Visible-Light-Promoted Oxidative Annulation of Naphthols and Alkynes: Synthesis of Functionalized Naphthofurans.

A visible-light-mediated site-selective oxidative annulation of naphthols with alkynes for the synthesis of functionalized naphthofurans has been developed. The reaction relies on the in situ formation of an electron donor acceptor pair between phenylacetylene and thiophenol as the light-absorbing system to obviate the requirement of an added photocatalyst. The protocol facilitates the transformation of 1-naphthol and 2-naphthol as well as 1,4-naphthoquinone into a wide variety of highly functionalized naphthofurans.

Photocatalytic functionalizations of alkynes.

Visible light mediated functionalizations have significantly expanded the scope of alkynes by unraveling new mechanistic pathways and enabling their transformation to diverse structural entities. The photoredox reactions on alkynes rely on their innate capability to generate myriad carbon-centred radicals via single electron transfer (SET), thereby, allowing the introduction of new radical precursors. Moreover, an array of methods have been developed facilitating transformations such as vicinal or gem-difunctionalization, annulation, cycloaddition and oxidative reactions to construct numerous key building blocks of natural and pharmaceutically important molecules. In addition, the introduction of photoredox chemistry has successfully been used to deal with the challenges associated with alkyne functionalization such as stereoselective and regioselective control. This article accounts for several visible light mediated functionalization reactions of alkynes, wherein they have been transformed into α-oxo compounds, ß-keto sulfoxides, substituted olefins, N-heterocycles, internal alkynes and sulfur containing compounds. The article has been primarily categorized into various sections based on the reaction type with particular attention being paid to mechanistic details, advancement and future applications.

Photoredox-Mediated Generation of gem-Difunctionalized Ketones: Synthesis of α,α-Aminothioketones.

A photoredox-mediated gem-difunctionalization of alkynes leading to the synthesis of α,α-aminothio-substituted carbonyl compounds is reported. The work presents concomitant introduction of α-C-N and C-S bonds as a key enabling advance. Furthermore, the low bond dissociation energy of the C-S bond compared to that of C-N or C-C bonds has been exploited for selective functionalization using different nucleophiles to build diverse α,α-disubstituted carbonyl scaffolds. Mild reaction conditions, broad substrate scope, and good yields are some of the added advantages of this reaction.

Photoredox Generated Vinyl Radicals: Synthesis of Bisindoles and ß-Carbolines.

A photoredox catalyzed approach enabling use of alkynes as surrogate of 2-oxoaldehydes/1,2-diones is reported. The method overcomes the difficulty associated with application of unsubstituted aliphatic α-oxoaldehydes, which has hitherto limited their general use. Indoles, tryptamine, and tryptophan methyl ester participated in the reaction to give a variety of α-oxo based analogues. Quantum yield investigations support a radical chain mechanism.