Can Twisted Double Bonds Facilitate Stepwise [2 + 2] Cycloadditions?

Computational studies for a series of low to high strain anti-Bredt alkenes suggest that those with highly twisted bridgehead double bonds and a small singlet-triplet energy gap may undergo facile stepwise [2 + 2] cycloadditions to furnish four membered rings. A selection of reaction substrates, including ethylene, acetylene, perfluoroethylene, and cyclooctyne are considered.

Photoactivation of Hydrazones for the Synthesis of Diarylalkanes and Trialkylmethylboronates: The Key Role Played by Soluble Base.

The synthesis of diaryl alkanes and tertiary organoboronates via Barluenga coupling at room temperature occurred via photoactivated conversion of aryl sulfonyl hydrazones to diazo compounds in the presence of soluble bases. The combination of arylsulfonyl hydrazone and a soluble base is necessary to provide a near-UV chromophore. Using aromatic hydrazones and aromatic boronic acids resulted in rapid deboronation because of the instability of dibenzylic boron intermediates.

Regio- and enantioselective synthesis of acyclic quaternary carbons via organocatalytic addition of organoborates to (Z)-Enediketones.

The chemical synthesis of molecules with closely packed atoms having their bond coordination saturated is a challenge to synthetic chemists, especially when three-dimensional control is required. The organocatalyzed asymmetric synthesis of acyclic alkenylated, alkynylated and heteroarylated quaternary carbon stereocenters via 1,4-conjugate addition is here catalyzed by 3,3´-bisperfluorotoluyl-BINOL. The highly useful products (31 examples) are produced in up to 99% yield and 97:3 er using enediketone substrates and potassium trifluoroorganoborate nucleophiles.

Biosensing with Polymerase Chain Reaction-Stable DNA-Functionalized Magnetically Susceptible Carbon-Iron Microparticles.

We demonstrate a rapid and sensitive method for DNA detection without the need for fluorescence. This is based on carbon-coated magnetic iron (Fe) microparticles with a covalent surface attachment of DNA. We show that these magnetic microparticles can capture complementary DNA.

Correction: Voltammetric pH sensor based on electrochemically modified pseudo-graphite.

Correction for 'Voltammetric pH sensor based on electrochemically modified pseudo-graphite' by Haoyu Zhu , , 2020, , 7252-7259, https://doi.org/10.1039/D0AN01405B.

Plant traits poorly predict winner and loser shrub species in a warming tundra biome.

Climate change is leading to species redistributions. In the tundra biome, shrubs are generally expanding, but not all tundra shrub species will benefit from warming. Winner and loser species, and the characteristics that may determine success or failure, have not yet been fully identified.

Enantioselective Total Synthesis of Cannabinoids via a Tandem Conjugate Addition/Enolate Alkylation Annulation with Ambiphilic Organoboronates.

Cannabinoid research depends on synthesizing derivatives for structure-activity relationship studies. (-)-Δ-Tetrahydrocannabinol and cannabidiol were synthesized via a tandem enantioselective conjugate addition/enolate alkylation annulation with a novel ambiphilic trifluoroborate in seven steps. A new class of alkenyl and aryl ambiphilic trifluoroborates were synthesized and showed great compatibility with various functional groups, high yields, and excellent enantio- and diastereoselectivity.

Quaternary and Tertiary Carbon Centers Synthesized via Gallium-Catalyzed Direct Substitution of Unfunctionalized Propargylic Alcohols with Boronic Acids.

(IPr)GaCl/AgSbF, AgSbF, and GaCl catalyzed substitution of the hydroxyl of secondary and tertiary propargylic alcohols with organoboronic acids via C-C bond formation, and GaCl effectively synthesized all-carbon quaternary propargylic centers. These catalysts performed the substitution at carbons bearing alkyl substituents, which has been problematic for other systems. Highly hindered carbon stereocenters were thus produced, including quaternary centers bearing doubly ortho-substituted aryl rings, that are difficult to access with traditional methods.

Enantioselective Organocatalytic Conjugate Addition in a Tandem Synthesis of δ-Substituted Cyclohexenones and Four-Step Total Synthesis of Penienone.

A bisperfluorotoluyl-BINOL catalyzed conjugate addition of trifluoroborate salts to doubly vinylogous esters and aldol condensation synthesized chiral δ-substituted cyclohexenones with high yields and enantioselectivities (10 examples, up to 89% yield, 89-98% ee). Stepwise and single-pot sequences were developed, with the former also providing β-substituted masked ketoaldehydes containing a vinyl ether. The transformation was used in a four-step total synthesis of penienone (24% overall yield), ≤ half the steps as in previous syntheses.

