LongLarf

Nature Communications, Published online: 19 May 2020; doi:10.1038/s41467-020-16283-9

One way or another! Quantum chemical activation strain analyses reveal that the regioselectivity of the classical textbook acid‐ and base‐catalyzed epoxide ring‐opening reactions are controlled by either strain (acidic regime: weak interactions) or steric interactions (basic regime: strong interactions). Our findings provide a concrete quantitative framework for understanding these indispensable textbook reactions.

We have quantum chemically analyzed the ring‐opening reaction of the model non‐symmetrical epoxide 2,2‐dimethyloxirane under basic and acidic conditions using density functional theory at OLYP/TZ2P. For the first time, our combined activation strain and Kohn–Sham molecular orbital analysis approach have revealed the interplay of physical factors that control the regioselectivity of these chemical reactions. Ring‐opening under basic conditions occurs in a regime of strong interaction between the nucleophile (OH^–) and the epoxide and the interaction is governed by the steric (Pauli) repulsion. The latter steers the attack preferentially towards the sterically less encumbered C^β. Under acidic conditions, the interaction between the nucleophile (H₂O) and the epoxide is weak and, now, the regioselectivity is governed by the activation strain. Protonation of the epoxide induces elongation of the weaker (CH₃)₂C^α–O bond, and effectively predistorts the substrate for the attack at the sterically more hindered side, which goes with a less destabilizing overall strain energy. Our quantitative analysis significantly builds on the widely accepted rationales behind the regioselectivity of these ring‐opening reactions and provide a concrete framework for understanding these indispensable textbook reactions.

This minireview summarizes the recent applications of halogen, chalcogen, and pnictogen bonding in organocatalysis. In all these cases, a Lewis‐acidic region around an element of group 15–17 (the so‐called σ‐hole) interacts with the electron pair of a suitable Lewis base. Besides a discussion of recent applications, this review focuses on mechanistic investigations and comparisons between the different σ‐hole interactions.

Noncovalent interactions like halogen, chalcogen, and pnictogen bonding are known for a very long time. During the last decade, these interactions have found different applications in catalysis. These forces are often called σ‐hole interactions which can be explained by the anisotropic distribution of the electron density around these atoms. In this MiniReview, we will present recent applications of halogen, chalcogen, and pnictogen bonding in catalysis and discuss experimental and computational investigations to gain more insights into the underlying mechanisms.

Amines with the help of molybdenum: A series of Mo pincer complexes was synthesized and structurally characterized. Their potential as catalysts for the hydrogenation of aromatic nitriles to the corresponding primary amines is demonstrated.

Abstract

A series of molybdenum(0), (I) and (II) complexes ligated by different PNP and NNN pincer ligands were synthesized and structurally characterized. Along with previously described Mo−PNP complexes Mo‐1 and Mo‐2, all prepared compounds were tested in the catalytic hydrogenation of aromatic nitriles to primary amines. Among the applied catalysts, Mo‐1 is particularly well suited for the hydrogenation of electron‐rich benzonitriles. Additionally, two aliphatic nitriles were transformed into the desired products in 80 and 86 %, respectively. Moreover, catalytic intermediate Mo‐1a was isolated and its role in the catalytic cycle was subsequently demonstrated.

Two birds with one stone: A catalytic system for the synthesis of dicarboxylic acids from aqueous solutions of diols accompanied by the evolution of hydrogen is developed. An iridium complex bearing a functional bipyridonate ligand exhibits a high catalytic performance for this reaction. Selective dehydrogenative oxidation of vicinal diols to give α‐hydroxycarboxylic acids is also accomplished.

Abstract

A catalytic system for the synthesis of dicarboxylic acids from aqueous solutions of diols accompanied by the evolution of hydrogen was developed. An iridium complex bearing a functional bipyridonate ligand with N,N‐dimethylamino substituents exhibited a high catalytic performance for this type of dehydrogenative reaction. For example, adipic acid was synthesized from an aqueous solution of 1,6‐hexanediol in 97 % yield accompanied by the evolution of four equivalents of hydrogen by the present catalytic system. It should be noted that the simultaneous production of industrially important dicarboxylic acids and hydrogen, which is useful as an energy carrier, was achieved. In addition, the selective dehydrogenative oxidation of vicinal diols to give α‐hydroxycarboxylic acids was also accomplished.

