hexa-cata-hexabenzocoronene tetra perylenediimide (HBC-4-PDI), was designed and synthesized. HBC-4-PDI shows a wide and strong light absorption in the whole UV-vis region as well as suitable energy levels as an acceptor for OSCs. More importantly, the 3D construction effectively reduced the self-aggregation of c-HBC, leading to an appropriate scale phase separation of the blend film morphology in OSCs. A preliminary power conversion efficiency of 2.70 % with a champion open-circuit voltage of 1.06 V was obtained in OSCs with HBC-4-PDI as the acceptor, which was the highest among the previously reported OSCs based on c-HBC derivatives. The results indicated that HBC-4-PDI may serve as a good non-fullerene acceptor for OSCs.

Copper-Catalyzed Asymmetric Arylation of α-Substituted Cyanoacetates Enabled by Chiral Amide Ligands.

The (S)-nobin-embodied picolinamide and L-hydroxyproline-derived amide are effective ligands for Cu-catalyzed enantioselective coupling reaction of (hetero)aryl iodides with α-alkyl substituted cyanoacetates. This arylation reaction gave α-(heteroaryl)-α-alkyl cyanoacetates in good to excellent enantioselectivities (up to 95 % ee). A variety of functionalized (hetero)aryl and alkyl groups could be introduced to the quaternary center and therefore provided a valuable tool for preparing enantioenriched compounds with an all-carbon quaternary center tethered with convertible functional groups. The size of both α-alkyl and ester groups was proven as the key factor for asymmetric induction.

A Computational Study on the Reaction Mechanism of Stereocontrolled Synthesis of ß-Lactam within [2]Rotaxane.

The macrocycle effect of [2]rotaxane on the highly trans-stereoselective cyclization reaction of N-benzylfumaramide was extensively investigated by various computational methods, including DFT and high-level DLPNO-CCSD(T) methods. Our computational results suggest that the most favorable mechanism of the CsOH-promoted cyclization of the fumaramide into trans-ß-lactam within [2]rotaxane initiates with deprotonation of a N-benzyl group of the interlocked fumaramide substrate by CsOH, followed by the trans-selective C-C bond formation and protonation by one amide functional group of the macrocycle. Our distortion/interaction analysis further shows that the uncommon trans-stereoselective cyclization forming ß-lactam within the rotaxane may be attributed to a higher distortion energy (mainly from the distortion of the twisted cis-fumaramide conformation enforced by the rotaxane). Our systematic study should give deeper mechanistic insight into the reaction mechanism influenced by a supramolecular host.

Improved discrimination of phenylalanine enantiomers by surface enhanced Raman scattering assay: molecular insight into chiral interaction.

SERS spectroscopy, a nontraditional chiral analysis tool, is used to discriminate between D- and L-phenylalanine (Phe), with the Raman scattering enhancement degree of D-Phe being 50-fold greater than that of L-Phe. Such discrimination is achieved by chiral interactions between Phe and the chiral drug molecule of sertraline on silver coated gold nanorods.

Divergent Synthesis of Contorted Polycyclic Aromatics Containing Pentagons, Heptagon, and/or Azulene.

Divergent synthesis of four contorted aromatics containing pentagons, a heptagon, and/or an azulene from the same difluorenyl pentacenediene precursor were realized in one step. The subtle differences in molecular structure were confirmed by X-ray crystallography. The mechanisms for the control of different products, which involve a ring-expansion rearrangement, Scholl reactions, and/or Mallory cyclization were proposed on the basis of control experiments and DFT calculations. Such development adds good structure versatility and materials accessibility to the study of contorted aromatics.

Regio- and Steric Effects on Single Molecule Conductance of Phenanthrenes.

Here, six phenanthrene (the smallest arm-chair graphene nanoribbon) derivatives with dithiomethyl substitutions at different positions as the anchoring groups were synthesized. Scanning tunneling microscopy break junction technique was used to measure their single molecule conductances between gold electrodes, which showed a difference as much as 20-fold in the range of ∼10-2.82 G0 to ∼10-4.09 G0 following the trend of G2,7 > G3,6 > G2,6 > G1,7 > G1,6 > G1,8. DFT calculations agree well with this measured trend and indicate that the single molecule conductances are a combination of energy alignment, electronic coupling, and quantum effects. This significant regio- and steric effect on the single molecule conductance of phenanthrene model molecules shows the complexity in the practice of graphene nanoribbons as building blocks for future carbon-based electronics in one hand but also provides good conductance tunability on the other hand.

Detecting Individual Bond Switching within Amides in a Tunneling Junction.

Amides are essential in the chemistry of life. Detecting the chemical bond states within amides could unravel the nature of amide stabilization and planarity, which is critical to the structure and reactivity of such molecules. Yet, so far, no work has been reported to detect or measure the bond changes at the single-molecule level within amides. Here, we show that a transition between single and double bonds between N and C atoms in an amide can be monitored in real time in a nanogap between gold electrodes via the generation of distinctive conductance features. Density functional theory simulations show that the switching between amide isomers proceeds via a proton transfer process facilitated by a water molecule bridge, and the resulting molecular junctions display bimodal conductance states with a difference as much as nine times.

