Yuya Hu

Short hydrocarbons: An ionic liquid (1‐butyl‐3‐methylimidazolium hexafluorophosphate; [BMIm][PF₆])‐based Pd(PtBu₃)₂–FeCl₂ homogeneous catalytic system can realize CO₂ hydrogenation to C₂–C₄ hydrocarbons under mild conditions (e.g., 180 °C).

Abstract

Direct hydrogenation of CO₂ to C₂₊ hydrocarbons is very interesting, but achieving this transformation below 200 °C is challenging and seldom reported. Herein, a homogeneous catalytic system was developed composed of the ionic liquid 1‐butyl‐3‐methylimidazolium hexafluorophosphate ([BMIm][PF₆]), Pd(PtBu₃)₂, FeCl₂, and the ligand 4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene (Xantphos) for hydrogenation of CO₂ under mild conditions, which resulted in C₂–C₄ hydrocarbons in selectivities up to 98.3 C‐mol % at 180 °C. The combination of [BMIm][PF₆]) with Xantphos endowed the Pd–Fe catalysts with the ability of activating CO₂ and H₂ simultaneously via [HPd(PtBu₃)(BMIm‐COO)(BMIm)(PF₆)Fe]⁺ species, thus catalyzing the formation of C₂–C₄ hydrocarbons through CO₂ hydrogenation. In addition, this catalytic system is stable and recyclable, which may have promising applications.

Synlett
DOI: 10.1055/s-0037-1611912

The β-disubstituted ketone functionality is prevalent in biologically active compounds and in pharmaceuticals. A ruthenium-catalyzed direct synthesis of β-disubstituted ketones by cross-coupling of two different secondary alcohols is reported. This new protocol was applied to the synthesis of variety of β-disubstituted ketones from various cyclic, acyclic, symmetrical, and unsymmetrical secondary alcohols. An amine–amide metal–ligand cooperation in a Ru catalyst facilitates the activation and formation of covalent bonds in selective sequences to provide the products. Kinetic and deuterium-labeling experiments suggested that aliphatic alcohols oxidize faster than benzylic secondary alcohols. A plausible mechanism is proposed on the basis of mechanistic and kinetic studies. Water and H2 are the only byproducts from this selective cross-coupling of secondary alcohols.1 Introduction2 Catalytic Self- or Cross-Coupling of Alcohols and Selectivity Challenges3 Recent Developments in the Synthesis of β-Disubstituted Ketones4 Scope of Ruthenium-Catalyzed Cross-Couplings of Secondary Alcohols5 Mechanistic Studies and Proposed Mechanism6 Conclusion
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© Georg Thieme Verlag Stuttgart · New York

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

A variety of fused heterocycles are obtained from easily accessible ortho‐bromophenol and aniline precursors by intramolecular coupling of two C(sp³)−H bonds, enabled by 1,4‐Pd shift from a trisubstituted aryl bromide. Unlike most C(sp³)−C(sp³) cross‐dehydrogenative couplings, this reaction operates under redox‐neutral conditions, with the C−Br bond acting as an internal oxidant.

Abstract

The intramolecular coupling of two C(sp³)−H bonds to forge a C(sp³)−C(sp³) bond is enabled by 1,4‐Pd shift from a trisubstituted aryl bromide. Contrary to most C(sp³)−C(sp³) cross‐dehydrogenative couplings, this reaction operates under redox‐neutral conditions, with the C−Br bond acting as an internal oxidant. Furthermore, it allows the coupling between two moderately acidic primary or secondary C−H bonds, which are adjacent to an oxygen or nitrogen atom on one side, and benzylic or adjacent to a carbonyl group on the other side. A variety of valuable fused heterocycles were obtained from easily accessible ortho‐bromophenol and aniline precursors. The second C−H bond cleavage was successfully replaced with carbonyl insertion to generate other types of C(sp³)‐C(sp³) bonds.

Abstract

Described is a tetrabutylammonium fluoride‐mediated hydroxyalkylation reaction of phenols with cyclic carbonates. This operationally simple method enables the synthesis of a variety of aryl β‐hydroxyethyl ethers in good to excellent yields with a very small amount of catalyst loading (0.1–1 mol%). Of particular note is the efficient conversion of aromatic diols and phloroglucinol to the corresponding bis‐ and tris‐hydroxyethylated products. To further showcase the versatility of this protocol, guaifenesin was prepared with a single step by the condensation of guaiacol and glycerol carbonate. We also developed a flow ethoxylation process permitting the continuous synthesis of multiflorol.

Abstract

The isomerization of epoxides to aldehydes using the readily available Crabtree's reagent is described. The aldehydes were transformed into synthetically useful amines by a one‐pot reductive amination using pyrrolidine as imine‐formation catalyst. The reactions worked with low catalyst loadings in very mild conditions. The procedure is operationally simple and tolerates a wide range of functional groups. A DFT study of its mechanism is presented showing that the isomerization takes place via an iridium hydride mechanism with a low energy barrier, in agreement with the mild reaction conditions.

Pincers et Al: The intrinsic strong σ‐donation and Lewis acidity of a Lewis‐base‐free X‐type PAlP‐pincer Ir complex has been experimentally and theoretically elucidated (X‐ray diffraction study, NMR, IR, and XANES analysis). The thermally stable pincer‐Ir complex with a tetrahydrido‐Ir^V structure showed moderate activity in the catalytic transfer dehydrogenation of cyclooctane.

Abstract

A pincer‐iridium complex bearing a Lewis‐base‐free X‐type alumanyl ligand has been synthesized. X‐ray diffraction, NMR and IR spectroscopy, as well as XANES analysis confirmed its tetrahydrido‐Ir^V structure and Lewis acidity at the Al center as supported by DFT calculations. The resulting complex was applied as a catalyst for the transfer dehydrogenation of cyclooctane.

A unique chelation‐assisted, inner‐sphere pathway for the Pd‐catalyzed allylic amination of vinyl cyclic carbonates with aryl amines was revealed by DFT calculations and various experiments. A chelation effect enabled through a η²‐coordination of the N‐aryl group to the Pd centre was identified as a key interaction with profound implication for the selectivity parameters.

Abstract

A recently reported palladium‐catalyzed allylic substitution of vinyl‐substituted cyclic carbonates (VCCs) with aryl amines represents a rare example of a regio‐ and enantioselective synthesis of α,α‐disubstituted allylic N‐aryl amines. However, the underlying reasons for this unusual selectivity profile remain elusive. In the present work, density functional theory (DFT) calculations in combination with mechanistic control experiments were performed to elucidate in detail this allylic amination manifold and the origin of the regio‐ and enantioselectivity. The combined data show that after oxidative addition of the VCC to Pd⁰, the nucleophilic attack via an originally proposed outer‐sphere pathway gives, however, the opposite regioisomer compared to the experimental results. Instead, nucleophilic attack of the amine reagent via a unique type of chelation‐assisted, inner‐sphere pathway accounts for the experimentally observed “branched” regioselectivity and high enantio‐control.

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.