Imidazolium carbenes

Conditions (unless otherwise noted) 11.0 mL (83.8 mmol) chloropinacolone, 30.0 mL (0.126 mol) n-BusN, 110 mL MeOH, O.lmmol Pd, 5.4 atm CO, 105°C, 3 h Abbreviations MPA = methyl pivaloylacetate PA = Pinacolone TON = mol MPA produced/mol Pd used Me2lm = 1,3-dimethyl imidazoline-2-ylidene (dimethyl imidazolium carbene complex) Ms2lm = l,3-di-(2,4,6-trimethylphenyl) imidazoline-2-ylidene (dimesityl imidazolium carbene complex) cy-hex = cyclohexyl Used 0.127 mmol Pd catalyst... 

An N-heterocyclic carbene ligand, formed from l,3-bis-(2,4,6-trimethylphenyl)-3//-imidazol-l-ium chloride and cesium carbonate, with dipalladium tris(dibenzylideneacetone) gave excellent yields (93-96% yields) in the Suzuki coupling of 2-chloropurines and arylboronic acids in anhydrous dioxane <2001TL8751>. The combination of an imidazolium-carbene and nickel(O) bis(cyclooctadiene) formed a catalyst capable of insertion into the C-F bond of 6-fluoropurine nucleosides (Scheme 35) <20050L1149>. 

Both 6-alkanesulfanyl- and 6-alkanesulfonylpurines proved competent coupling partners in the palladium-catalyzed Suzuki reaction with boronic acids when an imidazolium carbene ligand was used <20050L1149>. The sulfones were found to be significantly more reactive (60 °C, THF) than the thioethers (90 °C, toluene) (Scheme 44). 

Similarly, Buchmeister and co-workers functionalized a monolithic carrier by an imidazolium carbene precursor and, following generation of the carbene by deprotonation, prepared the active ruthenium catalyst. 

Nolan reported the use of the 2,6-diisopropylphenyl imidazolium carbene precursor, which contains an unsaturated backbone, for the reaction of aryl chlorides with a variety of amines at 100 °C [165, 166]. This temperature is lower than those conventionally used for reactions of aryl chlorides, but is higher than those used with P(tBu)3 or the 2-biphenylyl di-tert-butylphosphines. Reaction yields were high when 2 mol % palladium was used. Reactions of primary amines occurred in good yield, even when unhindered aryl halides were used. The monoarylamine was obtained in 86 % yield, and only a 5 % yield of the diaryl-amine by-product was isolated. Notably, reactions of both aryl bromides and iodides proceeded at room temperature. 

Numerous methods for the generation of imidazole carbenes have been reported. For example, starting from an imidazolium halide, the use of systems such as sodium hydride in ammonia or dimethylsulphoxide (DMSO), sodium in ammonia, alkali metals in tetrahydrofuran (THF), metal ferf-butoxides in THF or DMSO, etc. Recently, Seddon and Earle reported a simple procedure for the generation of the imidazolium carbene 2 in 90-95% yield from an imidazolium chloride 1 which does not require solvents, filtrations, or produce noxious waste products (Scheme 4) [40],... 

CHO 0.25% [Pd(allyl)CI]2, imidazolium carbene ligand PhBp3K, CS2CO3, toluene-H20... 

Andrus, M.B., Song, C. and Zhang, J.Q. (2002) Palladium-imidazolium carbene catalyzed Mizoroki-Heck coupling with aryl diazonium ions. Org. Lett., 4, 2079-2. 

Carbenes are chemical species that possess a bivalent carbon atom with two nonbonding electrons. The divalent carbenes were considered as highly reactive intermediates, and the isolation of stable carbenes has been a challenge for a long time. Based on Wanzlick et al. s work in 1960, Arduengo et al. [1] reported the synthesis of stable imidazolium carbenes in 1991. Independently, Bertrand and coworkers [2] reported the synthesis of phosphinocarbene in 1988. In 1995, Enders et al. [3] reported the synthesis of triazolium carbenes. 

Non-chiral imidazolium carbenes were also used as Brpnsted bases to directly generate the aldehyde enolates for an intramolecular Michael reaction of 0=CH(CH2)4CH=CXY (X = COPh, CO2R Y = H, CO2R) to produce 1,2-rran5-disubstituted cyclopentanes. ... 

Scheme 4.41 shows the reaction of Ph2SiH2 with CO2 under imidazolium-carbene catalysis [178-180]. 

A similar reaction is also promoted by imidazolium carbene carboxylates [186] as shown in (4.38), where the activated-H migrates to imidazole and CO2 forms a C-C bond with acac. 

Two general classes of N-heterocyclic carbenes were investigated including the thiazolium and imidazolium carbenes (Scheme 5 f. The thiazolium carbenes were formed in situ from their respective salts with 5 equivalents of triethylamine, 11. The ROP of lactide was accomplished in CH2CI2 under mild conditions (2-4 days at 25 °C) producing polymers of controlled molecular weight and narrow polydispersity with -85%... 

Several mono-NHC complexes were obtained. For example, transmetallation from a silver-NHC complex afforded the preparation of an Rh-NHC complex, in which the NHC ligand was obtained from caffeine. Deprotonation of chiral ferrocenyl imidazolium salts allowed the preparation of chiral Rh(l)-NHC complexes that could also be P,C and S,C chelate. Dicyclophane imidazolium carbene ligands also provided an efficient way of obtaining chiral Rh-NHC catalysts. ... 

Ma, Y., Song, C., Ma, C., Sun, Z., Chai, Q., and Andrus, M.B. (2003) Asymmetric addition of aryl boron reagents to enones with rhodium dicyclophane imidazolium carbene catalysis. Angew. Chem. Int. Ed., 42, 5871 5874. 

The use of nucleophilic catalysts is promising. Indeed, ring-opening polymerisation of LA initiated by alcohols and catalysed by nucleophiles, such as tertiary amines (e.g., 4-dimethylaminopyridine, 4-pyrrolidinopyridine) (Nederberg et al, 2001) and N-imidazolium carbenes (Connor et al, 2002) is well controlled. A monomer activation mechanism (Fig. 4.11), similar to the one reported for biocatalysis, has been proposed. 

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