Experimental and Computational Mechanistic Study of Carbonazidate-Initiated Cascade Reactions.

A variety of Huisgen cyclization or nitrene/carbene alkyne cascade reactions with different types of termination were investigated. Accessible nitrene precursors were assessed, and carbonazidates were found to be the only effective initiators. Solvents, terminal alkynyl substituents, and catalysts can all impact the reaction outcome.

Simulated hurricane-induced changes in light and nutrient regimes change seedling performance in Everglades forest-dominant species.

Wind damage from cyclones can devastate the forest canopy, altering environmental conditions in the understory that affect seedling growth and plant community regeneration. To investigate the impact of hurricane-induced increases in light and soil nutrients as a result of canopy defoliation, we conducted a two-way factorial light and nutrient manipulation in a shadehouse experiment. We measured seedling growth of the dominant canopy species in the four Everglades forest communities: pine rocklands ( var ), cypress domes (), hardwood hammocks, and tree islands ( and ).

A Comparison of Solid Electrolyte Interphase Formation and Evolution on Highly Oriented Pyrolytic and Disordered Graphite Negative Electrodes in Lithium-Ion Batteries.

The presence and stability of solid electrolyte interphase (SEI) on graphitic electrodes is vital to the performance of lithium-ion batteries (LIBs). However, the formation and evolution of SEI remain the least understood area in LIBs due to its dynamic nature, complexity in chemical composition, heterogeneity in morphology, as well as lack of reliable in situ/operando techniques for accurate characterization. In addition, chemical composition and morphology of SEI are not only affected by the choice of electrolyte, but also by the nature of the electrode surface.

Engineering Escherichia coli for anaerobic alkane activation: Biosynthesis of (1-methylalkyl)succinates.

In anoxic environments, microbial activation of alkanes for subsequent metabolism occurs most commonly through the addition of fumarate to a subterminal carbon, producing an alkylsuccinate. Alkylsuccinate synthases are complex, multi-subunit enzymes that utilize a catalytic glycyl radical and require a partner, activating enzyme for hydrogen abstraction. While many genes encoding putative alkylsuccinate synthases have been identified, primarily from nitrate- and sulfate-reducing bacteria, few have been characterized and none have been reported to be functionally expressed in a heterologous host.

Experimental warming differentially affects vegetative and reproductive phenology of tundra plants.

Rapid climate warming is altering Arctic and alpine tundra ecosystem structure and function, including shifts in plant phenology. While the advancement of green up and flowering are well-documented, it remains unclear whether all phenophases, particularly those later in the season, will shift in unison or respond divergently to warming. Here, we present the largest synthesis to our knowledge of experimental warming effects on tundra plant phenology from the International Tundra Experiment.

Rate Dependence on Inductive and Resonance Effects for the Organocatalyzed Enantioselective Conjugate Addition of Alkenyl and Alkynyl Boronic Acids to β-Indolyl Enones and β-Pyrrolyl Enones.

Two key factors bear on reaction rates for the conjugate addition of alkenyl boronic acids to heteroaryl-appended enones: the proximity of inductively electron-withdrawing heteroatoms to the site of bond formation and the resonance contribution of available heteroatom lone pairs to stabilize the developing positive charge at the enone β-position. For the former, the closer the heteroatom is to the enone β-carbon, the faster the reaction. For the latter, greater resonance stabilization of the benzylic cationic charge accelerates the reaction.

Formation of β-Oxo--vinylimidates via Intermolecular Ester Incorporation in Huisgen Cyclization/Carbene Cascade Reactions.

Unusual intermolecular trapping of esters by carbenes generated via a Huisgen cyclization/retroelectrocyclization/dediazotization cascade reaction is presented. β-Oxo--vinylimidates could be obtained in one step from propargyl carbonazidates. Mechanistic control experiments suggested reversible dipole formation by ester addition to the carbene, and nitrogen attack to the ester carbonyl was irreversibly followed by stereoselective decarboxylative elimination to give the -vinyl imidate.

Voltammetric pH sensor based on electrochemically modified pseudo-graphite.

A nanocrystalline graphite-like amorphous carbon (graphite from the University of Idaho thermolyzed asphalt reaction, GUITAR) shares morphological features with classical graphites, including basal and edge planes (BP, EP). However, unlike graphites and other sp2-hybridized carbons, GUITAR has fast heterogenous electron transfer (HET) across its basal planes, and resistance to corrosion similar to sp3-C and boron-doped diamond electrodes. In this contribution, quinoid modified BP-GUITAR (q-GUITAR) is examined as a sensor for pH determination.

