LongLarf

Nature Catalysis, Published online: 06 January 2020; doi:10.1038/s41929-019-0404-6

The cover picture shows that NO_x removal from exhaust gases is very fast with catalysts containing vanadia supported on ceria while iron in the ceria support “throughs a wrench in the works”. In their Full Paper on p. XX ff., Angelika Brückner et al. show that iron boosts activity of bare Ce_1‐xFe_xO_2‐δ solid solutions while, unexpectedly, it is detrimental when vanadia is supported on their surface. In situ XANES, operando DRIFTS and EPR studies showed that Fe³⁺ is partly reduced to inactive Fe²⁺, favours clustering of VO_y sites and damages active V‐O‐Ce junctions.

Abstract

Supported VO_y/Ce_1‐xFe_xO_2‐δ catalysts (x=0, 0.5, 0.1, 0.2) and bare supports were prepared and tested in selective catalytic reduction (SCR) of NO_x by NH₃ between 150 and 300 °C with a GHSV of 70 000 h⁻¹. Iron was found to be beneficial for the activity of the pristine supports, reaching 80 % conversion at 275 °C. When vanadium was additionally introduced into the system, iron was found to be detrimental for NO_x‐conversion. To derive structure‐reactivity relationships, V‐free supports and VO_y/Ce_1‐xFe_xO_2‐δ catalysts were characterized by XRD, XPS, Raman spectroscopy and TEM. In situ XANES, as well as operando DRIFTS and EPR measurements were performed to study the behavior of the catalysts under reaction conditions. Up to an iron content of x=0.1, a solid Ce_1‐xFe_xO_2‐δ solution was formed. Higher iron contents led to formation of iron oxide agglomerates. These agglomerates, as well as an increased amount of surface oxygen species were found to be responsible for increased NO_x‐conversion over of pure supports. For V‐containing catalysts, an interaction of Fe and V centers could be found. Under reaction conditions, Fe³⁺ was preferentially reduced instead of V⁵⁺, decreasing the catalytic activity of VO_y/Ce_1‐xFe_xO_2‐δ.

Nature Catalysis, Published online: 18 December 2019; doi:10.1038/s41929-019-0407-3

Pincer complexes studied: Group 10 POCOP (κ ³‐C₆HR₂‐2,6‐(OPtBu₂)₂; R=H, tBu) fluoride complexes show pronounced halogen bonding with iodopentafluorobenzene in toluene. Along an isostructural series of complexes, Pd fluorides show the strongest interactions.

Abstract

The thermodynamics of halogen bonding of a series of isostructural Group 10 metal pincer fluoride complexes of the type [(3,5‐R₂‐ ^tBuPOCOP ^tBu)MF] (3,5‐R₂‐ ^tBuPOCOP ^tBu=κ ³‐C₆HR₂‐2,6‐(OPtBu₂)₂ with R=H, tBu, COOMe; M=Ni, Pd, Pt) and iodopentafluorobenzene was investigated. Based on NMR experiments at different temperatures, all complexes 1‐tBu (R=tBu, M=Ni), 2‐H (R=H, M=Pd), 2‐tBu (R=tBu, M=Pd), 2‐COOMe (R=COOMe, M=Pd) and 3‐tBu (R=tBu, M=Pt) form strong halogen bonds with Pd complexes showing significantly stronger binding to iodopentafluorobenzene. Structural and computational analysis of a model adduct of complex 2‐tBu with 1,4‐diiodotetrafluorobenzene as well as of structures of iodopentafluorobenzene in toluene solution shows that formation of a type I contact occurs.

Higher‐order is coming! Recent advancements in the field of organocatalytic higher‐order cycloadditions involving fulvene‐ and tropone‐derived systems are summarized in this Minireview. Based on various catalytic activation modes, novel reactivities have been identified providing new opportunities for organic synthesis.

Abstract

The great progress that took place in the field of higher‐order cycloadditions involving fulvene‐ and tropone‐derived systems in the last few years is astonishing. By application of organocatalytic activation modes, new higher‐order reactivities have been identified and described in the literature. These approaches take advantage of the high reliability of organocatalysis, at the same time expanding its potential and paving new directions for its further evolution. In this Minireview, the progress in the field of organocatalytic higher‐order cycloadditions involving fulvene‐ and tropone‐derived systems is summarized and insights into mechanistic aspects of the developed reactivities are provided. Furthermore, the discussion on the nomenclatural issues related to cycloaddition reactions has been conducted and solutions to clarify the picture proposed.