Efficient Asymmetric Biomimetic Aldol Reaction of Glycinates and Trifluoromethyl Ketones by Carbonyl Catalysis.

The direct asymmetric aldol reaction of glycinates represents an intriguing and straightforward strategy to make biologically significant chiral ß-hydroxy-α-amino-acid derivatives. But it is not easy to realize the transformation due to the disruption of the reactive NH2 group of glycinates. Inspired by the enzymatic aldol reaction of glycine, we successfully developed an asymmetric aldol reaction of glycinate 5 and trifluoromethyl ketones 4 with 0.1-0.0033 mol % of chiral N-methyl pyridoxal 7 a as the catalyst, producing chiral ß-trifluoromethyl-ß-hydroxy-α-amino-acid esters 6 in 55-82 % yields (for the syn-diastereomers) with up to >20:1 dr and 99 % ee under very mild conditions. The reaction proceeds via a catalytic cycle similar to the enzymatic aldol reaction of glycine. Pyridoxal catalyst 7 a activates both reactants at the same time and brings them together in a specific spatial orientation, accounting for the high efficiency as well as excellent diastereo- and enantioselectivities.

Enantioselective α-Carbonylative Arylation for Facile Construction of Chiral Spirocyclic ß,ß'-Diketones.

We herein describe the first enantioselective α-carbonylative arylation, providing a diverse set of chiral spiro ß,ß'-diketones bearing various ring sizes and functionalities in high yields and good to excellent enantioselectivities. Calculations suggest the transformation proceeds through reductive elimination instead of nucleophilic addition pathway.

Enantioselective Synthesis of Pyroglutamic Acid Esters from Glycinate via Carbonyl Catalysis.

Direct α-functionalization of NH2 -free glycinates with relatively weak electrophiles such as α,ß-unsaturated esters still remains a big challenge in organic synthesis. With chiral pyridoxal 5 d as a carbonyl catalyst, direct asymmetric conjugated addition at the α-C of glycinate 1 a with α,ß-unsaturated esters 2 has been successfully realized, to produce various chiral pyroglutamic acid esters 4 in 14-96 % yields with 81-97 % ee's after in situ lactamization. The trans and cis diastereomers can be obtained at the same time by chromatography and both of them can be easily converted into chiral 4-substituted pyrrolidin-2-ones such as Alzheimer's drug Rolipram (11) with the same absolute configuration via tert-butyl group removal and subsequent Barton decarboxylation.

High Quality Pyrazinoquinoxaline-Based Graphdiyne for Efficient Gradient Storage of Lithium Ions.

N-doping of graphdiyne with atomic precision is very important for the study of heteroatom doping effect and the structure-properties relationships of graphdiyne. Here we report the bottom-up synthesis and characterizations of high-quality pyrazinoquinoxaline-based graphdiyne (PQ-GDY) film. First-principle studies of the layered structure were performed to examine the stacking mode, lithium binding affinity, and bulk lithium storage capacity. Three-stage insertion of 14 lithium atoms with binding affinities in the order of pyrazine nitrogen > diyne carbon > central aromatic ring were confirmed by both lithium-ion half-cell measurements and DFT calculations. More than half of the lithium atoms preferentially bind to pyrazine nitrogen, and a reversible capacity of 570.0 mA h g-1 at a current density of 200 mA g-1 after 800 cycles was achieved. Such a high capacity utilization rate of 97.2% provides a good case study of N-doped GDY with atomic precision.

Au Sputtered Paper Chromatography Tandem Raman Platform for Sensitive Detection of Heavy Metal Ions.

Surface-enhanced Raman scattering (SERS) is a powerful technique for sensitive detection, but it normally has difficulty in multicomponent detection in a complex system, especially for simultaneous analysis of mixture of heavy metal ions. In this work, a simple paper chromatography tandem SERS (PC-SERS) separation/detection platform is proposed by ion-sputtering gold on a filter paper. Based on SEM results, the great electromagnetic field inside nanogaps of Au nanoislands on the paper surface is evaluated with FDTD simulation. It is found that the PC-SERS platform has good uniformity (RSD = 10.12%) and long-time stability. The as-prepared PC-SERS platform was applied to efficiently separate and detect a mixture of pesticides (MG, MB, and CV) in pond water without any pretreatment process, and the limits of detection (LODs) were down to 10 nM. As a crucial application for food safety, several heavy metal ions such as Cd2+, Cu2+, and Ni2+ in grinded rice were successfully detected by the PC-SERS method taking advantage of the sandwich structure based on 4-mercaptobenzoic acid (4-MBA) molecules, which were modified onto sputtering the Au filter paper and gold nanoparticles (Au NPs) to link metal ions and acted as Raman signal molecules. All the LODs for metal ions were down to 1 µM. Due to the easiness of fabrication, good reproducibility, and simple pretreatment step, the PC-SERS platform holds promise in multicomponent detection in a real sample.

