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Principles Of Fluorescence Spectroscopy

Mesoionic carbenes are a subclass of the family of N-heterocyclic carbenes that generally feature less heteroatom stabilization of the carbenic carbon and hence impart specific donor properties and

Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications.

Chiral ligands play a central role in enantioselective transition-metal catalysis. The success of achiral N-heterocyclic carbenes (NHCs) as stable electron-rich neutral ligands in homogeneous catalysis led to the development of a manifold of chiral NHCs as stereodirecting ancillary ligands for various enantioselective transformations. Due to the modular design of NHCs and the ease of access to their azolium salt precursors, tailor-made NHCs are readily available. Many chiral NHC scaffolds have been synthesised and tested in catalysis. Herein, we highlight only those NHC structures which have enabled high degrees of enantioselectivity in transition-metal catalysis. Following a brief introduction to the field of chiral NHCs, this tutorial review introduces different categories of chiral NHCs and provides a guide to the structural fine-tuning of ligand requirements and stereochemical models.

Privileged chiral N-heterocyclic carbene ligands for asymmetric transition-metal catalysis

N-heterocyclic carbene (NHC)-metal complexes have become known as efficient catalysts for numerous transition metal catalyzed processes. An important application of many NHC-metal complexes is in the field of asymmetric catalysis, and this is achieved through the introduction of chiral information on the NHC ligands. Among the asymmetric transformations catalyzed by NHC-metal complexes is asymmetric hydrogenation, which is an attractive process for the synthesis of optically active compounds due to its high atom economy. However, to date, few chiral NHC-metal catalysts have been reported to be highly stereoselective for asymmetric hydrogenation. Over the past few years our group has made some significant breakthroughs within the field of asymmetric hydrogenation using chiral NHC catalysts. We have reported the NHC-Ru catalyzed asymmetric hydrogenation of a wide range of heterocyclic compounds with high regio- and enantioselectivity. The field of chiral NHC-metal complex catalyzed asymmetric hydrogenation ...

N-Heterocyclic Carbenes in Asymmetric Hydrogenation

Even though the existence of 1,2,3-triazoles has been known for more than a century, the recent discovery of a copper(I) catalyzed version of this reaction has attributed unprecedented importance to these compounds. Coordination and organometallic chemists have benefited from this modular synthetic route, and have accessed ligands based on both the triazoles as well as the triazolylidenes. The wide variation of steric and electronic properties that can be achieved for this ligand class has made them useful for generating metal complexes with various applications. Examples include, among others: magnetically switchable molecules, electro- and photo-active molecules, molecules for light-emitting electrochemical cells, dyes for dye-sensitized solar cells and a host of homogeneous catalytic processes. In this contribution, we present recent developments in this field with examples of some selected ligands. The focus is on systems developed in our group over the past few years.

Metal Complexes of Click‐Derived Triazoles and Mesoionic Carbenes: Electron Transfer, Photochemistry, Magnetic Bistability, and Catalysis

Halogenated triarylboranes (BAr3) have been known for decades, however it has only been since the surge of interest in main group catalysis that their application as strong Lewis acid catalysts has been recognised. This review aims to look past the popular tris(pentafluorophenyl)borane [B(C6F5)3] to the other halogenated triarylboranes, to give a greater breadth of understanding as to how tuning the Lewis acidity of BAr3 by modifications of the aryl rings can lead to improved reactivity. In this review, a discussion on Lewis acidity determination of boranes is given, the synthesis of these boranes is discussed, and examples of how they are being used for catalysis and frustrated Lewis pair (FLP) chemistry are explained.

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Halogenated triarylboranes: synthesis, properties and applications in catalysis.

Borane-Catalyzed Stereoselective C–H Insertion, Cyclopropanation, and Ring-Opening Reactions

Progress in frustrated Lewis pair (FLP) chemistry has revealed the importance of the main group elements in catalysis, opening new avenues in synthetic chemistry. Recently, new reactivities of frustrated Lewis pairs have been uncovered that disclose that certain combinations of Lewis acids and bases undergo single-electron transfer (SET) processes. Here an electron can be transferred from the Lewis basic donor to a Lewis acidic acceptor to generate a reactive frustrated radical pair (FRP). This minireview aims to showcase the recent advancements in this emerging field covering the synthesis and reactivities of frustrated radical pairs, with extensive highlights of the results from Electron Paramagnetic Resonance (EPR) spectroscopy to explain the nature and stability of the different radical species observed.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/ange.202010633

Frustrated Radical Pairs: Insights from EPR Spectroscopy.

Chiral 1,2,3-triazol-5-ylidene–Pd complexes with the planar chiral [22]paracyclophane wing tip group have been synthesized and structurally characterized The complex with a labile acetonitrile co-ligand is an excellent catalyst for chemoselective hydrogenation of alkynes and alkenes and enantioselective hydrogenation of prochiral alkenes at ambient conditions and 10 atmosphere of H2

Catalytic asymmetric hydrogenation using a [2.2]paracyclophane based chiral 1,2,3-triazol-5-ylidene–Pd complex under ambient conditions and 1 atmosphere of H2

Advances in the chemistry of metal-free systems known as frustrated Lewis pairs (FLPs) has exposed new reactivity of the p-block elements, particularly in small-molecule activation and catalysis. Typically, the mode of activation by FLPs has been predicated on a heterolytic two-electron process, although recently, select FLPs have been shown to participate in single-electron processes. Here, we report the reaction of diaryl substituted esters with FLPs. This results in divergent pathways, one whereby the diaryl moiety is stabilized by the Lewis basic phosphine, and the alternative pathway, wherein a single-electron transfer process occurs, generating the [Mes3P]+⋅/[C(H)Ar2]⋅ radical ion pair. The latter species undergoes a homocoupling reaction to yield tetraphenylethane derivatives. In the presence of olefins, this reactivity can be harnessed through an sp2-sp3 C–C heterocoupling reaction to generate α,β-substituted olefins. Notably, this work showcases an FLP approach to metal-free radical C–H bond activation with subsequent C–C bond formation, which also displays complementary reactivity to other approaches.

Ayan Dasgupta

Papers

Halogenated triarylboranes: synthesis, properties and applications in catalysis.

Borane-Catalyzed Stereoselective C–H Insertion, Cyclopropanation, and Ring-Opening Reactions

Frustrated Radical Pairs: Insights from EPR Spectroscopy.

Catalytic asymmetric hydrogenation using a [2.2]paracyclophane based chiral 1,2,3-triazol-5-ylidene–Pd complex under ambient conditions and 1 atmosphere of H2

Radical Reactivity of Frustrated Lewis Pairs with Diaryl Esters