NDVI Changes Show Warming Increases the Length of the Green Season at Tundra Communities in Northern Alaska: A Fine-Scale Analysis.

A warming Arctic has been associated with increases in aboveground plant biomass, specifically shrubs, and changes in vegetation cover. However, the magnitude and direction of changes in NDVI have not been consistent across different tundra types. Here we examine the responsiveness of fine-scale NDVI values to experimental warming at eight sites in northern Alaska, United States.

Synthesis of Bridged Azacycles and Propellanes via Nitrene/Alkyne Cascades.

A nitrene/alkyne cascade reaction terminating in C-H bond insertion to form functionalized bridged azacycles from carbonazidates is presented. Due to an initial Huisgen cyclization, all carbonazidates reacted with the alkyne in an exo mode in contrast to the use of sulfamate esters, which react predominately in an endo mode. Substrates with different ring sizes as well as different aryl and heteroaryl groups were also explored.

Relay Catalysis To Synthesize β-Substituted Enones: Organocatalytic Substitution of Vinylogous Esters and Amides with Organoboronates.

Organocatalysis was shown to facilitate conjugate additions to vinylogous esters and amides for the first time. Subsequent elimination of a β-alcohol or amine provided π-conjugated β-substituted enones. Remarkably, nucleophile addition to the electron-rich vinylogous substrates is more rapid than classical enones, forming monosubstituted products.

Oxyallyl Cation Capture via Electrophilic Deborylation of Organoboronates: Access to ,'-Substituted Cyclic Ketones.

An umpolung strategy to synthesize ,'-substituted cyclic ketones through the nucleophilic addition of organoboronates to -hydroxyl silyl enol ethers is described. The reaction proceeds via the trapping of in situ generated oxyallyl cations via the electrophilic deborylation of C(sp) and C(sp) borates. This efficient and straightforward method provides direct access to -substituted silyl enol ethers in high yield with complete regioselectivity.

Chiral Diol-Based Organocatalysts in Enantioselective Reactions.

Organocatalysis has emerged as a powerful synthetic tool in organic chemistry in the last few decades. Among various classes of organocatalysis, chiral diol-based scaffolds, such as BINOLs, VANOLs, and tartaric acid derivatives, have been widely used to induce enantioselectivity due to the ability of the hydroxyls to coordinate with the Lewis acidic sites of reagents or substrates and create a chiral environment for the transformation. In this review, we will discuss the applications of these diol-based catalysts in different types of reactions, including the scopes of reactions and the modes of catalyst activation.

Branched Amine Synthesis via Aziridine or Azetidine Opening with Organotrifluoroborates by Cooperative Brønsted/Lewis Acid Catalysis: An Acid-Dependent Divergent Mechanism.

A practical catalytic method to synthesize β,β- and γ,γ-substituted amines by opening aziridines and azetidines, respectively, using alkenyl, alkynyl, or aryl/heteroaryl trifluoroborate salts is described. This reaction features simple open-flask reaction conditions, the use of transition-metal-free catalysis, complete regioselectivity, and high diastereoselectivity. Preliminary mechanistic studies suggest that carbocation formation is disfavored.

Tertiary and Quaternary Carbon Formation via Gallium-Catalyzed Nucleophilic Addition of Organoboronates to Cyclopropanes.

GaCl and (IPr)GaCl/AgSbF formed γ-tertiary and γ-quaternary carbons via homoconjugate addition of organoboron nucleophiles to diester- and ketone-functionalized cyclopropanes. Electron donor group cyclopropane substituents were not needed, allowing electron-deficient aryl, alkenyl, alkyl, and hydrogen-substituted cyclopropanes to be used. The catalytic conditions were compatible with alkenyl, alkynyl, and aryl nucleophiles, including ortho-substituted aromatics, to synthesize highly hindered quaternary carbons.

Synthesis of azasilacyclopentenes and silanols Huisgen cycloaddition-initiated C-H bond insertion cascades.

An unusual transition metal-free cascade reaction of alkynyl carbonazidates was discovered to form azasilacyclopentenes. Mild thermolysis afforded the products a series of cyclizations, rearrangements, and an α-silyl C-H bond insertion (rather than the more common Wolff rearrangement, 1,2-shift, or β-silyl C-H insertion) to form silacyclopropanes. A mechanistic proposal for the sequence was informed by control experiments and the characterization of reaction intermediates.