Coupling reactions palladium carbene complexes

It is not necessary to use preformed and isolated palladium carbene complexes they can also be generated in situ [31]. When Pd(OAc)2 is dissolved in the presence of a base in [BMIm]Br, styrene was efficiently coupled with iodobenzene affording stilbene (complete conversion and 99% selectivity). The authors were able to isolate the thus-formed palladium carbene complex, derived from the ionic liquid solvent. The counter ion of the ionic liquid also plays an important role. Mizoroki-Heck reactions proceeded much faster in [BMIm]Br than in [BMIm]BF4. In the latter, precipitation of palladium black was encountered. The fact is explained by the necessity of the presence of a halide ion for the stabilization of the carbene-palladium complex [32]. 

The application of ultrasound in Mizoroki-Heck reactions is not well described. The first application of ultrasound irradiation in Mizoroki-Heck reactions performed in ionic liquids catalysed by palladium-carbene complexes and palladium nanoparticles was published in 2001 [ 190]. The Pd/C-catalysed reaction of iodobenzene and methyl methacrylate in NMP at room temperature resulted in an increased reaction rate under sonochem-ical conditions [191]. The catalyst showed high activity under phosphine-free ambient conditions and could be reused. The coupling products could be obtained in up to 70% yield. 

Schonfelder, D., Fischer, K., Schmidt, M. et al. (2(X)5) Poly(2-oxazoline)s functionalized with palladium carbene complexes soluble, amphiphilic polymer supports for C-C coupling reactions in water. Macromolecules, 38, 254-62. 

Palladium complexes featuring A -heterocyclic carbene ligands (NHCs, Fig. 11.1) are a class of compounds of supreme importance in modern chemistry, which are extensively applied, in particular, in catalysis of cross-coupling reactions [1-4], Complexes with these species have challenged the dominance of the commonly used tertiary phosphine-based catalysts in a wide range of palladium-catalyzed organic processes [3, 4],... 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

A silver(i) complex having the heterosubstituted 3-methyl-l-(2-pyridylmethyl)imidazol-2-ylidene ligand, [Ag(carbene)2] [I/Agl2] 37, was reported, which was further reacted to give a series of palladium(n) carbene complexes that were demonstrated to be active catalysts toward Heck, Suzuki, and Sonogashira coupling reactions.87... 

This article presents the principles known so far for the synthesis of metal complexes containing stable carbenes, including the preparation of the relevant carbene precursors. The use of some of these compounds in transition-metal-catalyzed reactions is discussed mainly for ruthenium-catalyzed olefin metathesis and palladium-Znickel-catalyzed coupling reactions of aryl halides, but other reactions will be touched upon as well. Chapters about the properties of metal- carbene complexes, their applications in materials science and medicinal chemistry, and their role in bioinorganic chemistry round the survey off. The focus of this review is on ZV-heterocyclic carbenes, in the following abbreviated as NHC and NHCs, respectively. 

Steel PG, Teasdale CWT. Polymer supported palladium A-heterocyclic carbene complexes long lived recyclable catalysts for cross coupling reactions. Tetrahedron Lett 2004 45 8977-8980. 

In a related transformation, a palladium-benzothiazole carbene complex has been reported to efficiently catalyze the arylation of allylic alcohols [79]. Carrying the reaction in an ionic liquid, the authors could couple aryl bromides and activated aryl chlorides with terminal allylic alcohols with remarkable regioselectivity (Scheme 10). The interest of this methodology was also highlighted by its application to the synthesis of three intermediates in the synthesis of medicinal products [80-82],... 

Through the direct coupling of 2-bromo-4,4-dimethyloxazoline 189 and 1-mesityl imidazole (188) the corresponding imidazolium salt 190 was obtained and used for the preparation of a mono-carbene-palladium complex 191 active as a catalyst in Heck and Suzuki C-C coupling reactions <020M5204>. 

Kantchev EAB, O Brien CJ, Organ MG (2007) Palladium complexes of A-heterocyclic carbenes as catalysts for cross-coupling reactions - a synthetic chemist s perspective. Angew Chem Int Ed 46 2768-2813 Kerr MS, Rovis T (2004) Enantio selective synthesis of quaternary stereocenters via a catalytic asymmetric Stetter reaction. J Am Chem Soc 126 8876-8877 Kerr MS, Read de Alaniz J, Rovis T (2002) A highly enantio selective catalytic intramolecular Stetter reaction. J Am Chem Soc 124 10298-1029 Kerr MS, Read de Alaniz J, Rovis T (2005) An efficient synthesis of achiral and chiral 1,2,4-triazolium salts bench stable precursors for A-heterocyclic carbenes. J Org Chem 70 5725-5728... 

Comparable reaction conditions were applied in the coupling of activated and non-activated arylchlorides with styrene or 2-ethylhexyl acrylate, using the palladium carbene catalysts shown in Scheme 6.6. While compounds 22 and 23 were found to be highly active catalysts, complex 21 was thermally unstable and decomposed to palladium black during the catalysis.1671 The yield and selectivity were only moderate in DMA, but results improved markedly when the reaction was carried out in [(C4)4N]Br. 

Depending on the alkyne, both six- and five-membered rings may be obtained npon reaction of B-carbomethoxy-alkenyl-ammo carbenes (Scheme 46). Dihydropyridme and indole carbene complexes participate in benzannnlation reactions (Scheme 47). Alkynes containing boronic esters may be used, affording boronic ester-fimctionalized quinones that can be used in palladium-catalyzed coupling reactions (Scheme 48). ... 

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... 

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