Dehydrierende Oxidation von Indolinen und anderen Heterocyclen durch frustrierte Lewis-Paare

Enhancing polyol/sugar cascade oxidation to formic acid with defect rich MnO2 catalysts

Oxidation of renewable polyol/sugar into formic acid using molecular O₂ over heterogeneous catalysts is still challenging due to the insufficient activation of both O₂ and organic substrates on coordination-saturated metal oxides. In this study, we develop a defective MnO₂ catalyst through a coordination number reduction strategy to enhance the aerobic oxidation of various polyols/sugars to formic acid. Compared to common MnO₂, the tri-coordinated Mn in the defective MnO₂ catalyst displays the electronic reconstruction of surface oxygen charge state and rich surface oxygen vacancies. These oxygen vacancies create more Mn^δ+ Lewis acid site together with nearby oxygen as Lewis base sites. This combined structure behaves much like Frustrated Lewis pairs, serving to facilitate the activation of O₂, as well as C-C and C-H bonds. As a result, the defective MnO₂ catalyst shows high catalytic activity (turnover frequency: 113.5 h^-1) and formic acid yield (>80%) comparable to noble metal catalysts for glycerol oxidation. The catalytic system is further extended to the oxidation of other polyols/sugars to formic acid with excellent catalytic performance.

Sigmatropic [1,5] Carbon Shift of Transient C3 Ammonium Enolates

The stereospecific sigmatropic [1,5] carbon shift of C3 ammonium enolates is discovered. According to mechanistic, kinetic and computational experiments, this new rearrangement proceeds via the catalytic generation of a transient C3 ammonium enolate by intramolecular aza‐Michael addition. This intermediate rapidly undergoes [1,5] sigmatropic carbon migration to furnish the respective tetrahydroquinoline‐4‐ones with excellent diastereoselectivities of d.r. >99 : 1 and in 61–98 % yield.

Recent advances in the synthesis of N-heteroarenes via catalytic dehydrogenation of N-heterocycles

Bio-active molecules having N-heteroarene core are widely used for numerous medicinal applications and as lifesaving drugs. In this direction, dehydrogenation of partially saturated aromatic N-heterocycles shows utmost importance for the synthesis of heterocycles. This feature article highlights the recent advances, from 2009 to April 2021, on the dehydrogenation of N-heteroaromatics. Notable features considering the development of newer catalysis for dehydrogenations are: (i) approaches based on precious metal catalysis, (ii) newer strategies and catalyst development technology using non-precious metal-catalysts for N-heterocycles having one or more heteroatoms, (iii) Synthesis of five or six-membered N-heterocycles using photocatalysis, electrocatalytic, and organo-catalytic approaches using different homogeneous and heterogeneous conditions' (iv) metal free (base and acid-promoted) dehydrogenation along with I₂, N-hydroxyphthalimide (NHPI) and bio catalyzed miscellaneous examples have also been discussed, (v) mechanistic studies for various dehydrogenation reactions and (vi) synthetic applications of various bio-active molecules including post-drug derivatization are discussed.

Bifunctional Borane Catalysis of a Hydride Transfer/Enantioselective [2+2] Cycloaddition Cascade

Herein, we present a mild and efficient method for synthesizing enantioenriched tetrahydroquinoline-fused cyclobutenes through a cascade reaction between 1,2-dihydroquinolines and alkynones with catalysis by chiral spiro-bicyclic bisboranes. The bisboranes served two functions: first they catalyzed a hydride transfer to convert the 1,2-dihydroquinoline substrate to a 1,4-dihydroquinoline, and then they activated the alkynone substrate for an enantioselective [2+2] cycloaddition reaction with the 1,4-dihydroquinoline generated in situ.

Multi-Component Metal-Organic Frameworks Significantly Boost Visible-Light-Driven Hydrogen Production Coupled with Selective Organic Oxidation

Visible-light-driven hydrogen production coupled with selective organic oxidation has attracted increasing attention, as it not only provides clean and renewable energy, but also utilizes the other half reaction to achieve some value-added organic chemicals. Metal-organic frameworks based on metal clusters and organic ligands self-assembly give a perspective on the formation of multifunctional heterogeneous photocatalyst to significantly boost visible-light photocatalytic activities under mild conditions. By incorporating two types of photoactive units, tricarboxytriphenylamine (H₃ TCA) and tris(4-(pyridinyl)phenyl)amine (NPy₃ ), into a single metal-organic frameworks, a multi-component MOF Co-MIX was obtained. With the redox active metal centers enabling the photoexcitation reduction of protons into hydrogen and the photogenerated holes promoting considerable oxidation of substrates, the resulting Co-MIX exhibits high catalytic activity for the photocatalytic hydrogen production coupled with selective oxidation of benzylamine or 1,2,3,4-tetrahydroisoquinoline. Importantly, the photocatalytic experiments of single-component Co-TCA and Co-NPy₃ verified the positive synergistic effects on stability and photocatalytic ability of the two ligands (H₃ TCA and NPy₃ ) in one single MOF, revealing that the multi-component strategy is very important for the efficient charge separation and excellent photocatalytic activity of the catalyst.

