Imidazole enolate arylation

Imidazole-containing compounds have been utilized as reagents for various synthetic transformations. A convenient access to substituted allyl enol carbonates was established through the reaction of ketone enolates with the complex of allyl l//-imidazole-l-carboxylates 74 and boron trifluoride etherate <07JOC9372>. Relatively mild and highly efficient Cul-catalyzed /V-arylation procedures for imidazoles with aryl and heteroaryl bromides or chlorides have been developed in the presence of ligands 75 and cesium carbonate <07JOC2737>. a, -Unsaturated 2-acyl imidazoles 76 are an alternative and practical class of dienophiles for the DNA-based catalytic asymmetric Diels-Alder reaction in... 

Ketz et al. synthesised a keto functionalised imidazolium salt using the standard protocol by reacting the appropriate N-aryl substituted imidazole with the respective keto halide [95], Their intention was to use the corresponding nickel(II) carbene enolate complex in olefin polymerisation reactions similar to the phosphino enolates in the SHOP process [1 ]. It proved difficult to prepare the intended precatalyst with only one carbene enolate ligand attached to the nickel centre. Initially, the homoleptic complex with two carbene enolate ligands was formed. The authors pointed out that the high proportion of n-olefins is unusual. 

Unlike formamidine, acetamidine and benzamidine react with both aromatic and aliphatic a-hydroxyketones to give imidazoles exclusively. It has been suggested that aryl groups favour the enolic form (2) of the tautomeric mixture, resulting in the formation of oxazoles as major products. Aliphatic groups favour the keto form (1), from which imidazoles are derived. That amidines more complex than formamidine favour imidazole formation may be a consequence of steric hindrance to reaction of the enolic hydroxy groups with the amidine carbon in (2). The general reaction has been used to prepare such compounds as 4,5-dipropyl imidazole (25% yield from tris(formylamino)-methane and 5-hydroxyoctan-4-one), and a variety of 2-imidazolones and 2-aminoimidazoles [8]. The fact that oxazoles can be converted into imidazoles with some ease extends the applicability of this reaction. 

Imidazole and benzimidazole esters are quite reactive towards lithium enolates of ketones. Kinetic studies show that the reaction involves primarily monomers of the enolates <19990L145>. This is in contrast to reactions of aryl esters where both monomers and aggregates of the enolates participate in the reaction. Although (thio)carbonylimidazole esters are normally formed between alcohols and Kl -carbonyldiimidazole or l,l -(thiocarbonyl)diimidazole 1039, transfer of the imidazole group has been reported in reactions with activated alcohols to give, for example, 1040 (Scheme 250) <1997JOC7319>. 

Scheme 8.95. The formation of a generic silyl enol ether (R = alkyl, alkaryl, aryl, etc.) from a generic carbonyl compound (aldehyde or ketone) with imidazole as base in methylene chloride solvent. It is argued that the silylation occurs preferentially on oxygen because of the strength of the silicon-oxygen bond. The silyl enol ether can be reconverted to the corresponding aldehyde or ketone by treatment with tetra- -butylammonimn fluoride ( -Bu4N" E).

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