Visible light-induced transition metal-catalyzed transformations: beyond conventional photosensitizers

Abstract

Employment of simple transition metal (TM = Co, Fe, Cu, Pd, Pt, Au)-based photocatalyst (PC) has led to the dramatic acceleration of known TM-catalyzed reactions, as well as to the discovery of unprecedented chemical transformations. Compared to the conventional cooperative/dual photocatalysis (type B), this new class of unconventional PCs operates via a single photoexcitation/catalytic cycle, where the TM complex plays a "double duty" role by harvesting light and catalyzing the chemical transformation. Also, these TM photocatalysts participate in the bond-forming/breaking event in the transformation via a substrate-TM interaction, an aspect that is uncommon for conventional photocatalysis (type A). This tutorial review highlights the recent advances in this emerging area.

Figure 1

Photocatalytic approaches: a) Traditional photocatalysis. b) Cooperative/dual photocatalysis. c) Visible light-induced transition metal catalysis: focus of this tutorial review.

Scheme 1

a) Scheffold’s photoinduced/electrochemical Co-catalyzed reductive cascade reaction. b) Catalytic cycle.

Scheme 2

Carreira’s photoinduced Co-catalyzed Heck reaction.

Scheme 3

Scope of Carreira’s photoinduced Co-catalyzed intramolecular Heck reaction of alkyl halides.

Scheme 4

Mechanism of Carreira’s photoinduced Co-catalyzed intramolecular Heck reaction of alkyl halides.

Scheme 5

Carreira’s photoinduced Co-catalyzed intermolecular Heck reaction of iodomethyltrifluoromethanes with styrenes.

Scheme 6

Darcel and Sortais’ photocatalytic hydrosilylation of ketones and aldehydes using Fe(II) catalyst.

Scheme 7

Scope of Hwang’s photoinduced Cu-catalyzed, “Pd-free,” Sonogashira reaction.

Scheme 8

Catalytic-cycle of the Hwang’s photoinduced Cu-catalyzed, “Pd-free,” Sonogashira reaction.

Scheme 9

a) Scope of Hwang’s photoinduced synthesis of unsymmetrical conjugated diynes. b) Catalytic-cycle.

Scheme 10

(a) Comparison of the Jiao and Hwang’s method for the synthesis of α-ketoamides (b) Hwang’s synthesis of epoxide hydrolase inhibitor.

Scheme 11

Mechanism of Hwang’s oxidative coupling of aryl amines and terminal alkynes.

Scheme 12

Hwang’s three-component coupling.

Scheme 13

Catalytic-cycle for Hwang’s three-component coupling.

Scheme 14

Scope of Fu and Peters’ asymmetric coupling.

Scheme 15

Mechanism of Fu and Peters’ photoinduced asymmetric coupling.

Scheme 16

Hwang synthesis of α-ketoimides and α-ketoamides.

Scheme 17

Hwang’s photocatalytic synthesis of aryl ketones.

Scheme 18

Mechanism of Hwang’s photocatalytic synthesis of aryl ketones.

Scheme 19

Catalytic Cycle of the photoinduced Heck reaction.

Scheme 20

Scope of Gevorgyan’s photoinduced Pd-catalyzed desaturation reaction.

Scheme 21

(a) Mechanistic studies for Gevorgyan’s photoinduced Pd-catalyzed desaturation reaction. (b) Proposed catalytic cycle.

Scheme 22

Tung’s Pt-catalyzed photochemical evolution of H₂.

Scheme 23

Catalytic cycle of the Pt-catalyzed photochemical evolution of H₂.

Scheme 24

(a) Hashmi’s visible light-induced Au-catalyzed 1,2-difunctionalization of alkynes. (b) Further applications of Hashmi’s method. (c) Oxyarylation of alkenes under photoinduced Au-catalysis with (right) and without (left) exogenous photosensitizers.

Scheme 25

a) Mechanistic studies and b) Proposed mechanism of Hashmi’s visible light-induced Au-catalyzed 1,2-difunctionalization of alkynes.

Scheme 26

Hashmi’s visible light-induced Au-catalyzed cross-coupling reaction.

Scheme 27

Proposed mechanism of Hashmi’s visible light-induced Au-catalyzed cross-coupling reaction.