Palladium carbenes

The first examples of microwave-assisted cross-couplings with organozinc compounds were recently reported [47]. In addition, the first high-speed synthesis of aryl boronates (Suzuki coupling reactants) has been performed under the action of single-mode irradiation with an in-situ-generated palladium carbene catalyst [48],... 

The synthesis of this salt started with the enantiomerically pure 1,2-diamine 106, that was converted into the corresponding thiourea derivative 107 (Scheme 12). Exposure of the thiourea 107 to oxalyl chloride in toluene at 60 °C cleanly afforded the desired imidazolinium chloride 108. These two salts were used to produce new palladium and nickel carbene complexes. The structure of both palladium carbene complexes 96a and 96b has been elucidated by X-ray diffraction <2005CEJ1833>. 

Another strategy was successfully implemented by synthetic deprotonation of the acidic C2 group of the imidazolium cation by basic ligands of metal complexes, forming carbenes (Scheme 12). When Pd(OAc)2 was heated in the presence of [BMIM]Br, a mixture of palladium imidazolylidene complexes formed. The palladium carbene complexes have been shown to be active and stable catalysts for the... 

The telomerization of the lower reactive isoprene with glycerol was achieved in the presence of palladium-carbene complex but in a dioxane/PEG solvent [15]. Under such conditions, both glycerol and PEG are converted. After 24 h, in the presence of 0.06% [Pd(acac)2/IMes.Cl] (1/1.5) at 90°C, the telomerization of isoprene with glycerol (glycerol/PEG/dioxane/isoprene = 1/2.5/2.5/5) yields 70% of the linear monoether glycerol together with 29% of PEG telomer. 

The telomerization of butadiene by means of water in ILs was described by Dullius et Rottger et al. report a process for the telomerization of acyclic olefins having at least two conjugated double bonds, or their mixtures, using a palladium-carbene complex as catalyst in an IL solvent. The nucleophiles included water, alcohols, phenols, polyols, carboxylic acids, ammonia and primary and secondary amines. The acycylic olefins could be either 1,3-butadiene or isoprene. 

But not only palladium(O) complexes can activate CO or O2, also palla-dium(II) complexes have been reported to be active in the presence of carbon monoxide or dioxygen as it was shown in the direct synthesis of polycarbonate from CO and phenol or bisphenol A [79,80]. The authors could confirm the positive influence of the NHC ligand comparing the activity and reactivity of the palladium-carbene complex with the corresponding PdBr2 catalyst. The molecular weights and yields of the polycarbonates improved with increasing steric hindrance of the substituents in the l,T-position of the car-bene complex. 

Even more interesting is the oxidative kinetic resolution of alcohols under aerobic conditions. The system Pd(lI)/sparteine/02 was reported to convert a racemic alcohol with high selectivity into the ketone and the alcohol [97-99]. This has also been shown to work with palladium carbene complexes (Scheme 16). 

Members of the tetracyclic dibenzopyrrocoline alkaloid family can be prepared by the intramolecular ring closure of l-(o-halobcnzyl)-tetrahydroisoquinoline derivatives. (3.38.)47 The analogous transformation of dihydroisoquinolines (3.39.) proceeds probably through the isomeric enamine form obtained by the tautomeric shift of the double bond 48 The palladium-carbene catalyst system applied in these reactions was also effective in the preparation of indoline, indolizidine and pyrrolizidine derivatives 49... 

Methane, C-H activation, via palladium carbenes, 8, 243 Methanol carbonylations to acetic acid, 1, 133 iridium-catalyzed... 

Type III reactions proceed by attack of a nucleophile at the central sp carbon of the allenyl system of the complexes 5. Reactions of soft carbon nucleophiles derived from active methylene compounds, such as /i-kcto esters or malonates, and oxygen nucleophiles belong to this type. The attack of the nucleophile generates the intermediates 9, which are regarded as the palladium-carbene complexes 10. The intermediates 9 pick up a proton from the active methylene compound and n-allylpalladium complexes 11 are formed, which undergo further reaction with the nucleophile, as expected, and hence the alkenes 12 are formed by the introduction of two nucleophiles. 

Similarly, vinylstannanes can also yield products of cine-substitution (Scheme 8.17), specially if tin and an electron-withdrawing or aryl group are bound to the same carbon atom [40, 137-141]. It has been suggested that formation of these products proceeds via intermediate formation of a palladium carbene complex [138, 140] or via reversible /3-hydride elimination [141], and can be avoided by addition of Cu(I) salts [142], which increase the rate of Stille coupling, or by protecting vinylic C-H groups by transient silylation [143]. 

Clement ND, Routaboul L, Grotevendt A, Jackstell R, Beller M (2008) Development of palladium-carbene catalysts for telomerization and dimerization of 1,3-dienes from basic research to industrial applications. Chem Eur J 14 7408-7420... 

Jackstell R, Grotevendt A, Michalik D, El Firdoussi L, Beller M (2007) Telomerization and dimerization of isoprene by in situ generated palladium-carbene catalysts. J Organomet Chem... 

Grotevendt A, Jackstell R, Michalik D, Gomez M, Beller M (2009) Efficient and selective telomerization of 1,3-butadiene with diols catalyzed by palladium-carbene complexes. ChemSusChem 2 63-70... 

Scheme 29 Selectivity in telomerization or dimerization product by modification of the palladium carbene catalyst...

Major trends can be discerned for Pd-catalysts, aimed at increasing the stability and activity. First is the use of palladium-carbene complexes [178]. Although activities are still modest, much can be expected in this area. Second is the synthesis and use of palladium nanoparticles. For example, the giant palladium cluster, Pd561phen6o(OAc)i8o [179], was shown to catalyze the aerobic oxidation of primary allylic alcohols to the corresponding a,/funsaturated aldehydes (Fig. 4.66) [180]. 

C4Ciim][BF4] [QCpmJBr Pd(OAc)2 NaOAc 90-125 °C. Phosphine-free arylation of acrylates higher reaction rates in due to in situ formation of palladium carbene halides product extracted with ethyl acetate. [20]... 

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. 

The number of alkyl substituents attached to the imidazolium cation was found to be of great importance in the telomerisation of butadiene and methanol with a palladium/phosphine catalyst system, see Scheme 8.8.[27] In the presence of imidazolium ionic liquids with an acidic proton in the 2-position, rapid deactivation of the catalyst took place and it was proposed that formation of stable and inactive palladium-carbene species occurred. In contrast, both a pyridinium-based ionic liquid as well as imidazolium ionic liquids bearing a methyl substituent in the 2-position led to active systems. 

The same reaction was also investigated using the palladium-carbene complex, 58, as catalyst (Scheme 9.12).1401 Of the different ionic liquids studied, tetrabutylammonium bromide gave by far the best results while imidazolium based solvents afforded only poor conversions. At atmospheric pressure, only iodobenzene was carbonylated. The conversion of less reactive arylhalides not only required higher CO-pressures, but also the addition of a phosphine ligand. Reuse of the catalyst after extraction of the product with diethyl ether was possible for at least 6 runs with only a moderate decrease in activity. 

Note The compound in Figure 6.38 is not a palladium carbene complex but could be transformed into one by facile protonation (CF SO H) or methylation (CF SO Me). 

Palladium isocyanide complexes react with amines and alcohols to give palladium Carbene Complexes (equation 15) in a reaction that is a nucleophihc attack on the isonitrile carbon. These complexes easily lose a halide if it is trans to the carbene hgand, since the carbene carbon has a high trans influence. The structures of these compounds have the carbene carbon and its two heteroatom groups in one plane, which is perpendicular to the coordination plane of the Pd. 

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