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.

Keywords: Umpolung; Wolff-Kishner reaction; carbanions; photocatalysis.

Scheme 1

Catalytic generation of carbanions by carbonyl Umpolung.

Scheme 2

Generation of carbanions by catalytic WK processes.

Scheme 3

Ru‐catalyzed trapping of carbanionic species generated in the WK process.

Scheme 4

Metal‐catalyzed trapping of carbanionic species generated in the WK process.

Scheme 5

Intramolecular radical functionalization of sulfonylhydrazones.

Scheme 6

Catalytic radical functionalization of hydrazones.

Scheme 7

Iron‐catalyzed hydromethylation of alkenes.

Scheme 8

Photo‐WK reaction for the catalytic difunctionalization of sulfonylhydrazones.

Scheme 9

Photoredox catalytic generation of α‐amino carbanions—Fan/Walsh pathway.

Scheme 10

Photoredox catalytic system developed by Fan and Walsh for the generation of carbanions from imines.

Scheme 11

Photoredox catalytic hydrocarboxylation of imines developed by Yu and co‐workers.

Scheme 12

Generation of carbanions by the sequential reduction of carbonyl compounds.

Scheme 13

Photoinduced Ru‐catalytic generation of carbanions by the sequential reduction of carbonyl compounds.

Scheme 14

Photocatalytic enantioselective hydrogenation of 1,2‐diketones and α‐keto ketimines.

Scheme 15

Photocatalytic enantioselective reduction of ketones.

Scheme 16

Photocatalytic generation of α‐boryl carbanions.