Asymmetric α‑Photoalkylation of β‑Ketocarbonyls by Primary Amine Catalysis: Facile Access to Acyclic All-Carbon Quaternary Stereocenters

We describe the direct construction of all-carbon quaternary stereocenters via α-photoalkylation of β-ketocarbonyls with high efficacy and enantioselectivities by merging photoredox catalysis and primary amine catalysis. The open-shell photoradical approach enables asymmetric α-alkylations that are difficult under thermal conditions

Asymmetric Retro-Claisen Reaction by Chiral Primary Amine Catalysis

The communication describes an enamine-based asymmetric retro-Claisen reaction of β-diketones by primary amine catalysis. The reaction proceeds via a sequence of stereoselective C–C formation, C–C cleavage, and a highly stereospecific enamine protonation to afford chiral α-alkylated ketones or macrolides with high yields and enantioselectivities. A detailed mechanism was explored on the basis of experimental evidence and computational studies to account for the observed stereocontrol

Enantioselective Decarboxylative α‑Alkynylation of β‑Ketocarbonyls via a Catalytic α‑Imino Radical Intermediate

A distinctive aminocatalysis via α-imino radical is reported on the basis of SET oxidation of a secondary enamine. The combination of chiral primary amine catalysis and visible-light photoredox catalysis enables the enantioselective decarboxylative coupling of propiolic acid and β-ketocarbonyls to afford alkynylation adducts with high enantioselectivity. Mechanism studies indicate the reaction proceeds via an α-imino radical addition

Chiral Primary Amine Catalyzed Asymmetric Michael Addition of Malononitrile to α‑Substituted Vinyl Ketone

The first efficient and highly enantioselective Michael addition–protonation reaction of malononitriles to α-substituted vinyl ketones has been developed by using a chiral primary amine as the organocatalyst. With a Hantzsch ester as the hydride source, an enantioselective tandem reduction, Michael addition–protonation reaction of benzylidene­malono­nitrile has also been achieved with good yields and high enantioselectivities

Fabrication of a Novel and High-Performance Mesoporous Ethylene Tar-Based Solid Acid Catalyst for the Dehydration of Fructose into 5‑Hydroxymethylfurfural

In this article, a novel and high-performance mesoporous carbon-based solid acid catalyst was prepared using ethylene tar (ET) as a precursor, which is a byproduct of ethylene production. First, ET was carbonized at 550 °C by using magnesium acetate as the template. After that, the mesoporous ET-based solid acid catalyst was obtained by a one-step sulfonation process that removes the templates simultaneously. On the basis of these facts, the maximum yield of 5-hydroxymethylfurfural (5-HMF) in the presence of an ET catalyst during the dehydration of fructose can reach 87.8%. This effective catalytic activity is mainly attributed to the large specific surface area and high density of sulfonic acid groups existing in the ET catalyst. Moreover, no distinct activity drop was observed during five recycling runs that confirmed good recyclability and thermal stability of the ET catalyst. This research provides a novel and promising method for the utilization of ET as a low-cost, recyclable, and high-performance catalyst

Three-Coordinate Cobalt(IV) and Cobalt(V) Imido Complexes with N‑Heterocyclic Carbene Ligation: Synthesis, Structure, and Their Distinct Reactivity in C–H Bond Amination

The reaction of the cobalt(0) alkene complex [(IMes)­Co­(η²:η²-dvtms)] (<b>1</b>) (IMes = 1,3-bis­(1′,3′,5′-trimethylphenyl)­imidazol-2-ylidene, dvtms = divinyltetramethyldisiloxane) with 2 equiv of DippN₃ (Dipp = 2,6-diisopropylphenyl) afforded the cobalt­(IV) imido complex [(IMes)­Co­(NDipp)₂] (<b>2</b>), which could be oxidized by [Cp₂Fe]­[BAr^F₄] (Ar^F = 3,5-di­(trifluoromethyl)­phenyl) to give the cobalt­(V) imido species [(IMes)­Co­(NDipp)₂]­[BAr^F₄] (<b>3</b>). The molecular structures of all these complexes were established by single-crystal X-ray diffraction studies. Characterization data and theoretical calculations suggest ground spin states of <i>S</i> = ¹/₂ and <i>S</i> = 0 for the cobalt­(IV) and cobalt­(V) species, respectively. When heated, the cobalt­(IV) imido species was converted to a cobalt­(II) diamido complex via an intramolecular C–H bond amination reaction, but the cobalt­(V) species was stable under similar conditions. The different outcomes suggest that a high oxidation state does not guarantee C–H bond activation reactivity of late-transition-metal imido species

