LongLarf

Publication date: Available online 29 October 2020

Source: Chem

Author(s): Haoyue Li, Ning Yan

Nature, Published online: 28 October 2020; doi:10.1038/s41586-020-2831-6

CO₂ fixation: Phenol‐functionalized phosphonium salts are efficient catalysts for the synthesis of cyclic carbonates under mild and solvent‐free conditions. Their superior activity is elucidated by kinetic investigations and DFT calculations.

Abstract

A series of hydroxy‐functionalized phosphonium salts were studied as bifunctional catalysts for the conversion of CO₂ with epoxides under mild and solvent‐free conditions. The reaction in the presence of a phenol‐based phosphonium iodide proceeded via a first order rection kinetic with respect to the substrate. Notably, in contrast to the aliphatic analogue, the phenol‐based catalyst showed no product inhibition. The temperature dependence of the reaction rate was investigated, and the activation energy for the model reaction was determined from an Arrhenius‐plot (E _a=39.6 kJ mol⁻¹). The substrate scope was also evaluated. Under the optimized reaction conditions, 20 terminal epoxides were converted at room temperature to the corresponding cyclic carbonates, which were isolated in yields up to 99 %. The reaction is easily scalable and was performed on a scale up to 50 g substrate. Moreover, this method was applied in the synthesis of the antitussive agent dropropizine starting from epichlorohydrin and phenylpiperazine. Furthermore, DFT calculations were performed to rationalize the mechanism and the high efficiency of the phenol‐based phosphonium iodide catalyst. The calculation confirmed the activation of the epoxide via hydrogen bonding for the iodide salt, which facilitates the ring‐opening step. Notably, the effective Gibbs energy barrier regarding this step is 97 kJ mol⁻¹ for the bromide and 72 kJ mol⁻¹ for the iodide salt, which explains the difference in activity.

The use of hexafluoroisopropanol (HFIP) as an additive enables the halofunctionalization of unactivated alkenes with a remarkable functional‐group tolerance under mild reaction conditions. DFT computations were carried out to shed light on the mechanism of haloamidation, featuring alkene activation assisted by a nitrogen nucleophile as a key step. EWG=electron‐withdrawing group.

Abstract

Pyrrolidine and piperidine derivatives bearing halide functional groups are prevalent building blocks in drug discovery as halides can serve as an anchor for post‐modifications. In principle, one of the simplest ways to build these frameworks is the haloamination of alkenes. While progress has been made in this field, notably with the development of enantioselective versions, this reaction is still fraught with limitations in terms of reactivity. Besides, a major question remaining is to understand the mechanism at work. The formation of a haliranium intermediate is typically mentioned, but limited mechanistic evidence supports it. Reported here is an efficient metal‐ and oxidant‐free protocol to achieve the haloamidation of olefins, promoted by hexafluoroisopropanol, along with a DFT investigation of the mechanism. These findings should guide the future development of more complex transformations in the field of halofunctionalization.

One fits all: Co/Co₃O₄ core–shell nanoparticles prepared by pyrolysis of a Co‐pyromellitic acid template on silica served as stable and reusable catalysts for the selective hydrogenation of pyridines, quinolines, and other heteroarenes.

Abstract

Herein, we report the synthesis of specific silica‐supported Co/Co₃O₄ core–shell based nanoparticles prepared by template synthesis of cobalt‐pyromellitic acid on silica and subsequent pyrolysis. The optimal catalyst material allows for general and selective hydrogenation of pyridines, quinolines, and other heteroarenes including acridine, phenanthroline, naphthyridine, quinoxaline, imidazo[1,2‐a]pyridine, and indole under comparably mild reaction conditions. In addition, recycling of these Co nanoparticles and their ability for dehydrogenation catalysis are showcased.

The advanced Pd catalyst system with Neolephos as a ligand enables highly selective hydroamidation of unbiased (un)symmetrical 1,3‐diynes. In this way, a wide range of synthetically useful α‐alkynyl‐α,β‐unsaturated amides are afforded in good to high yields with excellent chemo‐, regio‐, and stereoselectivities.

Abstract

A chemo‐, regio‐, and stereoselective mono‐hydroamidation of (un)symmetrical 1,3‐diynes is described. Key for the success of this novel transformation is the utilization of an advanced palladium catalyst system with the specific ligand Neolephos. The synthetic value of this general approach to synthetically useful α‐alkynyl‐α, β‐unsaturated amides is showcased by diversification of several structurally complex molecules and marketed drugs. Control experiments and density‐functional theory (M06L‐SMD) computations also suggest the crucial role of the substrate in controlling the regioselectivity of unsymmetrical 1,3‐diynes.

Nature, Published online: 23 September 2020; doi:10.1038/d41586-020-02565-1

Nature, Published online: 23 September 2020; doi:10.1038/s41586-020-2733-7

Mild at heart: A nanostructured catalytic material composed by cobalt nanoparticles, magnesium oxide and a biowaste‐derived carbon matrix is able to efficiently mediate the hydrogenation of nitriles. Not only the mild operative conditions are of note, but also the remarkable selectivity towards primary amines. This allows the preparation of a large array of molecules in high yields even under scaled‐up conditions.

