A One-Pot Photochemical Method for the Generation of Functionalized Aminocyclopentanes.

Detailed herein is the development of a photochemical intermolecular formal [3+2] cycloaddition between cyclopropylimines and substituted alkenes to generate aminocyclopentane derivatives. The Schiff base of the cyclopropylimine was designed to enable a masked N-centered radical approach in which the requisite open-shell character was achieved upon excitation with visible light. The cycloaddition products were directly converted to N-functionalized aminocyclopentanes via solvolysis and N-acylation. The photochemical component of this reaction sequence was demonstrated to operate in continuous flow.

Photochemically derived 1-aminonorbornanes provide structurally unique succinate dehydrogenase inhibitors with in vitro and in planta activity.

Agrochemical fungicidal leads have been prepared from photochemically derived 1-aminonorbornane building blocks. The unique 1-aminonorbornane core is generated via direct excitation of a Schiff base precursor, leveraging the N-centered radical character of the excited state species to facilitate a series of radical reactions that construct the norbornane core. This process requires no exogenous reagents, only solvent and photons; thus, it represents an exceptionally simple and efficient means of generating the key building blocks. These (hetero) arene-fused 1-aminonorbornanes are unprecedented in both the agrochemical and pharmaceutical discovery literature; therefore, photochemical advances have provided the unique opportunity to explore the functional utility of novel chemical space. Toward this end, the 1-aminonorbornanes were used to generate next-generation succinate dehydrogenase inhibitors. In vitro fungicidal activity is demonstrated against three fungal plant pathogens affecting field crops, specifically: Fusarium graminearum, Sclerotinia sclerotiorum, and Macrophomina phaseolina. The in vitro performance against F. graminearum was shown to translate into a greenhouse setting. The discovery of in planta fungicidal activity illustrates the interdisciplinary value available via photochemical innovation.

Quantum Transport in Air-Stable Na3Bi Thin Films.

Na3Bi has attracted significant interest in both bulk form as a three-dimensional topological Dirac semimetal and ultrathin form as a wide-band gap two-dimensional topological insulator. Its extreme air sensitivity has limited experimental efforts on thin and ultrathin films grown via molecular beam epitaxy to ultrahigh vacuum environments. Here, we demonstrate air-stable Na3Bi thin films passivated with magnesium difluoride (MgF2) or silicon (Si) capping layers. Electrical measurements show that deposition of MgF2 or Si has minimal impact on the transport properties of Na3Bi while in ultrahigh vacuum. Importantly, the MgF2-passivated Na3Bi films are air-stable and remain metallic for over 100 h after exposure to air, as compared to near instantaneous degradation when they are unpassivated. Air stability enables transfer of films to a conventional high-magnetic field cryostat, enabling quantum transport measurements, which verify that the Dirac semimetal character of Na3Bi films is retained after air exposure.

Exploiting Imine Photochemistry for Masked N-Centered Radical Reactivity.

This report details the development of a masked N-centered radical strategy that harvests the energy of light to drive the conversion of cyclopropylimines to 1-aminonorbornanes. This process employs the N-centered radical character of a photoexcited imine to facilitate the homolytic fragmentation of the cyclopropane ring and the subsequent radical cyclization sequence that forms two new C-C bonds en route to the norbornane core. Achieving bond-forming reactivity as a function of the N-centered radical character of an excited state Schiff base is unique, requiring only violet light in this instance. This methodology operates in continuous flow, enhancing the potential to translate beyond the academic sector. The operational simplicity of this photochemical process and the structural novelty of the (hetero)aryl-fused 1-aminonorbornane products are anticipated to provide a valuable addition to discovery efforts in pharmaceutical and agrochemical industries.

Electric-field-tuned topological phase transition in ultrathin Na3Bi.

