The chemistry of amine radical cations produced by visible light photoredox catalysis

Abstract

Amine radical cations are highly useful reactive intermediates in amine synthesis. They have displayed several modes of reactivity leading to some highly sought-after synthetic intermediates including iminium ions, α-amino radicals, and distonic ions. One appealing method to access amine radical cations is through one-electron oxidation of the corresponding amines under visible light photoredox conditions. This approach and subsequent chemistries are emerging as a powerful tool in amine synthesis. This article reviews synthetic applications of amine radical cations produced by visible light photocatalysis.

Keywords: amine radical cation; catalysis; distonic ion; free radical; iminium ion; photoredox; visible light; α-amino radical.

Scheme 1

Amine radical cations’ mode of reactivity.

Scheme 2

Reductive quenching of photoexcited Ru complexes by Et₃N.

Scheme 3

Photoredox aza-Henry reaction.

Scheme 4

Formation of iminium ions using BrCCl₃ as stoichiometric oxidant.

Scheme 5

Oxidative functionalization of N-aryltetrahydroisoquinolines using Eosin Y.

Scheme 6

Synthetic and mechanistic studies of Eosin Y-catalyzed aza-Henry reaction.

Scheme 7

Oxidative functionalization of N-aryltetrahydroisoquinolines using RB and GO.

Scheme 8

Merging Ru-based photoredox catalysis and Lewis base catalysis for the Mannich reaction.

Scheme 9

Merging Au-based photoredox catalysis and Lewis base catalysis for the Mannich reaction.

Scheme 10

Merging Ru-based photoredox catalysis and Cu-catalyzed alkynylation reaction.

Scheme 11

Merging Ru-based photoredox catalysis and NHC catalysis.

Scheme 12

1,3-Dipolar cycloaddition of photogenically formed azomethine ylides.

Scheme 13

Plausible mechanism for photoredox 1,3-dipolar cycloaddition.

Scheme 14

Photoredox-catalyzed cascade reaction for the synthesis of fused isoxazolidines.

Scheme 15

Plausible mechanism for the photoredox-catalyzed cascade reaction.

Scheme 16

Photoredox-catalyzed α-arylation of glycine derivatives.

Scheme 17

Photoredox-catalyzed α-arylation of amides.

Scheme 18

Intramolecular interception of iminium ions by sulfonamides.

Scheme 19

Intramolecular interception of iminium ions by alcohols and sulfonamides.

Scheme 20

Intermolecular interception of iminium ions by phosphites.

Scheme 21

Photoredox-catalyzed oxidative phosphonylation by Eosin Y.

Scheme 22

Conjugated addition of α-amino radicals to Michael acceptors.

Scheme 23

Conjugated addition of α-amino radicals to Michael acceptors assisted by a Brønsted acid.

Scheme 24

Conjugated addition of α-amino radicals derived from anilines to Michael acceptors.

Scheme 25

Oxygen switch between two pathways involving α-amino radicals.

Scheme 26

Interception of α-amino radicals by azodicarboxylates.

Scheme 27

α-Arylation of amines.

Scheme 28

Plausible mechanism for α-arylation of amines.

Scheme 29

Photoinduced C–C bond cleavage of tertiary amines.

Scheme 30

Photoredox cleavage of C–C bonds of 1,2-diamines.

Scheme 31

Proposed mechanism photoredox cleavage of C–C bonds.

Scheme 32

Intermolecular [3 + 2] annulation of cyclopropylamines with olefins.

Scheme 33

Proposed mechanism for intermolecular [3 + 2] annulation.

Scheme 34

Photoinduced clevage of N–N bonds of aromatic hydrazines and hydrazides.