LongLarf

The outstanding stability of the pictured catalyst system (>25 recycling runs in 32 days without measurable loss of activity), which is unprecedented for Pd‐catalyzed hydroxycarbonylations, is showcased in the preparation of an industrially relevant fatty acid. Because of its efficiency and generality, it provides a basis for new cost‐competitive processes for the industrial production of carboxylic acids.

Abstract

The synthesis of carboxylic acids is of fundamental importance in the chemical industry and the corresponding products find numerous applications for polymers, cosmetics, pharmaceuticals, agrochemicals, and other manufactured chemicals. Although hydroxycarbonylations of olefins have been known for more than 60 years, currently known catalyst systems for this transformation do not fulfill industrial requirements, for example, stability. Presented herein for the first time is an aqueous‐phase protocol that allows conversion of various olefins, including sterically hindered and demanding tetra‐, tri‐, and 1,1‐disubstituted systems, as well as terminal alkenes, into the corresponding carboxylic acids in excellent yields. The outstanding stability of the catalyst system (26 recycling runs in 32 days without measurable loss of activity), is showcased in the preparation of an industrially relevant fatty acid. Key‐to‐success is the use of a built‐in‐base ligand under acidic aqueous conditions. This catalytic system is expected to provide a basis for new cost‐competitive processes for the industrial production of carboxylic acids.

Powered by rearomatization: The adduct of a nucleophilic chiral phosphite with an azaarene N‐allyl salt undergoes a stereoselective base‐mediated aza‐Wittig rearrangement. This method efficiently generates tertiary and quaternary chiral centers in isoquinoline, quinoline, and pyridine systems along with an alkene handle for further functionalization.

Abstract

A phosphite‐mediated [2,3]‐aza‐Wittig rearrangement has been developed for the regio‐ and enantioselective allylic alkylation of six‐membered heteroaromatic compounds (azaarenes). The nucleophilic phosphite adducts of N‐allyl salts undergo a stereoselective base‐mediated aza‐Wittig rearrangement and dissociation of the chiral phosphite for overall C−H functionalization of azaarenes. This method provides efficient access to tertiary and quaternary chiral centers in isoquinoline, quinoline, and pyridine systems, tolerating a broad variety of substituents on both the allyl part and azaarenes. Catalysis with chiral phosphites is also demonstrated with synthetically useful yields and enantioselectivities.

Clearing the air: Pulling CO₂ out of ambient air requires sorbents with unusual properties. Recent progress in surface chemistry and material synthesis have resulted in a new generation of solid CO₂ sorbents tuned to the exacting demands of direct air capture and negative emissions on a global scale.

Abstract

The urgency to address global climate change induced by greenhouse gas emissions is increasing. In particular, the rise in atmospheric CO₂ levels is generating alarm. Technologies to remove CO₂ from ambient air, or “direct air capture” (DAC), have recently demonstrated that they can contribute to “negative carbon emission.” Recent advances in surface chemistry and material synthesis have resulted in new generations of CO₂ sorbents, which may drive the future of DAC and its large‐scale deployment. This Review describes major types of sorbents designed to capture CO₂ from ambient air and they are categorized by the sorption mechanism: physisorption, chemisorption, and moisture‐swing sorption.

Splitting the work: Energy transfer (EnT) and photoinduced single‐electron transfer (SET) allow the reduction of arenes. Subsequent hydrogen‐atom transfer (HAT) yields the reduced dearomatized products.

Abstract

The direct reduction of arenes and heteroarenes by visible‐light irradiation remains challenging, as the energy of a single photon is not sufficient for breaking aromatic stabilization. Shown herein is that the energy accumulation of two visible‐light photons allows the dearomatization of arenes and heteroarenes. Mechanistic investigations confirm that the combination of energy‐transfer and electron‐transfer processes generates an arene radical anion, which is subsequently trapped by hydrogen‐atom transfer and finally protonated to form the dearomatized product. The photoreduction converts planar aromatic feedstock compounds into molecular skeletons that are of use in organic synthesis.

Sustainable alcohol dehydrogenation: The acceptorless dehydrogenation of alcohols is achieved with heterogeneous catalysts and continuous reactors, resulting in the continuous production of carbonyl compounds and molecular hydrogen.

Abstract

Although the selective oxidation of alcohols to carbonyl compounds is a critical reaction, it is often plagued by several challenges related to sustainability. Here, the continuous, acceptorless dehydrogenation of alcohols to carbonyl compounds over heterogeneous catalysts was demonstrated, in the absence of oxidants, bases or acceptor molecules. In addition to improving selectivity and atom efficiency, the absence of an acceptor resulted in the co‐production of molecular H₂, a clean energy source, and permitted dehydrogenation to proceed at >98 % selectivity at turnover frequency values amongst the highest in the literature. Moreover, excellent durability was observed during continuous operation over 48 h, reaching space‐time yields of 0.683 g_(product) mL⁻¹ h⁻¹, better than the state of the art by over two orders of magnitude. Alongside these breakthroughs, the basic kinetic parameters of the reaction were also determined, allowing some of the elementary reaction steps to be identified.

Don't hydrolyze, convert: It is possible to convert polyesters in a one‐pot process into polyethers by using an in situ‐generated Ru‐Triphos catalyst in combination with a Lewis acid and molecular hydrogen. This reaction opens up a novel field for polyester recycling and makes polyether polyols accessible that are otherwise difficult to obtain from conventional fossil‐based feedstocks.

