Abstract
Vinylogous enolate and enolate-type carbanions, generated by deprotonation of α,β-unsaturated compounds and characterized by delocalization of the negative charge over two or more carbon atoms, are extensively used in organic synthesis, enabling functionalization and C–C bond formation at remote positions. Similarly, reactions with electrophiles at benzylic and heterobenzylic position are performed through generation of arylogous and heteroarylogous enolate-type nucleophiles. Although widely exploited in metal-catalysis and organocatalysis, it is only in recent years that the vinylogy and arylogy principles have been translated fruitfully in phase-transfer catalyzed processes. This review provides an overview of the methods developed to date, involving vinylogous and (hetero)arylogous carbon nucleophiles under phase-transfer catalytic conditions, highlighting main mechanistic aspects.
Highlights
It is well-known that the resonance stabilization effect of a functional group is propagated through the π-electrons of one or more conjugated multiple bonds, according to the principle of vinylogy [1,2]
Moderate conversion (30% yield) to the expected spiro cycloadduct 57a were instead achieved under Phase transfer catalysis (PTC) conditions, employing tetrabutyl ammonium bromide (TBAB)
In early years vinylogous synthesis was dominated by metal promoted processes, asymmetric organocatalysis has assumed an increasing pivotal role in this field, as witnessed by the most recent review papers on this topic
Summary
It is well-known that the resonance stabilization effect of a functional group is propagated through the π-electrons of one or more conjugated multiple bonds, according to the principle of vinylogy [1,2]. The inorganic base initiated reactions, involving the deprotonation of a moderately acidic substrate with the generation of a reacting organic anion, are the most commonly used phase transfer catalyzed processes in organic synthesis. This technique started to be applied to vinylogous precursors only recently. The presence of solid or highly concentrated aqueous alkali bases, makes it possible to generate strongly basic carbanions in non-hydrated form into the organic phase This characteristic turns out to be useful in reactions of arylogous substrates which, as already mentioned, are hardly enolizable.
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