Visible photons as ideal reagents for the activation of coloured organic compounds

In recent decades, the traceless nature of visible photons has been exploited for the development of efficient synthetic strategies for the photoconversion of colourless compounds, namely, photocatalysis, chromophore activation, and the formation of an electron donor/acceptor (EDA) complex. However, the use of photoreactive coloured organic compounds is the optimal strategy to boost visible photons as ideal reagents in synthetic protocols. In view of such premises, the present review aims to provide its readership with a collection of recent photochemical strategies facilitated via direct light absorption by coloured molecules. The protocols have been classified and presented according to the nature of the intermediate/excited state achieved during the transformation.

Visible-Light-Triggered Difluoroacetylation/Cyclization of Chromone-Tethered Alkenes Enabling Synthesis of Tetrahydroxanthones

Hydroxanthones have attracted considerable attention due to their significance in organic and biological chemistry, yet their synthesis remains a great challenge. In this study, a series of chromone-tethered alkenes are designed, and a radical cyclization reaction of these chromone derivatives has been achieved under photoredox conditions. The reaction uses bromodifluoroacetamides or bromodifluoroacetates as coupling partners, affording a broad range of functionalized tetrahydroxanthone products with up to 85% yields. The reaction is triggered via the generation of difluoroacetate radicals or alkene radical cations with fac-Ir(ppy)3 or 2,3,5,6-tetrakis(carbazol-9-yl)-1,4-dicyanobenzene as a photocatalyst. This approach offers access to various tetrahydroxanthone derivatives from readily available starting materials and enriches the research content of heteroarene-tethered alkenes.

Co-Catalyzed atom transfer radical addition of bromodifluoroacetamides, expanding the scope of radical difluoroalkylation

The atom transfer radical addition (ATRA) of bromodifluoroacetamides to arylalkynes and terminal alkenes was conducted using von Wangelin's Co catalyst system (CoBr₂/1,2-bis(diphenylphosphino)benzene/Zn) in acetone/H₂O at 30 °C to afford the corresponding functionalized difluoroacetamides in 33-89% yields. Moreover, the Co catalyst was successfully applied to the tandem addition/cyclization of 1,6-diene and -enyne substrates and intramolecular ATRA of N-allyl and N-propargyl bromodifluoroacetamides, significantly expanding the scope of radical difluoroalkylation.

Photoinduced inverse Sonogashira coupling reaction

A transition-metal and photocatalyst-free, photoinduced inverse Sonogashira coupling reaction was developed. Under visible-light irradiation, the excited state iodoalkyne acted as an “alkynyl radical synthetic equivalent”.