Catalytic Generation of Carbanions through Carbonyl Umpolung

Abstract

Carbonyl Umpolung is a powerful strategy in organic chemistry to construct complex molecules. Over the last few years, versatile catalytic approaches for the generation of acyl anion equivalents from carbonyl compounds have been developed, but methods to obtain alkyl carbanions from carbonyl compounds in a catalytic fashion are still at an early stage. This Minireview summarizes recent progress in the generation of alkyl carbanions through catalytic carbonyl Umpolung. Two different catalytic approaches can be utilized to enable the generation of alkyl carbanions from carbonyl compounds: the catalytic Wolff–Kishner reaction and the catalytic single‐electron reduction of carbonyl compounds and imines. We discuss the reaction scope, mechanistic insights, and synthetic applications of the methods as well as potential future developments.

Deoxygenative Functionalizations of Aldehydes, Ketones and Carboxylic Acids

“…Although the diazo compound can behave as a free carbene under metal‐free conditions, this reactivity is more frequently elaborated with transition metals which form metallocarbenoids. Given that copious examples have appeared in the literature and have been nicely reviewed, [35–39] herein, only three classic reactions of tosylhydrazone‐derived metallocarbenoids will be presented briefly, including cyclopropanation, C−H insertion and migratory insertion. Additionally, it is noticed that an increasing number of tosylhydrazone‐based methodologies have been applied in natural product synthesis; therefore, critical steps in these syntheses will be used to exemplify the utility of this chemistry.…”

Deoxygenative Functionalizations of Aldehydes, Ketones and Carboxylic Acids

“…More importantly, the generation of radicals from carbonyl compounds such as aldehydes and ketones can often be realized without prefunctionalization (Scheme 12 a). [38, 92] Despite this theoretical feasibility, a direct deoxygenative conversion of aldehydes and ketones into alkyl radicals or their equivalents with the newly generated radical center on the carbonyl carbon remains rare.…”

“…For example, Baran and his co‐workers utilized the formaldehyde‐derived N ‐(alkylsulfonyl)hydrazone to enable an Fe‐catalyzed, two‐step hydromethylation across the alkenes [43] . A recent work from König's laboratory showed an elegant photoredox strategy for carbonyl difunctionalizations via tosylhydrazones [38, 44] . In both instances, tosylhydrazones served as radical acceptors, capturing the radical generated under catalytic conditions.…”

“…[43] Ar ecent work from Kçnigsl aboratory showed an elegant photoredox strategy for carbonyl difunctionalizations via tosylhydrazones. [38,44] In both instances,t osylhydrazones served as radical acceptors,c apturing the radical generated under catalytic conditions.…”

Ruthenium(ii)-catalyzed regioselective 1,6-conjugate addition of umpolung aldehydes as carbanion equivalents

“… 40 Furthermore, the formation of carbanion species by carbonyl umpolung generates a “soft” nucleophile through polarity inversion of the carbonyl carbon. 41 To capitalize on the soft property of the nucleophile, we speculated a favourable, soft–soft interaction following the HSAB theory. 42 For the nucleophile, such an interaction can be realized with a late transition-metal, ruthenium( ii )-catalyst.…”