Asymmetric Enamine Catalysis with β‑Ketoesters by Chiral Primary Amine: Divergent Stereocontrol Modes

α-Branched ketones remain a challenging type of substrates in aminocatalysis due to their congested structures as well as the associated difficulties in controlling chemo- and stereoselectivity. In this work, we have explored asymmetric aminocatalysis with α-substituted β-ketoesters. A simple chiral primary amine catalyst was identified to enable unprecedentedly effective catalysis of β-ketoesters in α-hydrazination and Robinson annulation reaction with good yields and high enantioselectivities. Stoichiometric experiments with preformed enamine ester intermediates revealed their enamine-catalytic nature as well as the critical roles of acidic additives in facilitating catalytic turnovers and in tuning the chemo- and stereoselectivity. With the identical catalytic system, the two reactions demonstrated opposite chiral inductions in terms of the absolute configurations of the newly formed stereogenic centers. Investigations into this intriguing issue by DFT have revealed divergent stereocontrol modes. For α-hydrazination, H-bonding-directed facial attack determines the stereoselectivity, whereas a steric model is applied to the Robinson annulation where dual activations of both β-ketoester and vinyl ketone/aldehyde are involved

Medium-Range Order in Sol–Gel Prepared Al₂O₃–SiO₂ Glasses: New Results from Solid-State NMR

The medium-range order of 0.5Al₂O₃–<i>x</i>SiO₂ glasses (1 ≤ <i>x</i> ≤ 6) prepared via a new sol–gel route from the Al lactate precursor has been studied by ²⁹Si and ²⁷Al single- and double-resonance solid-state NMR techniques. For high-alumina samples Si–O–Al connectivities are detected by ²⁹Si MAS NMR as well as by ²⁹Si­{²⁷Al} rotational echo adiabatic passage double-resonance (REAPDOR) spectroscopy. To boost the signal-to-noise ratio, the REAPDOR experiment was combined with a Carr–Purcell–Meiboom–Gill (CPMG) echo train acquisition. While all five silicon units Q⁽⁴⁾_<i>m</i>Al (0 ≤ <i>m</i> ≤ 4) are detectable in appreciable concentrations for <i>x</i> = 1, the spectra indicate that the average number of Al species bound to silicon, ⟨<i>m</i>_Al⟩, gradually decreases toward higher <i>x</i> values, as expected. The ²⁷Al MAS NMR spectra reveal four-, five-, and six-coordinated aluminum in these glasses. For <i>x</i> ≥ 3, the Al species detected are essentially independent of sample composition indicating a constant structural environment of Al. In contrast, for <i>x</i> = 1 and 2, an increase in the ²⁷Al isotropic chemical shifts suggests an increasing number of Al···Al proximities. Consistent with this finding, two-dimensional ²⁷Al–²⁷Al double-quantum/single-quantum correlation spectroscopy reveals spatial proximities among and between all types of aluminum species present. On the basis of the complementary evidence from these single- and double-resonance experiments, a model for the medium-range order of these glasses is developed

Pressure-Induced Structural and Optical Properties of Inorganic Halide Perovskite CsPbBr₃

Perovskite photovoltaic materials are gaining sustained attention because of their excellent photovoltaic properties and extensive practical applicability. In this Letter, we discuss the changes in the structure and optical properties of CsPbBr₃ under high pressure. As the pressure increased, the band gap initially began to red shift before 1.0 GPa followed by a continuous blue shift until the crystal was completely amorphized. An isostructural phase transition at 1.2 GPa was determined by high-pressure synchrotron X-ray and Raman spectroscopy. The result could be attributed to bond length shrinkage and PbBr₆ octahedral distortion under high pressure. The amorphization of the crystal was due to the severe distortion and tilt of the PbBr₆ octahedron, leading to broken long-range order. Changes in optical properties are closely related to the evolution of the crystal structure. Our discussion shows that high-pressure study can be used as an effective means to tune the structure and properties of all-inorganic halide perovskites

Chiral Primary Amine Catalyzed Asymmetric Michael Addition of Malononitrile to α‑Substituted Vinyl Ketone

The first efficient and highly enantioselective Michael addition–protonation reaction of malononitriles to α-substituted vinyl ketones has been developed by using a chiral primary amine as the organocatalyst. With a Hantzsch ester as the hydride source, an enantioselective tandem reduction, Michael addition–protonation reaction of benzylidene­malono­nitrile has also been achieved with good yields and high enantioselectivities