Abstract

The amount of plastic waste is continuously increasing. Besides conventional recycling, one solution to deal with this problem could be to use this waste as a resource for novel materials. In this study, polyesters are hydrogenated to give polyether polyols by using in situ‐generated Ru‐Triphos catalysts in combination with Lewis acids. The choice of Lewis acid and its concentration relative to the ruthenium catalyst are found to determine the selectivity of the reaction. Monitoring of the molecular weight during the reaction confirms a sequential mechanism in which the diols that are formed by hydrogenation are etherified to the polyethers. To probe the applicability of this tandem hydrogenation etherification approach, a range of polyester substrates is investigated. The oligoether products that form in these reactions have the chain lengths that are appropriate for application in the adhesives and coatings industries. This strategy makes polyether polyols accessible that are otherwise difficult to obtain from conventional fossil‐based feedstocks.

Just a pinch: Efficient, recoverable, and durable polymer‐supported bifunctional cooperative PC(sp³)P pincer catalysts are described for use in acceptorless dehydrogenative coupling of alcohols and transfer hydrogenation of aldehydes, highlighting that carefully designing a link between the support and the catalytic moiety may lead to superior heterogeneous catalysis.

Abstract

A series of polymer‐supported cooperative PC(sp³)P pincer catalysts was synthesized and characterized. Their catalytic activity in the acceptorless dehydrogenative coupling of alcohols and the transfer hydrogenation of aldehydes with formic acid as a hydrogen source was investigated. This comparative study, examining homogeneous and polymer‐tethered species, proved that carefully designing a link between the support and the catalytic moiety, which takes into consideration the mechanism underlying the target transformation, might lead to superior heterogeneous catalysis.

Photoexcited electron-hole pairs on a semiconductor surface can engage in redox reactions with two different substrates. Similar to conventional electrosynthesis, the primary redox intermediates afford only separate oxidized and reduced products or, more rarely, combine to one addition product. Here, we report that a stable organic semiconductor material, mesoporous graphitic carbon nitride (mpg-CN), can act as a visible-light photoredox catalyst to orchestrate oxidative and reductive interfacial electron transfers to two different substrates in a two- or three-component system for direct twofold carbon–hydrogen functionalization of arenes and heteroarenes. The mpg-CN catalyst tolerates reactive radicals and strong nucleophiles, is straightforwardly recoverable by simple centrifugation of reaction mixtures, and is reusable for at least four catalytic transformations with conserved activity.

Abstract

The P‐stereogenic PN(H)P tridentate ligand HN(CH₂CH₂)₂P(Me)R (R=Cy, 1 a; R= ^t Bu, 1 b) gives the Mn(I) complexes [Mn(CO)₃(PN(H)P)]Br, which were tested in the asymmetric H₂ hydrogenation of ketones. The amido species [Mn(CO)₂(PNP)], hydrides [MnH(CO)₂(PN(H)P)], and the alkoxide complex were detected by NMR spectroscopy. The manganese(I) derivative [Mn(CO)₃(1 a)]Br was compared to its iron(II) analogue [FeHBr(CO)(1 a)] by kinetic and DFT studies. The DFT study suggests that both Mn(I) and Fe(II) operate via a bifunctional mechanism for H⁺/H⁻ transfer with structurally similar enantiodetermining transition states and hence comparable enantioselectivity. The Mn(I) catalyst is less active than its Fe(II) analogue (k(Fe)/k(Mn)=ca. 30), which we attribute to the higher stability of the Mn(I) resting species, the off‐cycle alkoxo complex [Mn(OCH(Me)Ph))(CO)₂(1 a)] that follows from the larger π delocalization onto the additional CO ligand as compared to [FeH(OCH(Me)Ph))(CO)(1 a)].

Nature Reviews Chemistry, Published online: 17 July 2019; doi:10.1038/s41570-019-0116-0

Strategic play: A strategy for visible‐light induced [4+2] annulation of thiophenes and alkynes, to afford benzene rings, is presented. Under mild reaction conditions, the ready availability and structural diversity of thiophenes and alkynes permit the facile synthesis of several substituted aromatic rings. Valuable drugs and amino acids are also well tolerated. Moreover, DFT calculations explain the high regioselectivity of the reaction.

Abstract

The [4+2] annulation represents an elegant and versatile synthetic protocol for the construction of benzene rings. Herein, a strategy for visible‐light induced [4+2] annulation of thiophenes and alkynes, to afford benzene rings, is presented. Under simple and mild reaction conditions, the ready availability and structural diversity of thiophenes and alkynes permit the facile synthesis of several substituted aromatic rings. Valuable drugs and amino acids are also well tolerated. Moreover, DFT calculations explain the high regioselectivity of the reaction.

Publication date: 23 August 2019

Source: Tetrahedron, Volume 75, Issue 34

Author(s): Chandrakant S. Gholap, Rekha Singh, Mukesh Kumar, Dilip K. Maity, Sunil K. Ghosh

Graphical abstract

The first synthesis and structural characterization of Fe^II based aminoborane complexes of the type [Fe(PNP)(H)(η²:η²‐H₂B=NR₂)]⁺ (R=H, Me) is reported. These species are formed upon protonation of the borohydride complex [Fe(PNP)(H)(η²‐BH₄)] by ammonium salts NH₂R₂ ⁺ (R=H, Me).

Abstract

Herein, we report on the first synthesis and structural characterization of the iron based aminoborane complexes [Fe(PNP)(H)(η²:η²‐H₂B=NR₂)]⁺ (R=H, Me). These species are formed upon protonation of the borohydride complex [Fe(PNP)(H)(η²‐BH₄)] by ammonium salts [NH₂R₂]⁺ (R=H, Me). For R=Me, the reaction proceeds via the cationic dinuclear intermediate [{Fe(PNP)(H)}₂(μ²,η²:η²‐BH₄)]⁺. A mechanism for the reaction is proposed based on DFT calculations that also indicate the final aminoborane complex as the thermodynamic product. All complexes were characterized by NMR spectroscopy, HRMS, and X‐ray crystallography.