Consecutive β,β'-Selective C(sp3 )-H Silylation of Tertiary Amines with Dihydrosilanes Catalyzed by B(C6 F5 )3

Tris(pentafluorophenyl)borane has been found to catalyze the two-fold C(sp3 )-H silylation of various trialkylamine derivatives with dihydrosilanes, furnishing the corresponding 4-silapiperidines in decent yields. The multi-step reaction cascade involves amine-to-enamine dehydrogenation at two alkyl residues and two electrophilic silylation reactions of those enamines, one inter- and one intramolecular.

More Twins in the Scientific Literature of the 21st Century

This Essay highlights the complex issue of twinning in science publications. Historical accounts present cases where two scientists focused on the same problem and came up with the same solution following different paths. This has changed in the present day. The concurrent publication of rather similar research papers from different groups has increased in frequency since 2010. In the past, twinning in research publications was serendipitous, and there was a healthy competition among teams working on similar projects. Today, twinning has become more frequent. This can be attributed to the urge of researchers to have publications on popular topics, the tendency to base research programs on popular keywords, and funding agencies preferentially supporting certain areas of research. With vast amounts of literature being generated, editorial offices and referees may not be able to find these twins very easily. As we inch away from human ingenuity towards artificial intelligence, twinning may become even more frequent.

Auto-Tandem Catalysis with Frustrated Lewis Pairs for Reductive Etherification of Aldehydes and Ketones

Herein we report that a single frustrated Lewis pair (FLP) catalyst can promote the reductive etherification of aldehydes and ketones. The reaction does not require an exogenous acid catalyst, but the combined action of FLP on H₂ , R-OH or H₂ O generates the required Brønsted acid in a reversible, "turn on" manner. The method is not only a complementary metal-free reductive etherification, but also a niche procedure for ethers that would be either synthetically inconvenient or even intractable to access by alternative synthetic protocols.

Intermolecular Redox-Neutral Amine C-H Functionalization Induced by the Strong Boron Lewis Acid B(C6 F5 )3 in the Frustrated Lewis Pair Regime

N,N-Dimethylmesitylamine undergoes an intermolecular redox-neutral C-H activation/C-C coupling process upon treatment with dimethyl acetylenedicarboxylate and the strong boron Lewis acid B(C₆ F₅ )₃ . Similarly, N,N-dimethylmesitylamine reacts with two molar equivalents of ethyl acrylate to give the respective unsaturated coupling product with H₂ transfer to the acrylic ester to form the ethyl propionate/B(C₆ F₅ )₃ adduct. N,N-Dimethylmesitylamine also undergoes a C-H activation at the benzylic ortho sp³ -carbon atom with dihydrogen formation upon treatment with Piers' borane [HB(C₆ F₅ )₂ ]. The last two reactions of N,N-dimethylmesitylamine were analyzed by DFT calculations.

Catalytic Friedel-Crafts C-H Borylation of Electron-Rich Arenes: Dramatic Rate Acceleration by Added Alkenes

In the electrophilic C-H borylation of electron-rich aromatic compounds with catecholborane, the catalytic generation of the boron electrophile is initiated by heterolysis of the B-H bond by various Lewis and Brønsted acids, with a boronium ion formed exclusively. After ligand dissociation, the corresponding borenium ion undergoes regioselective electrophilic aromatic substitution on aniline derivatives as well as nitrogen-containing heterocycles. The catalysis is optimized using B(C₆ F₅ )₃ as the initiator and proceeds without the addition of an external base or dihydrogen acceptor. Temperatures above 80 °C are generally required to secure efficient turnover in these Friedel-Crafts-type reactions. Mechanistic experiments reveal that regeneration of the boronium/borenium ion with dihydrogen release is rate-determining. This finding finally led to the discovery that, with added alkenes, catalytic C-H borylations can, for the first time, be carried out at room temperature.

A Frustrated and Confused Lewis Pair

We report a new class of frustrated Lewis pairs (FLPs) by the hydroboration of bulky isocyanates ^iPr2 ArNCO (^iPr2 Ar=2,6-iPr₂ C₆ H₃ ) and ^Ph2tBu ArNCO (^Ph2tBu Ar=2,6-Ph₂ -4-tBuC₆ H₂ ) with Piers' borane (HB(C₆ F₅ )₂ ). While hydroboration of smaller isocyanates such as ^iPr2 ArNCO leads to isocyanate-N/B FLP adducts, hydroboration of the bulkier ^Ph2tBu ArNCO allows isolation of the substrate-free aminoborane with a short, covalent N-B bond. This confused FLP reversibly binds unsaturated substrates such as isocyanates and isocyanides, suggesting the intermediacy of a "normal" FLP along the reaction pathway, supported by high-level DFT studies and variable-temperature NMR spectroscopy. These results underscore the possibility of FLP behavior in systems that possess no obvious frustrated Lewis acid-base interaction.