Palladium N-heterocyclic carbene complexes

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. 

Lewis, A.K.D., Caddick, S., Cloke, F.G.N. et al. (2003) Synthetic, structural, and mechanistic studies on the oxidative addition of aromatic chlorides to a palladium (N-heterocyclic carbene) complex relevance to catalytic amination. J.Am. Chem. Soc., 125, 10066-73. 

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. 

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],... 

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... 

Bis(pyrazolyl)borate copper complex 902 has been employed as a catalyst in homogeneous and heterogeneous styrene epoxidation reactions <2000CC1653>. Pyrazole palladacycles 903 have proved to be stable and efficient catalysts for Heck vinylations of aryl iodides <2000CG2053>. A hemilabile pyrazolyl-functionalized N-heterocyclic carbene complex of palladium(ll) such as 904 has been found to be an excellent catalyst for Heck and Suzuki crosscoupling reactions in ionic liquids <2006JOC426>. 

Figure 25. N-Heterocyclic Carbene Complexes of Palladium in the Heck Reaction in Ionic Liquids.

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... 

Ellul CE, Reed G, Mahon MF, Pascu SI, Whitdesey MK. Tripodal N-heterocyclic carbene complexes of palladium and copper syntheses, characterization, and catalytic... 

Munz D, Poethig A, Tronnier A, Strassner T. ortho-Phenylene bridged palladium bis-N-heterocyclic carbene complexes synthesis, structure and catalysis. Dalton Trans. 2013 42 7297-7304. 

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

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). 

N-Heterocyclic carbenes are relatively stable, electron-rich hgands and can be made sterically demanding. They are donor ligands with httle 7r-acid properties, hke trialkylphosphines. These new ligands are readily available by deprotonation of azolium precursors, and the deprotonation can be carried out in the presence of palladium salts to generate the complexes directly (Scheme 12). 

Others have investigated the kinetics of amination reactions mediated by catalyst systems employing the new electron-rich monodentate ligands. In particular, Hartwig has shown that for catalysis by a 1 1 palladium to Xn tert-butyl)phosphine system, a mechanism in which oxidative addition of aryl chlorides follows coordination of base to the palladium competes with the standard nonanionic pathway. Finally, Caddick, Cloke, and coworkers have studied amination reactions of aryl chlorides performed by palladium complexes of N-heterocyclic carbene ligands. They found the rate to be limited by the oxidative addition step, which occurs first through the dissociation of an NHC ligand. 

Other palladium acetylacetonate complexes have also been developed for cross-coupling reactions. Indeed, an acac paUadacycle complex has recently been observed to effectively catalyze both the Suzuki-Miyaura and Heck reactions. Likewise, the A-heterocyclic carbene complex Pd(acac)Cl(ipr) (96) is an excellent catalyst for the formation of C—N and C—C bonds (equation 27) These catalysts were effective for coupling of ketones... 

Herrmann, W. A., Ofele, K., von Preysing, D., Schneider, S. K. Phospha-palladacycles and N-heterocyclic carbene palladium complexes efficient catalysts for CC-coupling reactions. J. Organomet. Chem. 2003, 687, 229-248. 

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