Cycling with polymers: This Review discusses the recent advances made in the cycloaddition reaction of captured CO₂ with epoxides over a variety of imidazolium‐functionalized organic cationic polymers as a class of eminent heterogeneous catalysts.

Abstract

The cycloaddition reaction of CO₂ with various epoxides to generate cyclic carbonates is one of the most promising and efficient approaches for CO₂ fixation. Typical imidazolium‐based ionic liquids possessing electrophilic cations and nucleophilic halogen anions have been identified as excellent and environmentally friendly candidates for synergistically activating epoxides to convert CO₂. Therefore, the feasible construction of a series of imidazolium‐functionalized organic cationic polymers can bridge the gap between homogeneous and heterogeneous catalysis, thereby obtaining highly selective CO₂ adsorption and simultaneous conversion ability. This Review describes the recent advancements made with regard to the design and synthesis of this type of polymeric networks having imidazolium functionality. They are considered as an outstanding heterogeneous catalyst for the cycloaddition of CO₂ to epoxides. Based on the perspective from the design of building blocks to the synthesis of cationic polymers, the focus mainly lies on how to introduce imidazole units into the material backbone via a covalent linking approach and how to incorporate other active sites capable of activating CO₂ and/or epoxides into such polymeric materials.

A very old reaction in a new light: A very old reaction is the formation of the hexapotassium salt of hexahydroxybenzene C₆(OK)₆ by the interaction of metallic K and CO. To date, only speculations exist about the reaction pathway for its formation. A novel concept is presented, based on metalla‐dioxo‐cyclobutadiynes (cis‐2,5‐dioxo‐butatriene as formal ethylenedione O=C=C=O complexes) in analogy to the experience to the well characterized all‐C‐metallacyclopentynes.

Abstract

For linear and cyclic coupling reactions of CO, among other products, the formation of the hexapotassium salt of hexahydroxybenzene is of particular interesting. The interaction of metallic potassium and CO offers, via the assumed K[OC≡CO]K as the result of several carbon monoxide coupling reactions, the formation of C₆(OK)₆ among other products. To date, only speculations exist about the reaction pathway for these products, which were first described by Liebig in 1834. A novel concept is suggested here, which consists of the single steps (i) reductive coupling of CO, (ii) formation of dihetero‐metallacyclopentynes (cis‐2,5‐diheterobutatriene as formal ethylenedione O=C=C=O complexes), (iii) formation of its dinuclear 1‐metalla‐2,5‐dioxo‐cyclopentyne complexes by external coordination of the triple bond, (iv) insertion of CO into the M−C bond of the formed metallacyclopropene, and (v) the reductive elimination of C₆(OK)₆. The novel aspect of this concept is the formation of dihetero‐metallacyclopentynes (in analogy to the well characterized all‐C‐metallacyclopentynes), which have not been considered in the mechanism of reductive CO coupling reactions. It is expected that the presence of transition‐metal impurities would promote the reaction.

Diazaphospholenes and diazaarsolenes are heterocyclic phosphorus and arsenic species, respectively. Recently these compounds and their corresponding cationic analogues have been used as pre‐catalysts for a wide range of reduction transformations, owing to the hydridic P‐H/ As−H bond of the active catalyst. This review discusses why these heterocycles make such good catalysts and highlights the examples currently present in the literature.

Abstract

The past 20 years has seen significant advances in main group chemistry and their use in catalysis. This Minireview showcases the recent emergence of phosphorus and arsenic containing heterocycles as catalysts. With that, we discuss how the Group 15 compounds diazaphospholenes, diazaarsolenes, and their cationic counterparts have proven to be highly effective catalysts for a wide range of reduction transformations. This Minireview highlights how the initial discovery by Gudat of the hydridic nature of the P−H bond in these systems led to these compounds being used as catalysts and discusses the wide range of examples currently present in the literature.

