Palladium nucleophilic carbenes

Hillier AC, Grasa GA, Viciu MS, Lee HM, Yang CL, Nolan SP (2002) Catalytic cross-coupling reactions mediated by palladium/nucleophilic carbene systems. J Qrganomet Chem 653 69-82... 

Hillier, A.C., Nolan, S.P. Palladium/nucleophilic carbene catalysts for cross-coupling reactions.Plat. Met. Rev. 46, 50 (2002)... 

Jafarpour, L., Grasa, G. A., Viciu, M. S., Hillier, A. C., Nolan, S. P. Convenient and efficient cross-coupling of aryl halides mediated by palladium/bulky nucleophilic carbenes and related ligands. Chimica Oggi200, 19,10-16. 

Chiral imidazolium or triazolium salt reacted with Pd(OAc)2 in the presence of Nal and KO Bu, giving a mixture of carbene-Pd complexes (Scheme These nucleophilic carbenes are regarded as replacements of phosphines with stronger Lewis basicity and attracted considerable attention recentlyThe palladium complexes formed as mixture of cis and trans isomers, which were separable by column chromatography. Upon heating in DMF at 100 °C, the cis isomer is completely isomerized into the trans isomer. In these complexes, rotations around the carbene-Pd bonds are restricted, probably due to double bond characters of the C—Pd bonds, and several diastereomers were detected. 

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. 

Keywords Absolute configuration, Amines, Amino acids, Carbenes, Cascade reactions, 2-chloro-2-cyclopropylideneacetates. Combinatorial libraries. Cycloadditions, Cyclobutenes, Cyclopropanes, Diels-Alder reactions. Heterocycles, Michael additions. Nitrones, Nucleophilic substitutions, Peptidomimetics, Palladium catalysis. Polycycles, Solid phase synthesis, Spiro compounds. Thiols... 

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. 

As an alternative to addition of anionic nucleophiles followed by reoxidation, rhodium(l)-catalyzed C-H activation allowed the nucleophilic addition of alkenes to the intermediate Rh(i) carbene complex <2002JA13964, 2004JOC7329>. Purine behaved anomalously compared to other heterocycles, for which selective monoalkylation was observed, and underwent sequential substitution first at C-8 and then at C-6 (Equation 8). Caffeine was monoalkylated at C-8 in low yield (15%). Selectivity for C-8-arylation was also observed in the palladium-catalyzed C-H activation of 6-phenyl-9-benzylpurine (aryl iodides, 0.05 equiv Pd(OAc)2, 3 equiv Cul, 2.5 equiv CS2CO3, DMF, 160 °C, 60 h, 48-95% yields) <2006OL5389>. 

Arasabenzene, with chromium, 5, 339 Arcyriacyanin A, via Heck couplings, 11, 320 Arduengo-type carbenes with titanium(IV), 4, 366 with vanadium, 5, 10 (Arene(chromium carbonyls analytical applications, 5, 261 benzyl cation stabilization, 5, 245 biomedical applications, 5, 260 chiral, as asymmetric catalysis ligands, 5, 241 chromatographic separation, 5, 239 cine and tele nucleophilic substitutions, 5, 236 kinetic and mechanistic studies, 5, 257 liquid crystalline behaviour, 5, 262 lithiations and electrophile reactions, 5, 236 as main polymer chain unit, 5, 251 mass spectroscopic studies, 5, 256 miscellaneous compounds, 5, 258 NMR studies, 5, 255 palladium coupling, 5, 239 polymer-bound complexes, 5, 250 spectroscopic studies, 5, 256 X-ray data analysis, 5, 257... 

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. 

Fischer-type carbenes can also be modified via transition metal catalyzed reactions. Fischer chromium aminocarbene complexes can be used as nucleophiles in palladium-catalyzed allyUc substitution reactions with aUylic acetates and carbonates, alFording the corresponding allyl-substituted aminocarbenes. For example, reaction of the Uthiated carbene (15) gives (16) in good yield (Scheme 25). ... 

Doyle has put forward arguments against the intermediacy of such complexes in catalytic cyclopropanation . Firstly, metal coordination activates the alkene to nucleophilic attack. Hence, an electrophilic metal carbene would add only reluctantly or not at all. Secondly, the stable PdCl2 complexes of dienes 8 and 428 do not react with ethyl diazoacetate, even if Rh fOAc) or PdCljfPhCbOj is added. The diazoester is decomposed only when it is added to a mixture of the Pd complex and excess diene. These results exclude the metal-carbene-olefin intermediate, but they leave open the possibility of metal carbene interaction with an uncomplexed olefin molecule. The preferred formation of exo-cyclopropanes in the PdCyPhCN) -catalyzed reactions between 8 and N2CHCOOEt or N2CPh2, with exo. endo ratios virtually identical to those observed upon cyclopropanation of monoolefin 429, also rule out coordination of a palladium carbene to the exocyclic double bond of 8 prior to cyclopropanation of the endocyclic double bond. 

Very common Cl fragments bound to a polymeric support are carbenoids. The simplest carbenoid is CO itself, which can be used in transition metal-catalyzed insertions between benzofuran-3yl palladium and a nucleophile such as trifluoroethanol [161]. One of the most common carbenoids is isonitrile, which is used in a large number of multicomponent reactions. Diazoalkanes have often been used as carbene precursors, since they can be readily obtained from support-bound anions and sulfonyl azides [252]. Commonly, diazoalkanes are converted transi-... 

The main advantage of these coordinating anions is that they stabilize the active species. This is particularly obvious in the case of palladium complexes, whose tendency to decompose into black metal is well documented. Imidazolium-based ionic liquids can generate in situ formation of metal-imidazolylidene carbene complexes by a deprotonation of the imidazolium cation. The ease of deprotonation depends on the nucleophilicity of the anions. In this case, NAILs may act as both solvents and catalyst ligand precursors [13],... 

Palladium isocyanide complexes react with amines and alcohols to give palladium Carbene Complexes (equation 15) in a reaction that is a nucleophilic attack on the isonitrile carbon. These complexes easily lose a halide if it is trans to the carbene ligand, 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. 

Recently, palladium complexes of carbene ligands have been recognized as highly reactive catalysts for palladium-promoted reactions, in particular for the Heck reaction [42-44]. The polymer-supported palladium carbene complexes 18 and 19 were prepared by the nucleophilic substitution of the bromomethy-lated Wang resin with 17 under basic reaction conditions (Scheme 7) [45]. The catalytic activity of 18 and 19 was examined for the Heck reaction of aryl bromides with acrylates or styrene to exhibit high TONs up to 5,000. The polymer-supported palladium-carbene complexes are air-stable and recyclable. 

Various bulky phosphines and AT-heterocydic carbene ligands have proved to be effective in a number of palladium-catalyzed reactions of aryl chlorides and bromides [28-30], e.g., Mizoroki-Heck reaction, Suzuki-Miyaura reaction, Migita-Kosugi-Stille reaction, amination, and alkoxylation, as well as the reaction with various carbon nucleophiles as described below. The ligands are considered to enhance both the initial oxidative addition of aryl halides and the reductive elimination of products. The identity of bases is also an important factor to obtain satisfactory results commonly in the arylation of carbon nucleophiles NaH,NaO-t-Bu, K3PO4,M2CO3 (M=K, Cs), and MN(SiMe3)2 (M= Li, Na, K) are typical choices. 

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