Certified organic: An organophosphorus‐catalyzed reaction was developed for the modular synthesis of diverse N‐aryl and N‐alkyl azaheterocycles (indoles, oxindoles, benzimidazoles, and quinoxalinediones) from readily available building blocks. A regiospecific approach to N‐substituted benzimidazoles and quinoxalinediones is also demonstrated.

Abstract

An organocatalytic method for the modular synthesis of diverse N‐aryl and N‐alkyl azaheterocycles (indoles, oxindoles, benzimidazoles, and quinoxalinediones) is reported. The method employs a small‐ring organophosphorus‐based catalyst (1,2,2,3,4,4‐hexamethylphosphetane P‐oxide) and a hydrosilane reductant to drive the conversion of ortho‐functionalized nitroarenes into azaheterocycles through sequential intermolecular reductive C−N cross coupling with boronic acids, followed by intramolecular cyclization. This method enables the rapid construction of azaheterocycles from readily available building blocks, including a regiospecific approach to N‐substituted benzimidazoles and quinoxalinediones.

The direct carbonylation of 1,3-butadiene offers the potential for a more cost-efficient and environmentally benign route to industrially important adipic acid derivatives. However, owing to the complex reaction network of regioisomeric carbonylation and isomerization pathways, a selective practical catalyst for this process has thus far proven elusive. Here, we report the design of a pyridyl-substituted bidentate phosphine ligand (HeMaRaphos) that, upon coordination to palladium, catalyzes adipate diester formation from 1,3-butadiene, carbon monoxide, and butanol with 97% selectivity and 100% atom-economy under industrially viable and scalable conditions (turnover number > 60,000). This catalyst system also affords access to a variety of other di- and triesters from 1,2- and 1,3-dienes.

Nature Communications, Published online: 13 December 2019; doi:10.1038/s41467-019-13701-5

Synlett
DOI: 10.1055/s-0039-1691405

Cheap and readily available 2-aminopyridine and related compounds can be used as organocatalysts for the conversion of epoxides into cyclic carbonates. This reaction gives high conversions under solvent-free conditions and is amenable to a kilogram-scale conversion under mild conditions.
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© Georg Thieme Verlag Stuttgart · New York

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

How does it really work? The mechanism of N‐heterocyclic carbene organocatalysis has been the subject of debate throughout the last century. Novel results put this old discussion back into focus, revealing more details about these valuable processes. This review summarizes the key elements of the progress, highlighting the arguments that led to the presently accepted mechanistic picture, and underlining the still open conceptual questions.

Abstract

The term “N‐Heterocyclic carbene organocatalysis” is often invoked in organic synthesis for reactions that are catalyzed by different azolium salts in the presence of bases. Although the mechanism of these reactions is considered today evident, a closer look into the details that have been collected throughout the last century reveals that there are many open questions and even contradictions in the field. Emerging new theoretical and experimental results offer solutions to these problems, because they show that through considering alternative reaction mechanisms a more consistent picture on the catalytic process can be obtained. These novel perspectives will be able to extend the scope of the reactions that we call today N‐heterocyclic carbene organocatalysis.

Organocatalytic acylfluoroalkylation: A multicomponent radical acylfluoroalkylation of olefins through NHC organocatalysis was developed, and over 120 examples of fluoroketones were facilely accessed from simple materials. Moreover, a dearomative difunctionalization of indoles could be readily achieved in a highly diastereoselective manner. The generality and practicality were highlighted by the late‐stage modification of drug skeletons.

Abstract

Fluorinated ketones are widely prevalent in numerous biologically interesting molecules, and the development of novel transformations to access these structures is an important task in organic synthesis. Herein, we report the multicomponent radical acylfluoroalkylation of a variety of olefins in the presence of various commercially available aromatic aldehydes and fluoroalkyl reagents through N‐heterocyclic carbene organocatalysis. With this protocol, over 120 examples of functionalized ketones with diverse fluorine substituents have been synthesized in up to 99 % yield with complete regioselectivity. The generality of this catalytic strategy was further highlighted by its successful application in the late‐stage functionalization of pharmaceutical skeletons. Excellent diastereoselectivity could be achieved in the reactions forging multiple stereocenters. In addition, preliminary results have been achieved on the catalytic asymmetric variant of the olefin difunctionalization process.

Nature Catalysis, Published online: 09 December 2019; doi:10.1038/s41929-019-0392-6