Abstract

Cobalt‐doped hybrid materials consisting of metal oxides and carbon derived from chitin were prepared, characterized and tested for industrially relevant nitrile hydrogenations. The optimal catalyst supported onto MgO showed, after pyrolysis at 700 °C, magnesium oxide nanocubes decorated with carbon‐enveloped Co nanoparticles. This special structure allows for the selective hydrogenation of diverse and demanding nitriles to the corresponding primary amines under mild conditions (e.g. 70 °C, 20 bar H₂). The advantage of this novel catalytic material is showcased for industrially important substrates, including adipodinitrile, picolinonitrile, and fatty acid nitriles. Notably, the developed system outperformed all other tested commercial catalysts, for example, Raney Nickel and even noble‐metal‐based systems in these transformations.

A divergent protocol for the synthesis of various (hetero)aryl S(VI) fluorides from the same starting material is reported. The protocol proceeds through a novel metal‐free Ar−S bond formation, followed by a mild oxidation. The careful tuning of the oxidation conditions dictates the chemoselectivity of the oxidation step.

Abstract

A convenient protocol to selectively access various arylsulfur(VI) fluorides from commercially available aryl halides in a divergent fashion is presented. Firstly, a novel sulfenylation reaction with the electrophilic N‐(chlorothio)phthalimide (Cl‐S‐Phth) and arylzinc reagents afforded the corresponding Ar‐S‐Phth compounds. Subsequently, the S(II) atom was selectively oxidized to distinct fluorinated sulfur(VI) compounds under mild conditions. Slight modifications on the oxidation protocol permit the chemoselective installation of 1, 3, or 4 fluorine atoms at the S(VI) center, affording the corresponding Ar‐SO₂F, Ar‐SOF₃, and Ar‐SF₄Cl. Of notice, this strategy enables the effective introduction of the rare and underexplored ‐SOF₃ moiety into various (hetero)aryl groups. Reactivity studies demonstrate that such elusive Ar‐SOF₃ can be utilized as a linchpin for the synthesis of highly coveted aryl sulfonimidoyl fluorides (Ar‐SO(NR)F).

Biomass to fuel: Recent developments in transformations of biomass‐derived 5‐hydroxymethylfurfural to 2,5‐dimethylfuran acid, a potential liquid fuel, are critically summarized. The main challenges in transformation of 5‐hydroxymethylfurfural to 2,5‐dimethylfuran are low reactant concentration, overhydrogenation and hydrogenolysis, and catalyst deactivation. A proper catalyst should have optimized metal dispersion and number of acidic sites. Results from continuous operation and kinetic modelling in transformation of 5‐hydroxymethylfurfural to 2,5‐dimethylfuran are also summarized.

Abstract

Recent developments in transformations of biobased 5‐hydroxymethylfurfural to 2,5‐dimethylfuran, a potential liquid fuel, are critically summarized. The highest yield of 2,5‐dimethylfuran (more than 98 %) from 5‐hydroxymethylfurfural are obtained over bimetallic Cu−Co supported on carbon at 180 °C under 5 bar hydrogen in 2‐propanol and over Ni supported on mesoporous carbon at 200 °C under 30 bar hydrogen in water in a batch reactor. The desired catalyst should have relatively high metal dispersion and some acidity to facilitate both hydrogenation and hydrogenolysis. However, overhydrogenation and overhydrogenolysis forming 2,5‐dimethyltetrahydrofuran and methylfuran, respectively, should be suppressed. Furthermore, a hydrophobic support is more selective than oxide‐based support. After a careful adjustment of the residence time in a continuous reactor it is also possible to produce high yields of 2,5‐dimethylfuran even over Pt/C. The main challenges limiting the industrial feasibility of these reactions are relatively low initial reactant concentration, catalyst deactivation by sintering, leaching and coking. In addition to selection of optimum reaction conditions and catalyst properties, kinetic modelling was also summarized.

The Cover Feature shows the ammonia‐free hydrogenation of aromatic and aliphatic nitriles to primary amines, catalyzed by a molybdenum pincer complex. In their Full Paper, T. Leischner et al. demonstrate that, upon pre‐activation with NaBHEt₃, the applied catalyst is deprotonated at the coordinated acetonitrile ligand. The resulting organometallic species was isolated and structurally characterized by means of X‐ray crystallography. Its active role as part of the catalytic cycle was subsequently demonstrated in control experiments.More information can be found in the https://doi.org/10.1002/cctc.202000736Full Paper by T. Leischner et al..

High‐performance hypoiodite catalysis was developed for the chemoselective oxidative or decarboxylative oxidative α‐azidation of 1,3‐dicarbonyl compounds (see scheme). By this method, the late‐stage azidation of complex molecules with NaN₃ and hydrogen peroxide as the azide source and oxidant, respectively, was possible under extremely mild conditions.

