Coupling reactions with carbene complexes

Pyridine compounds 45 can also be produced by the NHC-Ni catalysed cycloaddition between nitriles 43 and diynes 44 (Scheme 5.13) [16]. The SIPr carbene was found to be the best ligand for the nickel complex in this reaction. The reaction required mild reaction conditions and low catalyst loadings, as in the case of cycloaddition of carbon dioxide. In addition to tethered aUcynes (i.e. diynes), pyridines were prepared from a 3-component coupling reaction with 43 and 3-hexyne 23 (Scheme 5.13). The reaction of diynes 44 and nitriles 43 was also catalysed by a combination of [Ni(COD)J, NHC salts and "BuLi, which generates the NHC-Ni catalyst in situ. The pyridines 45 were obtained with comparable... 

In a study of reductive McMurry-type coupling of carbonyl compounds with (OR)3WsW(OR)3 (R = 2,2-dimethylpropyl), a dinuclear cyclopropylmethylidene complex 14 was isolated and structurally characterized by X-ray.The cyclopropyl group remained intact (also in subsequent coupling reactions with other carbonyl compounds) thus, no long-lived carbene intermediate (or cation or anion) is involved and the tungsten carbon bond must have considerable covalent [Pg.1893]

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],... 

Diisopropyl- and l,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene ligands and complexes of Pd(ii) have been synthesized. The complexes were obtained via an Ag-carbene transfer reaction with PdCl2(NGMe)2 and X-ray structures determined. The complexes were found to be extremely effective in Heck coupling reactions with aryl bromides but much less so with aryl chlorides. Palladium complexes of the triazole-based carbenes, 1,4-dimethyl-l,2,4-triazolin-2-ylidene, and chelating l,T-methylenebis(4-alkyl-l,2,4-triazolin-2-ylidene) have been synthesized by... 

The reactivity of A-heterocyclic carbenes (NHC) with iron has been presented through the various types of complexes that have been described. Their catalytic applications in cross-coupling, allylation, aziridination, hydrosilylation, and as hydrogenase mimics are detailed. A report on cross-coupling reactions catalysed by complexes of iron group metals with NHC features the reaction mechanisms. ... 

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. 

Another example of a [2s+2sh-1c+1co] cycloaddition reaction was observed by Barluenga et al. in the sequential coupling reaction of a Fischer carbene complex, a ketone enolate and allylmagnesium bromide [120]. This reaction produces cyclopentanol derivatives in a [2S+2SH-1C] cycloaddition process when -substituted lithium enolates are used (see Sect. 3.1). However, the analogous reaction with /J-unsubstituted lithium enolates leads to the diastereoselective synthesis of 1,3,3,5-tetrasubstituted cyclohexane- 1,4-diols. The ring skeleton of these compounds combines the carbene ligand, the enolate framework, two carbons of the allyl unit and a carbonyl ligand. Overall, the process can be considered as a for-... 

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