Enolate anions, arylation substrates

Although non-enolizable substrates are not arylated by arylbismuth(V) compounds under neutral conditions, their derived enolate anions react easily with Ph3BiCl2, Ph3BiC03 and tetraphenylbismuth(V) compounds to produce... 

The erythro isomer cannot eliminate entirely from an anti conformation because only the cis olefin would be produced. Of the two hydrogen atoms which could be abstracted by a base, the more acidic atom is removed and the elimination probably proceeds through an enolate anion (32). These anions however, have the same configuration as their precursors as isomerisation of the erythro to threo substrate before elimination is not observed, presumably due to steric interactions which cause rotation about the 2-3 carbon-carbon bond to be a slower process than proton capture from the solvent. To minimise steric interactions in the erythro dichloride, the bulky aryl residues will separate to a maximum by occupying the "anti positions (32). 

By carefully tuning the structure of the starting materials, Orru et al. [21] developed an efficient synthesis of imidazoline 18 by using essentially the same reaction partners (Scheme 15.8). The key to substrate design is the use of a-aryl-substituted a-isocyanoacetate (17) and the primary amine instead of the secondary amine as reaction partner of the aldehyde. The reaction was initiated by nucleophihc addition of the enolate anion of 17 onto the imine 19, generated in situ. However, the lack of an additional acidic proton a to ester in the Mannich adduct 20 made the P-elimination impossible. On the other hand, the Mannich adduct having a secondary amine remained nucleophilic and could add intramolecularly to the divalent isocyano carbon, leading, after protonation, to the imidazoline 18 in... 

The factors affecting the relative reactivity of aryl halides in SrnI reactions have been analysed and compared645. Competition experiments of pairs of substrates, in photo-stimulated reactions with pinacolone enolate ion in liquid ammonia, reveal a spread of reactivity exceeding three powers of ten. The ease of formation of the radical anion of the substrate appears to dominate the overall reactivity. The rate of dehalogenation of the radical anion may become important when its stability exceeds a certain threshold. When the fragmentation rate of the radical anion intermediate is fairly slow, the overall reactivity diminishes. 

In subsequent studies, a number of enolisable substrates, such as phenols and enols as well a various anions, were found to be arylated under mild conditions by triarylbismuth carbonates, as well as by other pentavalent triarylbismuth or tetraphenylbismuth compounds and by pentaphenylbismuth. 

Diaryliodonium salts have found synthetic application as arylating reagents in reactions with various organic substrates under polar, catalytic, or photochemical conditions. Typical examples of arylations of nucleophiles under polar, non-catalytic conditions are shown in Scheme 3.271 and include the reactions of diaryliodonium salts with thiosulfonate anions 675 [861], fluoride anion [862,863], malonates 676 [864] and silyl enol ethers 677 [865]. 

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