Abstract

We report high‐performance I⁺/H₂O₂ catalysis for the oxidative or decarboxylative oxidative α‐azidation of carbonyl compounds by using sodium azide under biphasic neutral phase‐transfer conditions. To induce higher reactivity especially for the α‐azidation of 1,3‐dicarbonyl compounds, we designed a structurally compact isoindoline‐derived quaternary ammonium iodide catalyst bearing electron‐withdrawing groups. The nonproductive decomposition pathways of I⁺/H₂O₂ catalysis could be suppressed by the use of a catalytic amount of a radical‐trapping agent. This oxidative coupling tolerates a variety of functional groups and could be readily applied to the late‐stage α‐azidation of structurally diverse complex molecules. Moreover, we achieved the enantioselective α‐azidation of 1,3‐dicarbonyl compounds as the first successful example of enantioselective intermolecular oxidative coupling with a chiral hypoiodite catalyst.

The advanced Pd/L11 system enables highly regioselective carbonylation of alkynols for synthesis of a wide range of 4‐membered α‐methylene‐β‐lactones.

Abstract

The first general and regioselective Pd‐catalyzed cyclocarbonylation to give α‐methylene‐β‐lactones is reported. Key to the success for this process is the use of a specific sterically demanding phosphine ligand based on N‐arylated imidazole (L11) in the presence of Pd(MeCN)₂Cl₂ as pre‐catalyst. A variety of easily available alkynols provide under additive‐free conditions the corresponding α‐methylene‐β‐lactones in moderate to good yields with excellent regio‐ and diastereoselectivity. The applicability of this novel methodology is showcased by the direct carbonylation of biologically active molecules including natural products.

Hydrocarbon cracking is an important process to make fuels and chemicals. Fluid catalytic cracking (FCC) catalysts are capable of doing this and are complex materials. They consist of a matrix (alumina, silica and clay) and an active component, a zeolite. Efficient cracking requires not only that there are a sufficient number of active sites, but also that these sites are accessible for the hydrocarbons. In this work, Velthoen and co‐workers explore different cooking recipes to assess the influence of Brønsted and Lewis acidity, as well as their accessibility and structure on FCC performance. For more information, see the Full Paper on https://doi.org/10.1002/chem.201905867page 11995 ff.

A new organocatalyst that has unprecedented reactivity for ring‐opening polymerization of epoxides follows a novel intramolecular ammonium cation assisted mechanism. The bifunctional catalyst incorporates two 9‐borabicyclo[3.3.1]nonane centers on the two ends as Lewis acidic sites for epoxide activation and a quaternary ammonium halide in the middle as the initiating site.

Abstract

This manuscript describes a kind of bifunctional organocatalyst with unprecedented reactivity for the synthesis of polyethers via ring‐opening polymerization (ROP) of epoxides under mild conditions. The bifunctional catalyst incorporates two 9‐borabicyclo[3.3.1]nonane centers on the two ends as Lewis acidic sites for epoxide activation and a quaternary ammonium halide in the middle as the initiating site. The catalyst could be easily prepared in two steps from commercially available stocks on up to kilogram scale with ≈100 % yield. The organoboron catalyst mediated ROP of epoxides displays living behavior with low catalyst loading (5 ppm) and enables the synthesis of polyethers with molecular weights of over a million grams per mole (>10⁶ g mol⁻¹). Based on the investigations on crystal structure of catalyst, MALDI‐TOF, and ¹¹B NMR spectroscopy, an intramolecular ammonium cation assisted S_N2 mechanism is proposed and verified by DFT calculations.

Quo vadis, CO₂ ? In a fossil‐C‐based economy only a selected class of reactions (low energy) can be exploited. Conversely, “Solar Chemistry” is a way to convert large volumes of carbon dioxide into a variety of high‐energy, added‐value species, either C₁ or C₂₊, by co‐processing with water.

Abstract

The utilization of carbon dioxide as building block for chemicals or source of carbon for energy products has been explored for over 40 years now, with varying allure. In correspondence with oil‐crises, the use of CO₂ has come into the spotlight, soon set aside when the crisis was over due to the low price of fossil carbon and the convenience of using established technologies. Nowadays, there is a continuous shift from fossil‐C‐based to perennial (solar, wind, geothermal, hydro‐power) energy‐driven processes that will also have a great potential to convert large amounts of carbon dioxide. The integration of biotechnology and catalysis will be a key player towards the utilization of CO₂ in several different applications, reducing both the extraction of fossil carbon and the carbon transfer to the atmosphere.

Mechanical entanglement of the substrate within the coordination sphere of a reactive transition‐metal complex is used as a strategy to access the organometallic chemistry of carbon—carbon bond activation reactions.

Abstract

By use of a macrocyclic phosphinite pincer ligand and bulky substrate substituents, we demonstrate how the mechanical bond can be leveraged to promote the oxidative addition of an interlocked 1,3‐diyne to a rhodium(I) center. The resulting rhodium(III) bis(alkynyl) product can be trapped out by reaction with carbon monoxide or intercepted through irreversible reaction with dihydrogen, resulting in selective hydrogenolysis of the C−C σ‐bond.