Porphyrins as Photoredox Catalysts in Csp²–H Arylations: Batch and Continuous Flow Approaches

Abstract

We have investigated both batch and continuous flow photoarylations of enol-acetates to yield different α-arylated aldehyde and ketone building blocks by using diazonium salts as the aryl-radical source. Different porphyrins were used as SET photocatalysts, and photophysical as well as electrochemical studies were performed to rationalize the photoredox properties and suggest mechanistic insights. Notably, the most electron-deficient porphyrin (meso-tetra­(pentafluorophenyl)­porphyrin) shows the best photoactivity as an electron donor in the triplet excited state, which was rationalized by the redox potentials of excited states and the turnover of the porphyrins in the photocatalytic cycle. A two-step continuous protocol and multigram-scale reactions are also presented revealing a robust, cost-competitive, and easy methodology, highlighting the significant potential of porphyrins as SET photocatalysts.

Photocatalysis with organic dyes: facile access to reactive intermediates for synthesis

“…The key aryl radical was trapped with heteroarenes, such as 11.2, to give the arylation products 11.3. Recently, similar methodologies for aryl radical generations have been developed exploiting flow techniques [74] and different organophotocatalysts, such as a metal-free porphyrin [75] and rhodamine 6G (OD14) [76].…”

Leaving Groups in Metal‐Free Arylations: Make Your Choice!

“…Furthermore, a two-step continuous flow protocol in which the diazonium salt is generated in situ and then subjected to photoarylation has also been developed. [99] Finally, both diazotated anthranilic acid and (more rarely) 2aminobenzensulfonic acids are mainly known as thermal precursors of arynes. [100] As a recent example, biomass derived methyl 2,5-furandicarboxylate was smoothly converted to 1,4-naphthalenedicarboxylic acid 21 (Scheme 21) via benzyne Scheme 21.…”

Recent applications of porphyrins as photocatalysts in organic synthesis: batch and continuous flow approaches

“…The adequate tuning of the porphyrin properties can enable them to absorb light in almost all of the UV-vis spectral range. Porphyrins also have elevated molar absorptivity (ca 10 5 L•mol −1 •cm −1 ) and appropriate electronic levels for both energy transfer (ET) and single electron transfer (SET) in many photoprocesses [9][10][11]. Additionally, it is possible to realize tuning in terms of chemical properties by changing substituents, thus producing robust, soluble or heterogeneous, readily available and low-cost photocatalysts.…”