Umpolung reactions at the α-carbon position of carbonyl compounds

Abstract

Umpolung chemistry has been a focus of research in organic synthesis because of its usefulness and unusual reactivity. Above all, umpolung of enamine properties enables the formation of highly substituted carbon-carbon bonds and the installation of aryl groups which is difficult to achieve using normal enolate chemistry. This short review describes reports of carbon-carbon bond formation using umpolung reactions of enol and enamine derivatives.

Environmentally Friendly Protocol for the Oxidative Iodofunctionalization of Olefins in a Green Solvent

“…Inspired by umpolung chemistry, − we envisaged that an environmentally friendly iodofunctionalization of unfunctionalized olefins could be realized via the oxidative umpolung of iodide using the readily available and more practical iodide salt and inorganic oxidant (Scheme , bottom). Although oxidative transformations via the umpolung of halides (Cl – , Br – , and I – ) into halonium ions (X + ) using oxidants have attracted much attention as a sustainable protocol in organic synthesis, , we assumed that two issues will have to be taken into consideration in this process.…”

An α‐Cyclopropanation of Carbonyl Derivatives by Oxidative Umpolung

“…Some umpolung reagents such as the cyanide anion or thiazolium based NHCs (both react as C1 nucleophiles and derive formally from formic acid) can transfer this umpolung‐reactivity through catalysis (Scheme a–III) . A third approach to reverse the polarity of a given functional group consist in its oxidation or reduction by an external or internal reagent (Scheme a–IV) . Ketones and their derivatives for instance are commonly employed in oxidative umpolung reactions.…”

Sulfur(iv)-mediated umpolung α-heterofunctionalization of 2-oxazolines

“…In contrast, reversing the polarity of carbonyls with halogens, hypervalent iodines, transition metals and other oxidative mediators provides them with unnatural electrophilic properties and consequently greatly expands the scope of coupling partners from electrophiles to nucleophiles. 2 In this context, umpolung tactics using hypervalent iodine reagents enabling α-functionalization of ketones with nucleophiles have gained enormous attention since their discovery in 1978 by Mizukami et al 3 In principle, there are two typical carbonyl sources used for iodine( iii )-mediated umpolung reactions. 4 First, ketones can be treated with nucleophile-substituted aryl iodanes to implement the umpolung event ( Fig.…”