Getting poly(aminoborane)s : The well‐known Fe amido complex [(PNP)Fe(H)(CO)] (PNP=N(CH₂CH₂Pi Pr₂)₂) is a potent catalyst for the selective formation of poly(aminoborane)s from methylamine borane and a SiMe₃‐substituted analogue. Mechanistic studies support a chain‐growth mechanism with polymer formation by nucleophilic attack at the terminus of a growing B−N chain.

Abstract

Dehydropolymerisation of methylamine borane (H₃B⋅NMeH₂) using the well‐known iron amido complex [(PNP)Fe(H)(CO)] (PNP=N(CH₂CH₂Pi Pr₂)₂) (1 ) gives poly(aminoborane)s by a chain‐growth mechanism. In toluene, rapid dehydrogenation of H₃B⋅NMeH₂ following first‐order behaviour as a limiting case of a more general underlying Michaelis–Menten kinetics is observed, forming aminoborane H₂B=NMeH, which selectively couples to give high‐molecular‐weight poly(aminoborane)s (H₂BNMeH) _n and only traces of borazine (HBNMe)₃ by depolymerisation after full conversion. Based on a series of comparative experiments using structurally related Fe catalysts and dimethylamine borane (H₃B⋅NMe₂H) polymer formation is proposed to occur by nucleophilic chain growth as reported earlier computationally and experimentally. A silyl functionalised primary borane H₃B⋅N(CH₂SiMe₃)H₂ was studied in homo‐ and co‐dehydropolymerisation reactions to give the first examples for Si containing poly(aminoborane)s.

Driven by light: A mild and facile photoredox approach towards asymmetrically substituted phosphines and phosphonium salts is reported. Blue‐light irradiation of mono‐ and diphenylphosphine with various aryl and alkyl iodides, diisopropylethylamine (DIPEA), and the organic photocatalyst 3DPAFIPN affords the desired products in good‐to‐excellent yields. In addition, the same method transforms white phosphorus (P₄) directly into symmetrical aryl phosphines and phosphonium salts.

Abstract

Asymmetrically substituted tertiary phosphines and quaternary phosphonium salts are used extensively in applications throughout industry and academia. Despite their significance, classical methods to synthesize such compounds often demand either harsh reaction conditions, prefunctionalization of starting materials, highly sensitive organometallic reagents, or expensive transition‐metal catalysts. Mild, practical methods thus remain elusive, despite being of great current interest. Herein, we describe a visible‐light‐driven method to form these products from secondary and primary phosphines. Using an inexpensive organic photocatalyst and blue‐light irradiation, arylphosphines can be both alkylated and arylated using commercially available organohalides. In addition, the same organocatalyst can be used to transform white phosphorus (P₄) directly into symmetrical aryl phosphines and phosphonium salts in a single reaction step, which has previously only been possible using precious metal catalysis.

The Pudovik reaction is an atom‐economic addition of phosphane oxides across alkynes, strongly depending on the nature of the s‐block metal, the solvent, the bulkiness of P‐bound groups, and the concentration. Bulky mesityl groups can interact with the newly formed C=C bond of the alkenylphosphane oxides.

Abstract

The hydrophosphorylation of phenylacetylene with di(aryl)phosphane oxides Ar₂P(O)H (Pudovik reaction) yields E /Z ‐isomer mixtures of phenylethenyl‐di(aryl)phosphane oxides (1 ). Alkali and alkaline‐earth metal di(aryl)phosphinites have been studied as catalysts for this reaction with increasing activity for the heavier s‐block metals. The Pudovik reaction can only be mediated for di(aryl)phosphane oxides whereas P‐bound alkyl and alcoholate substituents impede the P−H addition across alkynes. The demanding mesityl group favors the single‐hydrophosphorylated products 1‐Ar whereas smaller aryl substituents lead to the double‐hydrophosphorylated products 2‐Ar . Polar solvents are beneficial for an effective addition. Increasing concentration of the reactants and the catalyst accelerates the Pudovik reaction. Whereas Mes₂P(O)H does not form the bis‐phosphorylated product 2‐Mes , activation of an ortho ‐methyl group and cyclization occurs yielding 2‐benzyl‐1‐mesityl‐5,7‐dimethyl‐2,3‐dihydrophosphindole 1‐oxide (3 ).