The electric-field-induced quantum phase transition from topological to conventional insulator has been proposed as the basis of a topological field effect transistor1-4. In this scheme, 'on' is the ballistic flow of charge and spin along dissipationless edges of a two-dimensional quantum spin Hall insulator5-9, and 'off' is produced by applying an electric field that converts the exotic insulator to a conventional insulator with no conductive channels. Such a topological transistor is promising for low-energy logic circuits4, which would necessitate electric-field-switched materials with conventional and topological bandgaps much greater than the thermal energy at room temperature, substantially greater than proposed so far6-8. Topological Dirac semimetals are promising systems in which to look for topological field-effect switching, as they lie at the boundary between conventional and topological phases3,10-16. Here we use scanning tunnelling microscopy and spectroscopy and angle-resolved photoelectron spectroscopy to show that mono- and bilayer films of the topological Dirac semimetal3,17 Na3Bi are two-dimensional topological insulators with bulk bandgaps greater than 300 millielectronvolts owing to quantum confinement in the absence of electric field. On application of electric field by doping with potassium or by close approach of the scanning tunnelling microscope tip, the Stark effect completely closes the bandgap and re-opens it as a conventional gap of 90 millielectronvolts. The large bandgaps in both the conventional and quantum spin Hall phases, much greater than the thermal energy at room temperature (25 millielectronvolts), suggest that ultrathin Na3Bi is suitable for room-temperature topological transistor operation.

Room temperature in-plane ferroelectricity in van der Waals In2Se3.

Van der Waals (vdW) assembly of layered materials is a promising paradigm for creating electronic and optoelectronic devices with novel properties. Ferroelectricity in vdW layered materials could enable nonvolatile memory and low-power electronic and optoelectronic switches, but to date, few vdW ferroelectrics have been reported, and few in-plane vdW ferroelectrics are known. We report the discovery of in-plane ferroelectricity in a widely investigated vdW layered material, ß'-In2Se3. The in-plane ferroelectricity is strongly tied to the formation of one-dimensional superstructures aligning along one of the threefold rotational symmetric directions of the hexagonal lattice in the c plane. Surprisingly, the superstructures and ferroelectricity are stable to 200°C in both bulk and thin exfoliated layers of In2Se3. Because of the in-plane nature of ferroelectricity, the domains exhibit a strong linear dichroism, enabling novel polarization-dependent optical properties.

Lithium bis-catechol borate as an effective reductive quencher in photoredox catalysis.

The use of lithium bis-catechol borate (LiB(cat)2) as a reductive quencher for the photoredox mediated intermolecular C-H functionalization of various heteroaromatics with bromopyrroloindolines is described. LiB(cat)2 offers a financial benefit over state-of-the-art quenchers currently in use while eliminating the side reactions that typically plague these couplings. The advantage of this methodology is highlighted by the synthesis of C3-C2' (-) gliocladin C. Furthermore, additional examples of reactivity with various bromopyrroloindolines sets the stage for expedient routes towards other pharmaceutically active hexahydropyrroloindoline alkaloids and their analogues.

Calixarene-mediated liquid membrane transport of choline conjugates 3: The effect of handle variation on neurotransmitter transport.

Upper rim phosphonic acid functionalized calix[4]arene affects selective transport of multiple molecular payloads through a liquid membrane. The secret is in the attachment of a receptor-complementary handle to the payload. We find that the trimethylammonium ethylene group present in choline is one of several general handles for the transport of drug and drug-like species. Herein we compare the effect of handle variation against the transport of serotonin and dopamine. We find that several ionizable amine termini handles are sufficient for transport and identify two ideal candidates. Their performance is significantly enhanced in HEPES buffered solutions. This inquiry completes a series of 3 studies aimed at optimization of this strategy. In completion a new approach towards synthetic receptor mediated selective small molecule transport has emerged; future work in vesicular and cellular systems will follow.

Spatial charge inhomogeneity and defect states in topological Dirac semimetal thin films of Na3Bi.

Topological Dirac semimetals (TDSs) are three-dimensional analogs of graphene, with carriers behaving like massless Dirac fermions in three dimensions. In graphene, substrate disorder drives fluctuations in Fermi energy, necessitating construction of heterostructures of graphene and hexagonal boron nitride (h-BN) to minimize the fluctuations. Three-dimensional TDSs obviate the substrate and should show reduced EF fluctuations due to better metallic screening and higher dielectric constants. We map the potential fluctuations in TDS Na3Bi using a scanning tunneling microscope. The rms potential fluctuations are significantly smaller than the thermal energy room temperature (ΔEF,rms = 4 to 6 meV = 40 to 70 K) and comparable to the highest-quality graphene on h-BN. Surface Na vacancies produce a novel resonance close to the Dirac point with surprisingly large spatial extent and provide a unique way to tune the surface density of states in a TDS thin-film material. Sparse defect clusters show bound states whose occupation may be changed by applying a bias to the scanning tunneling microscope tip, offering an opportunity to study a quantum dot connected to a TDS reservoir.