N-Heterocyclic Carbenes in Late Transition Metal Catalysis

Palladium-catalyzed C-C and C-N bond-forming reactions are among the most versatile and powerful synthetic methods. For the last 15 years, N-heterocyclic carbenes (NHCs) have enjoyed increasing popularity as ligands in Pd-mediated cross-coupling and related transformations because of their superior performance compared to the more traditional tertiary phosphanes. The strong sigma-electron-donating ability of NHCs renders oxidative insertion even in challenging substrates facile, while their steric bulk and particular topology is responsible for fast reductive elimination. The strong Pd-NHC bonds contribute to the high stability of the active species, even at low ligand/Pd ratios and high temperatures. With a number of commercially available, stable, user-friendly, and powerful NHC-Pd precatalysts, the goal of a universal cross-coupling catalyst is within reach. This Review discusses the basics of Pd-NHC chemistry to understand the peculiarities of these catalysts and then gives a critical discussion on their application in C-C and C-N cross-coupling as well as carbopalladation reactions.

Palladium Complexes of N-Heterocyclic Carbenes as Catalysts for Cross-Coupling Reactions—A Synthetic Chemist's Perspective

It is well-recognized that N-heterocyclic carbene (NHC) ligands have provided a new dimension to the design of homogeneous catalysts. Part of the success of this type of ligands resides in the limitless access to a variety of topologies with tuned electronic properties, but also in the ability of a family of NHCs that are able to adapt their properties to the specific requirements of individual catalytic transformations. The term “smart” is used here to refer to switchable, multifunctional, adaptable, or tunable ligands and, in general, to all those ligands that are able to modify their steric or electronic properties to fulfill the requirements of a defined catalytic reaction. The purpose of this review is to comprehensively describe all types of smart NHC ligands by focusing attention on the catalytically relevant ligand-based reactivity.

Smart N-Heterocyclic Carbene Ligands in Catalysis

Preparation and application of N-heterocyclic carbene complexes of Ag(I)

Palladiumkatalysierte C-C- und C-N-Kupplungen zahlen zu den vielseitigsten und leistungsfahigsten Syntheseverfahren. Seit 15 Jahren erfreuen sich die N-heterocyclischen Carbene (NHCs) wachsender Beliebtheit als Liganden fur palladiumvermittelte Kreuzkupplungen und ahnliche Methoden. Sie sind in vielerlei Hinsicht den ublichen tertiaren Phosphanen uberlegen. Weil NHCs stark σ-elektronenschiebend sind, konnen sie auch anspruchsvolle Substrate zu oxidativen Insertionen bewegen. Andererseits fuhren ihre Sperrigkeit und ihre besondere Topologie zu einer raschen reduktiven Eliminierung. Schlieslich tragen die starken Pd-NHC-Bindungen zur hohen Stabilitat der aktiven Spezies bei – auch bei niedrigen Ligand/Pd-Verhaltnissen und hohen Temperaturen. Wenn erst stabile, anwenderfreundliche und leistungsstarke NHC-Pd-Prakatalysatoren kommerziell verfugbar sind, liegt das Ziel eines universellen Kreuzkupplungskatalysators in greifbarer Nahe. Dieser Aufsatz beschreibt zunachst die chemischen Grundlagen zu den NHC-Pd-Komplexen, um ihre Besonderheiten kennenzulernen. Anschliesend folgt eine umfassende Diskussion zur Anwendung dieser Katalysatoren in C-C- und C-N-Kreuzkupplungen sowie in der Carbopalladierung.

Aus der Sicht des Synthetikers: Palladiumkomplexe N-heterocyclischer Carbene als Katalysatoren für Kreuzkupplungen

A stable crystalline imino-N-heterocyclic carbene ligand and its corresponding palladium(II) and rhodium(I) complexes.

The imidazolium salts: 1-mesityl-3-(3-trimethoxysilylpropyl)imidazolium iodide and 1-tert-butyl-3-(3-trimethoxysilylpropyl)imidazolium iodide, abbreviated as (tmpMes)HI (3a) and (tmptBu)HI (3b), respectively, have been synthesised. The palladium(II) complexes (η3-C3H5) (tmpMes)PdCl (5a) and (η3-C3H5)(tmptBu)PdCl (5b), rhodium(I) and iridium(I) complexes (η4-1,5-COD) (tmpMes)MCl, M = Rh (6a), Ir (7a) and (η4-1,5-COD)(tmptBu)MCl, where M = Rh (6b), Ir (7b), were synthesised by silver transmetallation reactions using the silver(I) complexes (tmpMes)AgI (4a) and (tmptBu)AgI (4b). The iridium complex 7b has been structurally characterised. The Pd(II) and Rh(I) complexes have been immobilised by attachment to chemically modified MCM-41. The immobilised palladium(II) materials have been tested as recyclable catalysts for Suzuki type C–C bond formation reactions in water and the immobilised rhodium(I) materials have been examined for their catalytic ability for the hydroformylation of 1-octene.

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Heterogenised N-heterocyclic carbene complexes: synthesis, characterisation and application for hydroformylation and C–C bond formation reactions

Synthesis, Structure, and Temperature-Dependent Dynamics of Neutral Palladium Allyl Complexes of Annulated Diaminocarbenes and Their Catalytic Application for C−C and C−N Bond Formation Reactions†

A nonenolizable imino- N -heterocyclic carbene ligand and corresponding silver (I) metal complex

Stable annulated diaminocarbene ligands 7,9-bis(2,4,6-trimethylphenyl)-6b,9a-dihydroace naphtha[1,2-d]imidazolin-2-ylidene and 7,9-bis(2,6-diisopropylphenyl)-6b,9a-dihydroacenaphtho[1,2-d]imidazolin-2-ylidene; designated as (BIAN-SIMes, 5a,) and (BIAN-SIPr, 5b), respectively, have been prepared. The base dependent decomposition of imidazolinium salts via ring opening at the backbone was also observed. The corresponding rhodium(I) and iridum(I) complexes (η4-1,5-COD)M(BIAN-SIMes)Cl and (η4-1,5-COD)M(BIAN-SIPr)Cl; M= Rh (6a, 6b) and Ir (7a, 7b) have been synthesised by the reaction of free carbene with [M(η4-1,5-COD)(μ-Cl)]2; where M= Rh, Ir. The cationic Ir(I) complexes [(η4-1,5-COD)Ir(BIAN-SIMes)Py]BF48a and [(η4-1,5-COD)Ir(BIAN -SIPr)Py]PF68b have also been synthesised. Compounds 4b, 5a, 6a, 6b, 7b and 8b have been structurally characterised. The catalytic activities for the rhodium(I) complexes 6a and 6b were evaluated for the hydroformylation of 1-octene.

Sarim Dastgir

Papers

A stable crystalline imino-N-heterocyclic carbene ligand and its corresponding palladium(II) and rhodium(I) complexes.

Heterogenised N-heterocyclic carbene complexes: synthesis, characterisation and application for hydroformylation and C–C bond formation reactions

Synthesis, Structure, and Temperature-Dependent Dynamics of Neutral Palladium Allyl Complexes of Annulated Diaminocarbenes and Their Catalytic Application for C−C and C−N Bond Formation Reactions†

A nonenolizable imino- N -heterocyclic carbene ligand and corresponding silver (I) metal complex

Stable crystalline annulated diaminocarbenes: coordination with rhodium(I), iridium(I) and catalytic hydroformylation studies