Inverting the pyramid : Inspired by the aggregation‐induced emission (AIE) behavior of phosphanes, for the first time the photoinduced pyramidal inversion behavior of phosphanes in dilute solution was investigated. As expected, phosphanes are inverted under photoirradiation; the inversion mechanism could be explained by quantum chemical calculations (see figure).

Abstract

Aggregation‐induced emission (AIE) is a fascinating phenomenon because of the applications of luminescent materials in the aggregated state, which exploit the large structural changes of the molecules in the excited state. Recently, it was reported that triphenylphosphane derivatives show AIE behavior in which they undergo potentially large structural changes in the excited state. Inspired by this report, photoinduced pyramidal inversion behavior of phosphanes was investigated. In photochemical experiments, the prepared P‐stereogenic phosphanes exhibited photoracemization in dilute solution, and a negative correlation was observed between the photoracemization and the AIE phenomenon. Theoretical computations revealed that the inversion barrier in the excited state was much smaller than that in the ground state. This is the first report on the photoinduced pyramidal inversion behavior of phosphanes, which will provide new and unexplored applications.

Family reunion—the 7 kids of Dimethylorcinol ! A short and divergent total synthetic approach towards diorcinols and related structures yielded seven different diaryl ethers. A Pd‐catalyzed diaryl ether coupling was identified as the key step. Three members of the natural product class family exhibit anti‐MRSA and biofilm activity (see scheme).

Abstract

Diorcinols and related prenylated diaryl ethers were reported to exhibit activity against methicillin‐resistant clinical isolates of Staphylococcus aureus (MRSA). Within these lines, we report the first total synthesis of diorcinol D, I, J, the proposed structure of verticilatin and recently isolated antibacterial diaryl ether by using an efficient and highly divergent synthetic strategy. These total syntheses furnish the diaryl ethers in only five to seven steps employing a Pd‐catalyzed diaryl ether coupling as the key step. The total synthesis led to the structural revision of the natural product verticilatin, which has been isolated from a plant pathogenic fungus. Furthermore, these structures were tested in order to determine their antibacterial activities against different MRSA strains as well as further Gram‐positive and ‐negative bacteria.

Nature Reviews Chemistry, Published online: 03 June 2020; doi:10.1038/s41570-020-0191-2

Double formicable : A simple formamide catalyst enables the rapid transformation of formic acid into carbon monoxide (CO) using inexpensive trichlorotriazine (TCT). Application towards four different carbonylative cross couplings down to room temperature showcases high levels of synthetic utility and versatility. The organocatalytic ex situ formation of CO using commercial H¹³CO₂H allows the synthesis of radiolabeled compounds such as the drug Moclobemide.

Abstract

Herein, the first organocatalytic method for the transformation of non‐derivatized formic acid into carbon monoxide (CO) is introduced. Formylpyrrolidine (FPyr) and trichlorotriazine (TCT), which is a cost‐efficient commodity chemical, enable this decarbonylation. Utilization of dimethylformamide (DMF) as solvent and catalyst even allows for a rapid CO generation at room temperature. Application towards four different carbonylative cross coupling protocols demonstrates the high synthetic utility and versatility of the new approach. Remarkably, this also comprehends a carbonylative Sonogashira reaction at room temperature employing intrinsically difficult electron‐deficient aryl iodides. Commercial ¹³C‐enriched formic acid facilitates the production of radiolabeled compounds as exemplified by the pharmaceutical Moclobemide. Finally, comparative experiments verified that the present method is highly superior to other protocols for the activation of carboxylic acids.

