Palladium N-heterocyclic carbene

Independently, Caddick et al. reported microwave-assisted amination of aryl chlorides using a palladium-N-heterocyclic carbene complex as the catalyst (Scheme 99) [lOlj. Initial experiments in a domestic microwave oven (reflux conditions) revealed that the solvent is crucial for the reaction. The Pd source also proved very important, since Pd(OAc)2 at high power in DMF gave extensive catalyst decomposition and using it at medium and low power gave no reaction at all. Pd(dba)2/imidazohum salt (1 mol% catalyst loading) in DME with the addition of some DMF was found to be suitable. Oil bath experiments indicated that only thermal effects are governing the amination reactions. 

Utilizing more reactive discrete palladium-N-heterocyclic carbene (NHC) complexes (for example, Pd(carb)2) or in situ generated palladium/imidazolium salt complexes (1 mol% ligand A), Caddick and coworkers were able to extend the rapid amination protocols described above to electron-rich aryl chlorides (Scheme 6.61) [128],... 

Aryl chlorides are more reluctant to participate in amination than most other aryl halides/pseudohalides. To tackle this problem, Caddick et al. examined the effect of palladium-N-heterocyclic carbenes as catalysts in rapid microwave-promoted reactions [87]. Para-tolyl and -anisyl chloride were reacted with aromatic and aliphatic amines in mostly good yields within 6 minutes of heating at 160 °C. Reactions using anisyl, tolyl or phenyl chlorides and aliphatic amines have also been reported by Maes et al. using a phosphine ligand and a strong base, which creates the desired products after 10 minutes of heating at 110-200 °C [88]. 

Keywords Palladium N-heterocyclic carbene Cross-coupling reaction Catalysis... 

Caddick, S., Cloke, F.G.N., Hitchcock, P.B. et al. (2002) The first example of simple oxidative addition of an aryl chloride to a discrete palladium N-heterocyclic carbene amination precatalyst. 

Katayev D, Lariraiov E, Nakanishi M, Besnard C, Kiindig EP (2014) Palladium-N-heterocyclic carbene (NHC)-catalyzed asymmetric synthesis of indolines through regiodivergent C (sp3)-H activation scope and DFT study. Chem Eur J 20(46) 15021-15030... 

A tetracyclic bis(imidazolidine), containing two fused diazepine rings, was prepared and served as a building block for formation of iridium- and palladium-N-heterocyclic carbene (NHC) complexes (130M6445). The tetracyclic system 95 was formed via condensation of 1,4-diaminobutane 92, glyoxal 93, and formaldehyde 94.The reaction of 95 with [IrCl(COD)]2 in the presence of excess 1,5-cyclooctadiene led directly to the iridium-NHC complex 96.The palladium-NHC complex 97 was accessed via oxidation of bis(imidazolidine) 95 with N-bromosuccinimide and reaction with Pd(OAc)2 in the presence of KL... 

E. Maerten, M. Sauthier, A. Mortreux, Y. Castanet, Tetrahedron, 2007, 63, 682-689. Palladium-N-heterocyclic carbene an efficient catalytic system for the carbonylative cross-coupling of pyridine halides with boronic acids. 

Z. S. Qureshi, S. A. Revankar, M. V. Khedkar, B. M. Bhanage, Catal. Today 2012,198,148-153. Aminocarbonylation of aryl iodides with primary and secondary amines in aqueous medium using polymer supported palladium N-heterocyclic carbene complex as an efficient and heterogeneous recyclable catalyst. 

Demir S, Ozdemir I, Arslan H, VanDerveer D. Butylene linked palladium N-heterocyclic carbene complexes synthesis and catalytic properties. J Organomet Chem. 2011 696 2589-2593. 

Nolan SP, Viciu MS (2005) The Use of N-Heterocyclic Carbenes as Ligands in Palladium Mediated Catalysis. 14 241-278... 

Scheme 6.122 Preparation of a pyridyl bis-N-heterocyclic carbene palladium complex.

We reported the use of M-heterocyclic carbene complexes (NHC) for the catalytic activation of methane [55,56]. We found that solutions of N-heterocyclic carbene complexes of palladium(II) in carboxylic acids catalyze the conversion of methane to the corresponding methylesters. The high thermal stability of palladium(II) carbene complexes could be shown for complex 18 (Scheme 22), which we also structurally characterized [120]. An extraordinary feature is the unprecedented resistance of the palladium-NHC-complexes 18-22 under the acidic oxidizing conditions which are necessary for the CH-activation and functionalization. 

Peris E (2006) Routes to N-Heterocyclic Carbene Complexes. 21 83-116 Popp BV, Stahl SS (2007) Palladium-Catalyzed Oxidation Reactions Comparison of Benzo-quinone and Molecular Oxygen as Stoichiometric Oxidants. 22 149-189 Prashad M (2004) Palladium-Catalyzed Heck Arylations in the Synthesis of Active Pharmaceutical Ingredients. 6 181-204... 

Following the introduction to this volume provided herein, the following authors will continue the discussion of N-heterocyclic carbenes in this volume. Peris will discuss routes to NHC complexes, a prerequisite for doing catalysis. First and foremost, palladium catalysis has benefited from the use of NHC. The unique properties of NHC allow their use in oxidation catal-... 

Other bidentate N-heterocyclic carbenes were used to form stable chelate complexes. A fine example is the use of palladium NHC complex 24 in the catalytic conversion of methane to methanol (Fig. 10) [111]. In this case the stability of the complexes is a requirement, since the reaction takes place in an acidic medium (trifluoroacetic acid) at elevated temperatures (80 °C) mediated by strong oxidizing agents (potassium peroxodisulfate). 

Abstract N-heterocyclic carbenes (NHCs) have attracted increasing attention since their discovery. Notably, they have allowed for major advances in palladium-catalyzed reactions. Mainly known for their application in cross-coupling reactions, this review intends to provide a broader overview of (NHC)-palladium systems in organic transformations. 

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