Chiral Plasmonic Nanoparticle Assisted Raman Enantioselective Recognition.

Au nanoparticles (NPs) labeled with the handedness tag of "d-" or "l-", which were detached from inorganic chiral silica, showed both intrinsic chirality and surface enhanced Raman scattering (SERS) activity. In the presence of these chiral Au substrates, it was found that the enantiomer of cystine with the same handedness tag of Au NPs would show stronger Raman scattering signal intensities than those of the enantiomer with the opposite tag, where the differences could be over three times. Consequently, this work afforded a novel enantioselective recognition method on ordinary Raman spectroscopy by using chiral plasmonic metallic nanomaterials.

Exploiting the trifluoroethyl group as a precatalyst ligand in nickel-catalyzed Suzuki-type alkylations.

We report herein the exploitment of the partially fluorinated trifluoroethyl as precatalyst ligands in nickel-catalyzed Suzuki-type alkylation and fluoroalkylation coupling reactions. Compared with the [L n NiII(aryl)(X)] precatalysts, the unique characters of bis-trifluoroethyl ligands imparted precatalyst [(bipy)Ni(CH2CF3)2] with bench-top stability, good solubilities in organic media and interesting catalytic activities. Preliminary mechanistic studies reveal that an eliminative extrusion of a vinylidene difluoride (VDF, CH2[double bond, length as m-dash]CF2) mask from [(bipy)Ni(CH2CF3)2] is a critical step for the initiation of a catalytic reaction.

Brønsted-Acid-Promoted Rh-Catalyzed Asymmetric Hydrogenation of N-Unprotected Indoles: A Cocatalysis of Transition Metal and Anion Binding.

The incorporation of Brønsted acid, thiourea anion binding, and transition metal catalysis enables an efficient method to synthesize chiral indolines via hydrogenation of indoles. Catalyzed by a rhodium/ZhaoPhos complex, asymmetric hydrogenation of unprotected indoles is performed smoothly with excellent enantioselectivities (up to 99% ee, up to 400 TON). Brønsted acid HCl activates indoles to form iminium ion intermediates. Mechanistic studies support the assumption that anion binding plays a crucial role as a secondary interaction. DFT calculations reveal an outer-sphere mechanism in this chemical transformation.

Enantioselective palladium-catalyzed diboration of 1,1-disubstituted allenes.

A practical and enantioselective palladium-catalyzed diboration of 1,1-disubstituted allenes is developed by employing a P-chiral monophosphorus ligand, BI-DIME. A series of diboronic esters containing a chiral tertiary boronic ester moiety are formed in excellent yields and ee's with the palladium loading as low as 0.2 mol%. DFT calculations revealed a concerted mechanism of oxidative addition of bis(pinacolato)diboron and allene insertion, as well as a critical dispersion effect on the origins of the enantioselectivity. The method is successfully applied to the concise and enantioselective synthesis of brassinazole.

Efficient syntheses of (-)-crinine and (-)-aspidospermidine, and the formal synthesis of (-)-minfiensine by enantioselective intramolecular dearomative cyclization.

Polycyclic alkaloids bearing all-carbon quaternary centers possess a diversity of biological activities and are challenging targets in natural product synthesis. The development of a general and asymmetric catalytic method applicable to the efficient syntheses of a series of complex polycyclic alkaloids remains highly desirable in synthetic chemistry. Herein we describe an efficient palladium-catalyzed enantioselective dearomative cyclization which is capable of synthesizing two important classes of tricyclic nitrogen-containing skeleton, chiral dihydrophenanthridinone and dihydrocarbazolone derivatives bearing all-carbon quaternary centers, in excellent yields and enantioselectivities. The P-chiral monophosphorus ligand AntPhos is crucial for the reactivity and enantioselectivity, and the choice of the N-phosphoramide protecting group is essential for the desired chemoselectivity. This method has enabled the enantioselective total syntheses of three distinctive and challenging biologically important polycyclic alkaloids, specifically a concise and gram-scale synthesis of (-)-crinine, an efficient synthesis of indole alkaloid (-)-aspidospermidine and a formal enantioselective synthesis of (-)-minfiensine.

Practical and Asymmetric Reductive Coupling of Isoquinolines Templated by Chiral Diborons.

We herein describe a chiral diboron-templated highly diastereoselective and enantioselective reductive coupling of isoquinolines that provided expedited access to a series of chiral substituted bisisoquinolines in good yields and excellent ee's under mild conditions. The method enjoys a broad substrate scope and good functional group compatibility. Mechanistic investigation suggests the reaction proceeds through a concerted [3,3]-sigmatropic rearrangement.