Synlett
DOI: 10.1055/s-0040-1707126

Valorization of biomass has become an area of intense focus because of the diminishing reserves of crude oil and the ongoing problem of climate change. The principal strategies for the utilization of biomass as a feedstock are (i) to produce biofuels for the transportation sector and (ii) to produce organic commodity chemicals. In this respect, we have developed a serious of manganese-catalyzed dehydrogenative/deoxygenative coupling reactions of lower alcohols, obtainable from oxygen-rich lignocellulosic biomass, to deliver advanced liquid fuels and valuable chemicals.1 Introduction2 Manganese-Catalyzed Upgrading of Ethanol to Butan-1-ol3 Manganese-Catalyzed Selective Upgrading of Ethanol with Methanol to Isobutanol4 Manganese-Catalyzed Acceptorless Dehydrogenative Coupling of Alcohols with Hydroxides to Give Carboxylates5 Manganese-Catalyzed Dual-Deoxygenative Coupling of Primary Alcohols with 2-Arylethanols6 Conclusion
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© Georg Thieme Verlag Stuttgart · New York

Article in Thieme eJournals:
Table of contents  |  Abstract  |  Full text

Tetrahydrocarbazoles and perhydrocyclohepta[b]indoles undergo a catalytic cascade singlet oxygenation in alkaline medium, which leads to chiral tricyclic perhydropyrido‐ and perhydro‐azepino[1,2‐a]indoles in a single operation. These photooxygenation products are new synthetic equivalents of uncommon C,N‐diacyliminium ions and can be functionalized with the aid of phosphoric acid organocatalysis.

Asymmetric hydrogenations are among the most practical methods for the synthesis of chiral building blocks at industrial scale. The selective reduction of an oxime to the corresponding chiral hydroxylamine derivative remains a challenging variant because of undesired cleavage of the weak nitrogen-oxygen bond. We report a robust cyclometalated iridium(III) complex bearing a chiral cyclopentadienyl ligand as an efficient catalyst for this reaction operating under highly acidic conditions. Valuable N-alkoxy amines can be accessed at room temperature with nondetected overreduction of the N-O bond. Catalyst turnover numbers up to 4000 and enantiomeric ratios up to 98:2 are observed. The findings serve as a blueprint for the development of metal-catalyzed enantioselective hydrogenations of challenging substrates.

Precise synthesis : The new “built‐in‐base” ligand Neolephos was designed and applied in the Pd‐catalyzed monocarbonylation of 1,3‐diynes. The precise synthesis of conjugated enynes was achieved in good‐to‐high yield with excellent chemo‐ and stereoselectivity. The presented methodology can be used for simple diversification of natural products and pharmaceuticals.

Abstract

For the first time, the monoalkoxycarbonylation of easily available 1,3‐diynes to give synthetically useful conjugated enynes has been realized. Key to success was the design and utilization of the new ligand 2,2′‐bis(tert ‐butyl(pyridin‐2‐yl)phosphanyl)‐1,1′‐binaphthalene (Neolephos), which permits the palladium‐catalyzed selective carbonylation under mild conditions, providing a general preparation of functionalized 1,3‐enynes in good‐to‐high yields with excellent chemoselectivities. Synthetic applications that showcase the possibilities of this novel methodology include an efficient one‐pot synthesis of 4‐aryl‐4H ‐pyrans as well as the rapid construction of various heterocyclic, bicyclic, and polycyclic compounds.

MOF‐derived Co‐N‐C catalysts with isolated single cobalt atoms have been synthesized and compared with cobalt nanoparticles for formic acid dehydrogenation. The atomically dispersed Co‐N‐C catalyst achieves superior activity, better acid resistance and improved long‐term stability compared with nanoparticles synthesized by a similar route. HAADF‐STEM, XPS, EPR and XAFS characterizations reveal the formation of Co(II)Nx centers as active sites. The optimal low‐cost catalyst is a promising candidate for liquid H2 generation.

Ribosomes can produce proteins in minutes and are largely constrained to proteinogenic amino acids. Here, we report highly efficient chemistry matched with an automated fast-flow instrument for the direct manufacturing of peptide chains up to 164 amino acids long over 327 consecutive reactions. The machine is rapid: Peptide chain elongation is complete in hours. We demonstrate the utility of this approach by the chemical synthesis of nine different protein chains that represent enzymes, structural units, and regulatory factors. After purification and folding, the synthetic materials display biophysical and enzymatic properties comparable to the biologically expressed proteins. High-fidelity automated flow chemistry is an alternative for producing single-domain proteins without the ribosome.