Recent Advances on Metal-Free, Visible-Light- Induced Catalysis for Assembling Nitrogen- and Oxygen-Based Heterocyclic Scaffolds

Abstract

Heterocycles are important class of structures, which occupy a major space in the domain of natural and bioactive compounds. For this reason, development of new synthetic strategies for their controllable synthesis became of special interests. The development of novel photoredox systems with wide-range application in organic synthesis is particularly interesting. Organic dyes have been widely applied as photoredox catalysts in organic synthesis. Their low costs compared to the typical photocatalysts based on transition metals make them an excellent alternative. This review describes proceedings since 2015 in the area of application of metal-free, visible-light-mediated catalysis for assembling various heterocyclic scaffolds containing five- and six-membered rings bearing nitrogen and oxygen heteroatoms.

Keywords: heterocycles; organic dyes; photocatalysis; photoredox; photoredox cyclization; visible-light-induced catalysis.

Figure 1

Selected example of current drugs containing heterocyclic scaffold.

Figure 2

Examples of metal-free photoredox catalysts.

Scheme 1

(a) Vicinal thioamination of alkynes, mediated by Eosin Y; (b) Proposed mechanism for vicinal thioamination of alkynes, mediated by Eosin Y.

Scheme 2

(a) Visible-light-induced cyclization of arylsulfonyl chlorides; (b) Proposed mechanism.

Scheme 3

Dehydrogenative cascade trifluoromethylation and oxidation of 1,6-enynes.

Scheme 4

(a) A visible-light-induced cross-dehydrogenative cascade in the synthesis of tetrahydroisoquinoline derivatives; (b) Proposed mechanism.

Scheme 5

(a) [2 + 2 + 2] Cyclization of alkynes with nitriles; (b) Proposed mechanism.

Scheme 6

(a) Cycloaddition of N-propargyl aromatic amines, diaryliodonium salts, and sulfur dioxide; (b) Proposed mechanism.

Scheme 7

(a) Cyclization of O-(2,4-dinitrophenyl)oximes to phenanthridine; (b) Proposed mechanism.

Scheme 8

(a) Decarboxylative radical cyclization of vinyl azides with N-acyloxyphthalimides; (b) Proposed mechanism.

Scheme 9

(a) Hydroimination and iminohydroxylation cyclization of olefins; (b) Proposed mechanism.

Scheme 10

Synthesis of 3-(trifluoromethyl)indolin-2-one derivatives under photoredox conditions.

Scheme 11

(a) Synthesis of 3,4-dihydroisoquinolinones; (b) Proposed mechanism.

Scheme 12

[2 + 2] Photocycloaddition of enones.

Scheme 13

(a) Synthesis of tetrahydroquinoline derivatives; (b) Proposed mechanism for synthesis of tetrahydroquinoline derivatives.

Scheme 14

(a) [4 + 2] Radical cyclization of N-methylanilines with maleimides in the synthesis of tetrahydroquinolines; (b) Proposed mechanism.

Scheme 15

(a) Preparation of isoquinolinediones; (b) Proposed mechanism for preparation of isoquinolinediones.

Scheme 16

(a) Visible-light-mediated oxidative [3 + 2] cycloaddition in the synthesis of furans; (b) Proposed mechanism.

Scheme 17

(a) Synthesis of 3,4-di-substituted mercaptolactones; (b) Proposed mechanism.

Scheme 18

(a) Hydroetheryfication of alkenols; (b) Proposed mechanism.

Scheme 19

(a) Flavin-mediated visible-light [2 + 2] cycloaddition of dienes; (b) Proposed mechanism.

Scheme 20

(a) Visible-light-mediated radical cascade cyclization of polyenes; (b) Proposed mechanism.

Scheme 21

(a) Visible-light-mediated nitrone 1,3-dipolar cycloaddition; (b) Proposed mechanism.

Scheme 22

(a) [3 + 2] Cycloaddition of oxaziridines with alkynes; (b) Proposed mechanism.

Scheme 23

(a) Application of amidoximes in visible-light-driven synthesis of oxadiazolines; (b) Proposed mechanism.

Scheme 23

(a) Application of amidoximes in visible-light-driven synthesis of oxadiazolines; (b) Proposed mechanism.

Scheme 24

(a) Hydrofunctionalization of unsaturated amides; (b) Proposed mechanism.

Scheme 24

(a) Hydrofunctionalization of unsaturated amides; (b) Proposed mechanism.

Scheme 25

(a) Visible-light-mediated in situ generation and conversion of fluorophosgene; (b) Proposed mechanism.

Scheme 25

(a) Visible-light-mediated in situ generation and conversion of fluorophosgene; (b) Proposed mechanism.

Scheme 26

(a) Synthesis of quinazolines; (b) Proposed mechanism.

Scheme 26

(a) Synthesis of quinazolines; (b) Proposed mechanism.

Scheme 27

Dehydrogenation of amines to imines.

Scheme 28

(a) Photoredox alkoxycarbonylation–addition–cyclization sequence; (b) Proposed mechanism.

Scheme 29

(a) Synthesis of 1,3-oxazines under photoredox conditions; (b) Proposed mechanism.

Scheme 30

(a) Synthesis sulfonated quinazolinones via oxidative/reductive cyclization; (b) Proposed mechanism.